Problem of Acute ocular hypotonia in modern ophthalmosurgery

However in our opinion, not all of existing points of view concerning this problem are well-based. So, the opinion widely spread among ophthalmologists that emptying of anterior chamber and deviation of irido-lens diaphragm forward after opening of the eye are natural and they occur because of pressure predominance in posterior segment of the eye, is not conformed with hydrostatics laws. It becomes evident if to consider properties of liquids (in comparison from gases) to maintain their initial volume independently of the value of hydrostatic pressure. That is, if to consider the eye as a closed isolated hydrostatic system, no changes in position of membranes and irido-lens diaphragm because of changes of pressure in one of ocular segments, can occur (!) because liquids are not contracted and not widened under changes of pressure.

Just the same rather logical at the first sight (probably because of our social consciousness unvoluntarily is based on experience of existence in conditions of open air environment) but purely mechanical theories of development of detachment of retina or the choroid, detachment of posterior hyaloid membrane, prolapse of the vitreous, irido-lenticular block and so on because of internal tractions or changes in pressure in this or that ocular segment in fact clarify not much because of changes in position (form) of elastic tissues and changes in volume of interstitial spaces in liquid surroundings are possible, independently of gradient of hydrostatic pressure between them, only if conditions arise for additional flow-in (flow-out) of liquid or depressurization of one of intermembranous spaces.

We should take into consideration that for example while depressurising anterior chamber its shallowing and flowing of humor out, deviation of irido-lenticular diaphragm and the vitreous forward can occur only under significant reduction of volume of whole eyeball or sharp enlargement of the volume of its content. The vitreous can not move forward only because of the reason that pressure is dropped in anterior chamber because otherwise additional depot of liquid, gas or vacuum space equal to removed volume of the vitreous should immediately appear in posterior ocular segment or in retrobulbar space.

Practically there are no data about the following: how volumetric changes in the membranes and the vitreous could be compared in quantitative ratio with, for example, volume of the eye chambers. Dynamics of these changes in time and their link with concrete stages of operational intervention are not defined. Non-invasive and that is why high-informative methods of investigations such as: rheoophthalmography, ophthalmodopplerography, scanning echoophthalmography, computer and magnetoresonance tomography and others are used rather seldom for studying of above mentioned processes.

Extraocular factors

Unlike intraocular, extraocular factors leading to disorders of normal topographic relations of ocular tissues in conditions of acute hypotonia attract ophthalmosurgeons for a long time ago.

Fucks [59], Andreev L.A. [8] pointed out to the fact that course of operation much depends on arterial pressure and tonus of orbital muscles. Duke-Elder [39], Sallman [143] and others also paid attention to the role of elevated intraorbital pressure in pathogenesis of operational complications.

Many authors [30,40] paid great attention to full relaxation of blepharons in order to eliminate their pressure on the eyeball. Robinson [139] using special rubber container with manometer had defined that blepharons pressure on the eyes in conditions when they maintain their muscular tonus may fluctuate from 25 to 70 mm. of m.c.

Posagennikov A.P. [132] having made similar investigations with blepharodinamometer of personal construction, have received the following results: before making akinesia strength of blepharons contraction oscillates from 90 to 160 gr.; 5 minutes after akinesia in dependence on anaesthetics type - from 13,4 to 26,1 gr. The author also points out to the fact that the patients with developed face musculation even after qualitatively made akinesia can have blepharons contraction and their pressure on the eyeball because of contraction of facial musculature.

In accordance with opinions of many authors, one of the main factors of deformation of hypotonic eye is pressure of blepharostat [33,68]. Many ophthalmologists pay attention to favourable role of bridle suture, which relieves IOP and prevents the vitreous from prolapse [13,31,52,83]. At the same time, Karyagin V.F. [86] have investigated influence of means of the eye immobilisation on anatomic condition of the eyeball, and have made a conclusion that excessive stretching of bridle suture leads to deformation of the eyeball in the opened eye, and deformation itself leads to spasm of orbicular muscles and to more pressurisation of the eyeball.

Breslin and co-authors [22] observed drop of IOP of 50 patients after retrobulbar anaesthesia by 1,5 mm. of m.c. (at an average). They explained it by relaxation of extraocular muscles and reduction of their pressure on the eyeball. Shamsutdinova R.A. [148] is of similar opinion. As Pohjampelto [130] thinks drop of IOP after retrobulbar anaesthesia is connected with decrease of pulse volume of the eye under the influence of mechanical compression of the vessels in the orbit caused by injection of Novokainum.

In order to make full relaxation of extraocular muscles series of the authors [43,86] recommend to inject 5-6 ml of anaesthetics retrobulbarly. The others think that large amount of anaesthetics into retrobulbar space elevates intraorbital pressure in itself and intensifies deformation of the eyeball [134]. Atkinson [11] points out that practical significance of “intraorbital akinesia” resulting from retrobulbar injection is so important as anaesthetic effect.

Retrobulbar haematoma resulting from injury of vascular trunks by injection needle may also be the reason of sharp deformation and deviation of the eyeball forward [43,44,149].

More complete analysis of extraocular factors of deformation of the eyeball after its decompression can be found in the work of Shmeljova V.V. [149]. The author picked out the following reasons of such deformation: blepharon pressure on the eyeball, pressure of external commissure of blepharon, pressure of direct muscles, pressure of instruments, and also she describes a whole series of measures to eliminate the influence of such factors and prevent deformation of the eyeball.

So, in accordance with literature data the following extraocular factors leading to disorders of normal topographic relations of the eye tissues in condition of its acute hypotonia can be specified:

1) pressure of the blepharons and facial musculature; 2) pressure of extraocular muscles; 3) pressure of means of ocular immobilisation; 4) elevation in the intraorbital pressure because of excess amount of anaesthetics injected or development of retrobulbar haematoma; 5) pressure of the instruments at the time of manipulations in the ocular cavity.

All above mentioned factors cause deformation of the eyeball and as a result disparity occurs between the volume of the eye cavity and the volume of its liquid content. However, question of quantitative evaluation of the value of deformation and corresponding changes of volume of the eye cavity in conditions of acute hypotonia was not worked out in the literature. Dynamics of changes in the volume of the eye cavity after its decompression at the amount of natural contractive ability of sclera were not defined. Correlation of volumetric changes in fibrous and vascular tracts of the eye between themselves and in combination with external deforming factors was not investigated.

All of these points out to the necessity of creation a theory of the eye deformation in condition of acute hypotonia, similar to volumetric theory of Fridenvald [57,58] which is broadly used in the investigation of hydrodynamics of the eye. Experimental work of Sacharov Y.I. [142] also proves actuality of such investigations. The author has found difficult structure of deformation of elastic spherical membranes close to the fibrous capsule of the eye by their dynamic properties.

In particular, at deformation of such membranes in accordance with the data of Sacharov Y.I. there are three essential areas: area of pre-critical deformation, area of saltatory loss of steadiness and area of post-critical deformation. The most difficult is behaviour of sphere at the moment of saltatory change of deformation, because a part of sphere is deformed from convex to concave, passing, obviously through all the stages from transfiguration, applapation, to spherical impression. In spite of the fact that the author's work is connected to regularity of deformation of only fibrous capsule of the eye, it significantly widens our notion of the character of reaction of the eyeball content to decompression. However it's absolutely clear that questions of deformation of the eye as difficult integral system are waiting to be solved.

Relations between the level of ophthalmotonus and volume of intraocular humor, lost in the eye depressurization (independent investigations)

The main pathogenetic factor of development of acute ocular hypotonia during intraocular operations is its traumatic depressurization and loss of some part of its liquid content. There is direct link between the level of ophthalmotonus and the volume of intraocular humor in the eye chambers. However existing tables for defining of ophthalmotonus and showings of the eye hydrodynamics [57,120,145] can not be used for quantitative analysis of disorders of the eye hydrostatic resulting from its traumatic decompression.

It is connected with the following: existing tables were created only to define the volume of aqueous humor of the eye chambers displaced during tonometry, kept in the eye cavities and significantly influencing on the tension of its membranes and volume of uveal bloodstream. Besides, indices shown in the tables are defined for concrete types of tonometers and “averaged” eye anterior-posterior size of which is approximately 24,5 mm and radius of curvature of anterior surface of the cornea is 7.7mm [155].

During traumatic depressurization and eye decompression bodily machinery of volumetric changes in its cavity is of the other kind. Intraocular pressure is dropped usually during several seconds and , thus under the influence of elastic properties of membranes definite volume of liquid goes out of the eye ( but not redistributed inside). Hyperemia (thickening) of the choroid develops, fibrous capsule starts contracting gradually (lack of hydraulic compression), deformation of the eyeball from the outside arises. As a result of these changes the eye looses additional volume of liquid.

Relations between the level of ophthalmotonus and the volume of intraocular humor lost during the eye depressurization explored by us on human cadaveric eyes. Advantage of experimental investigations on fresh-enucleated eyes of people is in the following: influence of hydrodynamics disorders (the limits of hyperemia are defined mathematically) and deformation of the eyeball from outside on the character of volumetric changes of the eye is excluded.

The results of investigations for five sizes of the eyeball are shown in table № 2.

So, in physiological conditions difference between intraocular and hydrostatic pressure is stipulated by definite volume of intraocular humor. Hydraulic backwater created by this volume of humor in the eye cavity is balanced by some tension of the membranes of the eyeball. If not take into consideration external deformation of the eyeball and changes of blood-filling of the choroid so exactly this volume of liquid would become “excessive” and would go out of the eye immediately after its traumatic depressurization.

Calculations show (more detailed in publication 120) that in order to relieve ophthalmotonus to zero (if more precise to the level of atmospheric pressure + hydrostatic which creates in the lowest point of the eyeball its liquid content) the eye of average sizes should loose in total about 65 mm3. of humor, that is 6-7 times lesser than real volume of eye chambers. (!).

Thus, formed together balance of pressures in different segments of the eye and biomechanics (bodily machinery) of changes in the volume of intratissular spaces are so, that at dissection of the cornea loss of intraocular liquid and drop of ophthalmotonus occur mainly because of shallowing of posterior chamber. Shallowing of anterior chamber (if it's not connected with the block of the pupil and impossibility of emptying of the posterior chamber) is possible only at reduction of the volume of the eyeball itself on the account of external deformation. No internal reasons except intraocular haemorrhages can lead to shallowing of anterior chamber even in drop of ophthalmotonus to the level of atmospheric.

Table № 2

Volume of liquid in mm3 lost by the eye under the influence of elastic properties of the membranes in gradual drop of IOP by 1 mm. of m.c. in the limits of ophthalmotonus from 40 to 2mm. of m.c.

Changes of pressure by 1mm. of m.c.	Anterior-posterior size of the eye
	20mm	22mm	24mm	26mm	28mm
40-39	0,77	0,85	0,89	0,97	1,06
39-38	0,79	0,86	0,92	1.00	1.09
38-37	0,81	0,87	0,94	1,02	1,05
37-36	0,83	0,89	0,96	1,05	1,14
36-35	0,84	0,91	0,98	1,07	1,17
35-34	0,87	0,94	1,01	1,10	1,20
34-33	0,89	0,96	1,03	1,12	1,23
33-32	0,91	0,99	1,06	1,16	1,26
32-31	0,94	1,01	1,09	1,19	1,30
31-30	0,97	1,05	1,13	1,23	1,34
30-29	1,01	1,09	1,17	1,28	1,39
29-28	1,04	1,13	1,21	1,32	1,44
28-27	1,08	1,16	1,25	1,36	1,49
27-26	1,11	1,20	1,29	1,41	1,54
26-25	1,14	1,24	1,33	1,45	1,58
25-24	1,19	1,28	1,38	1,50	1,64
24-23	1,23	1,33	1,43	1,56	1,70
23-22	1,27	1,38	1,48	1,61	1,76
22-21	1,32	1,43	1,54	1,68	1,83
21-20	1,38	1,49	1,60	1,74	1,90
20-19	1,43	1,54	1,66	1,81	1,98
19-18	1,49	1,60	1,73	1,89	2,06
18-17	1,55	1,67	1,80	1,96	2,14
17-16	1,63	1,77	1,90	2,07	2,26
16-15	1,75	1,89	2,03	2,21	2,42
15-14	1,88	2,04	2,19	2,39	2,61
14-13	2,06	2,22	2,39	2,61	2,84
13-12	2,26	2,44	2,63	2,87	3,13
12-11	2,49	2,69	2,90	3,16	3,45
11-10	2,78	3,00	3,23	3,52	3,84
10-9	3,12	3,38	3,63	3,96	4,32
9-8	3,32	3,60	3,86	4,25	4,65
8-7	3,53	3,81	4,10	4,47	4,88
7-6	3,96	4,28	4,60	5,01	5,47
6-5	4,47	4,84	5,20	5,67	6,19
5-4	5,06	5,47	5,88	6,40	6,99
4-3	5,68	6,14	6,60	7,19	7,85
3-2	6,62	7,17	7,70	8,39	9,16
2-1	7,83	8,46	9,10	9,92	10,83

The table allows define theoretically before the operation by how many cubical millimetres the volume of the eye cavity would be reduced immediately after its decompression (for 10-12 seconds), resulting from contraction of elastic membranes, or what amount of liquid should be extracted from the eye in order to relieve ophthalmotonus to necessary value. The table is important for analysing the factors defining this or that reaction of the eye content for decompression in real conditions of intraocular operations and, in particular, defining the volumes of “natural” (connected with peculiar for condition of acute hypotonia changes in sizes of intraocular membranes and the vitreous) and “occasional” (external) deformation, arising under the influence of the pressure of the tissues surrounding the eye, surgical instruments or means of immobilisation.

The table has some systematic error connected with the following: to control the value of IOP we used tonograph, electronic transformer of which pressed the eyeball and deformed the cornea. As a result the volume of the liquid displaced caused additional tension of the eye membranes and some increasing of the data.

Dynamics of changes in the volume of cavity of ocular fibrous capsule after decompression (independent research)

While making cavitary ophthalmologic operations the membranes of the eye during long period of time are deprived of natural hydraulic compression. Above mentioned experimental researches have shown that at depressurization of the cavity of the eyeball at the first 10-12 seconds already because of only elastic properties looses to 65 mm3 of liquid (in accordance with table № 3).

It's natural to suppose that definite shrinkage of the eye membranes continues also in the following period and consequently the volume of the eye cavity during the operation gradually decreases. This, in particular, was proved by the work of Kozlov V.I. [94] in which the author investigated elastic properties of the ocular membranes at normal and increased IOP, and he came to a conclusion that when IOP is elevated tension occurs and when it is relieved- contraction of the fibrous capsule of the eye.

Because of absence of data in the literature concerning dynamics of changes in the surface area and volume of fibrous capsule of the eye during 30-40 minutes after its surgical depressurization (that is time corresponding to average longitude of the intraocular operation and the period of the most deep hypotonia of the eye), we have conducted independent researches.

The aim of these investigations was to define the dynamics of changes in the volume of the ocular cavity after its decompression at the expense of retraction of fibrous capsule during the period of time corresponding to longitude of intraocular operation.

Investigations were hold on cadaveric eyes of people died no more than 24 hours before with the help of special methods of microfilming of marked areas of sclera [120].

The dynamics of changes in the volume of cavity of the fibrous ocular capsule (V) during 35 minutes after its decompression (t) is shown on the diagram (pic. 7)

Pic. 7

As it is seen on the picture immediately after decompression of the eye progressive reduction of the volume of cavity of fibrous capsule was observed right up to 14 minutes, when the volume reached the less level (98,63 % from the initial). Further changes in the volume of cavity of fibrous capsule were not significant right up to 21-27 minutes when it again began gradually increasing and to the 35^th minute it increased by 0.44% from minimal level, however being by 0,93% below the initial level.

Decrease in the volume of cavity of the eyeball during the first 14-15 minutes after decompression has shown that the area of elastic sclera, lost hydraulic backwater from the inside is gradually retracted. Some enlargement of the volume arising from 24-27 minutes possibly is connected with beginning of swelling of the vitreous in conditions of hypotonia, however this supposition should be experimentally tested.

If to correlate change of the volume in the cavity of the eyeball with the volume of intraocular humor and ocular chambers (that is very important for correct understanding of topographic changes arising in the eye during cavitary operations) we will receive the following data. The volume of the cavity of the eyeball with diameter 24mm (radius 12mm.) to the 14^th minute after decompression will decrease from 5570 mm3 to 5494 mm3., that is by 247 mm3. It is approximately equal to 1/3 of the volume of anterior chamber.

Topographic changes arising in the tissues and structures of the operated eye after its decompression (independent research).

Character of topographic changes arising in the tissues and the structures of the eye in real conditions of cavitary operation after its decompression was investigated by the method of non-contact-drop echography (NCDE) worked out by Alekseev B.N. [3]. We have modified and simplified well-known method [119], having made it more suitable for usage during the operations [120].

For more graduate analysis of the data received we conventionally picked out four types of topographic changes of the eye after its decompression: 1) deformation of fibrous capsule of the eye; 2) changes in depth of anterior chamber; 3) lens deformation; 4) ciliary-choroid detachment. Let us examine every one from the mentioned types of topographic changes.

Deformation of fibrous capsule of the eye

As it goes out from our researches [120] deformation of fibrous capsule after drop of eye turgor is generally become apparent by diminishing of the distances between the membranes, however in some cases its widening also took place. Statistic analysis has shown that there is correlative link between separate types of deformation of fibrous capsule of the eye. We have picked out three types of such a link

diminishing of the size in the vertical meridian - enlargement in the horizontal - maintenance of anterior-posterior size (coefficient of correlation is 0.51 +/- 0.12);

diminishing of anterior-posterior size - maintenance or slight enlargement of sizes in the rest meridians (coefficient of correlation is 0,30 +/- 0,08);

enlargement or maintenance of anterior-posterior size -diminishing of sizes in the straight meridians - enlargement of the size in oblique meridians (coefficient of correlation is 0,44 +/- 0,16).

It let us determine three main types of deformation of the eyeball (pic. 8)

Pic. 8 The main types of the eye deformation

The first type of deformation was observed in 29,4 % cases, the second type - in 11,8% cases, the third - in 14,7 %. In 35,3% of cases there was combination of different types. Only in 8,8% cases considerable deformation of fibrous capsule was absent.

Analysis of topographic relations of the eyeball with surrounding tissues shows that the main deforming factor of the first type may be - pressure of blepharons on the eyes and means of immobilisation, the second type - elevation in the intraorbital pressure in consequence of injection of surplus quantity of anaesthetics retrobulbarly or development of haematoma, the third type - residual tonus of oculomotor muscles. The following circumstance also proves it in the most cases while making diagnostic during examination of deformation of the 1^st and the 2^nd types we have managed to eliminate it by slackening of bridle suture, lifting of loops of blepharostat (having put gauge balls under its arcs), additional akinesia. It's necessary to note that in many cases the form of the eyeball seemed to be absolutely normal and finding out of significant deformation of its fibrous capsule with the help of NCDE was sometimes unexpected. It proved us that it's necessary to pay much attention to the factors of external deformation during the intraocular operations.

Changes in the depth of anterior chamber

Dynamics of changes in the depth of anterior chamber after ocular decompression are shown in table №3.

The most of the patients (88,3%) had “typical” reaction of irido-lens diaphragm to decompression of the eye consisting in shallowing of the anterior chamber. However not in all cases this reaction was so simple.

Many of the authors [20,41,80,91] considers removal of irido-lens diaphragm forward after decompression of anterior chamber to be normal and logical consequence of arising overfall between the pressure in the anterior chamber and the vitreous. To our minds such point of view does not conform to the laws of hydrostatics. Fluid content of the eye is not compressed and not tensile (in contrast to gas) and change in pressure in the anterior chamber or in the vitreous can not lead itself to changes in the volume of aqueous humor of the eye chambers and the vitreous. Consequently, loss of some part of liquid content of the eye after drop of ophthalmotonus can have place only if: a) volume of cavity of the eyeball will be decreased; b) volume of fluid content of the eye will be increased [152].

Analysis of results of our investigations and literature data lets us make the following conclusion: in conditions of acute hypotonia both mentioned processes have place.

Table № 3

Deformation of the anterior chamber after decompression of the eye

Depth of anterior chamber after eye decompression
Without changes *)	Diminution	Enlargement
	Under 1mm	More than 1mm	Under 1mm	More than 1mm
5,9%	23,5%	64,7%	2,9%	2,9%

*)- variations of the size to this or that side in the limits of 0,2mm were not considered

Graduate “contraction” of the membranes lack of hydraulic compression and deformation of the eye from the outside resulting from pressure of muscles, instruments, intraorbital cellular tissue and so on stimulates decrease in volume of the eyeball cavity. Volume of fluid content of the eye increases in the result of flow of additional blood in the vessels of the choroid, excessive transsudation into intermembranous spaces and because of graduate swelling of the vitreous.

In some cases drop of ophthalmotonus was accompanied by deepening of the anterior chamber. We connect it with availability of intravitreous cavities on the background of rough filamentous destruction of the vitreous. After opening of the eye and removal of the lens, emptying of the cavities of the vitreous and retraction of its fibrillar stroma had place, this became apparent by sharp collapse of the eyeball after cryophakia.

Deformation of the lens

After ocular decompression we have found increase of anterior-posterior size of the lens by an average 0.5mm though such type of deformation was observed not in all cases.

We could not find descriptions of this effect in the available literature.

Besides, after lens removal degree of its density was approximately evaluated: the 1^st degree - not large nucleus, jelly-like or fluid cortical substance, the form of the lens after its removal from the eye approximates to spherical. After piercing the capsule is spontaneously bursting; the 2^nd degree- large nucleus, cortical substance is soft, but in destruction of the capsule it does not spread, the lens has the form of convex disk; the 3d degree - the lens, as a rule, of small sizes, of impressed form and wax density.

Marked correlation link was found (correlation coefficient -0.6 +/- 0.14, p<0.001) between the size of the lens deformation and degree of its density (table №5 on page 29).

Besides, it was peculiar that maximal deformation of the lens of the 1^st degree of consistence usually was defined immediately after drop of ophthalmotonus, and of the lens of the 2^nd degree of consistence - 5 minutes after.

Table № 4

Lens deformation after eye decompression

Anterior-posterior size of the lens after eye decompression
Without changes*)	Increase	Diminution
	Under 0,5mm	0,6-1,0mm	More than 1,0mm	Under 0,5mm
32,4%	26,5%	23,5%	14,7%	2,9%

*)- variations of the size to this or that side in the limits of 0,2mm were not considered

Primary increase of the lens thickness in the presence of “soft” cataract we connect with drop of the eye turgor, deformation of the membranes, hyperemia of uveal vessels and thickening of ciliary body. All these factors lead to contraction of diameter of the ciliary ring and diminution of tension of Cinn ligaments.

Obviously, sharp disorder of hydrostatic balance between the pressure inside the lens and the pressure in the ocular chambers arising after decompression of the eyeball, leads to peculiar “explosive effect” and that can not but influence on the form of the lens and condition of its capsule. So dependence found between the value of deformation and consistence of the lens substance should be taken into consideration when the method of cataract extraction is chosen (intra- or extracapsular). Literature data concerning spontaneous destruction of the lens after decompression of the eye prove it [150].

Table № 5. Deformation of the lens after eye decompression depending on degree of its density

Showing	Degree of density of the lens
	I	II	III
1. Percentage of total quantity	23,5%	53,0%	23,5%
2. Size before decompression (mm)	3,8+0,1	3,7+0,1	3,1+0,2
3. Size after decompression (mm)	4,9+0,2	4,1+0,1	3,2+0,2
4. Increase of thickness after decompression in mm.	1,1+0,2	0,4+0,1	0,1+0,1
5. Increase of thickness after decompression in percentage	28,9+5,3%	10,8+2,7%	3,2%+0,3%
6. Probability of error (P)	P<0,001	P<0,01	P<0,05

Ciliochoroidal detachment

One of unexpected results of our investigations was registration of detachment of ciliary body 5 minutes after decompression in 23,5% cases. Distinct splitting of echo-signal from membranes of anterior segment of the eye points out to this fact. Arising of ciliochoroidal detachment (CCD) turned out to be statistically true (p<0,05).

At present there is an opinion that ciliochoroidal detachment after cavitary operations develops in 100% of cases [63]. However in the literature we did not find data of arising of CCD just several minutes after eye decompression (!), when its fibrous capsule was not practically dissected and no manipulation in the cavity of the eyeball was made. Absence of mechanical trauma in the moment of CCD arising, on the one hand, and availability of sharp hyperemia of the choroid found by us in the first minutes after eye decompression, on the other hand, let us suppose that arising of humor into supraciliary space is connected with its ultrafiltration through overfilled (with blood) capillaries of ciliary body. Abundant fenestration of capillaries endothelium and high permeability of their walls for humor also promote it [76].

Chapter 4. General characteristics of deformations of ocular tissues in conditions of acute hypotonia

The role of factors of deformation in etiopathogenesis of operational complications

So, analysis if the literature and results of our experimental and clinical researches have shown that one of the main showing of acute hypotonia of the eye is considerable disorders of natural topographic relations between its separate tissues and structures. Great variety of internal and external factors of deformation influencing on the eye, under influence of which this or that reaction of ocular content is formed to its opening and decompression draws attention. Difficult character of interrelations and interinfluences of separate deforming factors demands their definite systematisation within the process of their studying.

While analysing this problem we proceeded from the following: in the basis of any deformation of fibrous capsule and intraocular tissues there is traumatic depressurization of the cavity of the eyeball and impairment of physiologic balance between intraocular (IOP) and extraocular (EOP) pressure. Because everyone of the mentioned types of pressure is determined by many components and disorder of separate components of the system IOP-EOP can not be studied separately from the others, all main types of pressures defining anatomic condition of the eye were combined in the diagram (look at pic.9, page 30). While making this diagram we considered ambiguity of influence of changes of IOP and EOP on anatomic condition of the eye, which to our minds have place in conditions of traumatic depressurization of the eyeball. Peculiarity is in the following.

Any factors elevating extraocular pressure (pressure of blepharons, means of eye immobilisation, extraocular muscles and so on) can lead to disorders of anatomic condition of the eye (tissue deformation) only in the case if it exceeds intraocular pressure.

Changes of intraocular pressure can cause deformation of separate intraocular tissues in spite of the fact whether general balance between IOP and EOP is upset or not.

So, when pressure is dropped in the ocular chambers for example to 10gPa summary intraocular pressure will be, as before, much more than extraocular, but under such circumstances, as it is seen from our researches thickening of the choroid at the expense of hyperemia will take place, additional volume of fluid will displace the vitreous forward, fluid transsudation from overloaded vessels may cause CCD, thickening of lens, shrinkage of fibrous capsule and other similar reactions will also take place.

Basing on the above stated we have picked out two independent forms of ocular deformation: extraocular and intraocular.

Conventionally we can suppose that intraocular deformation of tissues arises as a result of hydrostatic impairment in the eye cavity (balance impairment between hydrostatic, osmotic and colloidoosmotic pressures in the tissues and intertissular spaces and as a result changes of IOP. Extraocular deformation arises as a result of predominance of pressure from outside (EOP).

On the basis of personal independent rheographic and echographic investigations [71-75,117-120] and also literature data we pick out five main types of intraocular and extraocular deformation. Separately in the present chapter we also consider changes in anatomic conditions of the eye in accordance with type of volumetric and linear deformation. General classification of different types of deformation of the eyeball is presented in the diagram (pic. 10 on page 31).

Volumetric deformation

In conditions of acute hypotonia of the eye any deformation of its tissues is accompanied by arising of disparity in the volumes of the eyeball cavity and the volume of its fluid content. When extraocular deformation is arising the volume of the eyeball reduces. It is connected with the following: for any body of spherical form (to which the eyeball approximates) any given area of surface has the greatest capacity.

Intraocular deformation is connected with flow in the eye (hyperemia of the choroid, swelling of the vitreous, humor transsudation into the intertissular spaces) or flow out of the eye (emptying of the chambers, intravitreous spaces) of definite volume of fluid (that is changes in the volume of fluid content of the eye) and also with changes in the area of surfaces of the membranes (that is, changes in the volume of the ocular cavity).

Data of intraoperative echography have shown that not all the types of volumetric deformation are obligatory arising during the surgical intervention on each eye. Resulting from deprivation of the eye tissues of natural hydraulic compression, such changes as shrinkage of elastic sclera, sharp intensification of blood filling of the uveal tract, lens deformation and changes in the volume of the vitreous inevitably arise (the first and the third types of intraocular deformation on diagram 10). Such type of deformation was classified as natural that is not depending on the surgeon's will or operational circumstances, but being inevitable consequence of the eye decompression.

At the same time all types of extraocular deformation and the forth type of intraocular deformation (diagram 10) arise only in separate cases (for example as a result of mistakes in the technology of operation, at expressed initial defect of the vessels and so on), and their arising does not have obligatory character. Deformation of mentioned types we consider as occasional.

As for the fifth type of intraocular deformation that is deformation of intertissular spaces resulting from their depressurization and fluid redistribution, so present type of deformation depending on concrete circumstances can be referred to the both types: as to natural so to occasional.

So for example, making of any depressive cavitary operation is connected with planned depressurization at least one of intertissular spaces (more often ocular chambers). Deformation of this space connected with its loss of a part of fluid content will be natural. However, if resulting from rough manipulations or by any other reason “not-planned” depressurization of any other space occurred, for example suprachoroidal or subretinal, resulting from humor redistribution deformation of these spaces (detachment of uveal tract or retina) will be of occasional character.

Planned depressurization and as a result natural deformation of this or that intertissular space during the operation differs from occasional in the following: the surgeon makes it deliberately taking into consideration the character of arising volumetric changes. That is why after necessary manipulations the surgeon finishes the operation by restoration of leak-proofness and consequently by removal of deformation of this space (suturing of the cornea, restoration of ocular chambers, removal of subretinal humor, limiting diathermocoagulation of the sclera and so on.)

Occasional depressurization of intertissular space, opening of which was not supposed during the operation, (for example tear of scleral spur), as a rule is not eliminated during the operation (because it is not planned before and often is not found by the surgeon in time) and arising deformation in postoperative period (CCD) becomes apparent as complication.

The volume of occasional deformation (Vod) depends on concrete conditions of making the operation and can be varied in broad limits. The volume of natural deformation (Vnd) on the basis of revealed conformity to natural laws can be prognosticated to a certain extent and can be approximately counted at any stages of operation.

Let us consider for example what volume of humor (Vnd) will become excessive in the eye as a result of natural deformation in the time of intracapsular cataract extraction under the local anaesthesia, 2 minutes after limbal dissection of the cornea? The volume mentioned will be assembled of the humor volume causing initial intraocular pressure (Vp), volume by which ocular cavity will be reduced within 2 minutes as the result of “shrinkage” of the fibrous capsule (Vfc), and the value by which the volume of the uveal tract will increase as a result of its hyperemia (Vut):

Vnd = Vp + Vfc + Vut (1)

Let us suppose that the eye to be operated has anterior-posterior size 24,0 mm, radius of curvature of the cornea 7,0mm and diameter 11,0 mm, initial intraocular pressure (Po) - 21,0 gPa (16 mm of m.c.). Vp is found in table № 2 (page 22) - 67 mm3; Vfc basing on the diagram on pic. 6 (page 14) - 0,5% or 28 mm3; Vut may be approximately defined in the following way: average thickness of the choroid of people's eye is equal to 0,48 - 0,70 mm [154,85], consequently if to imagine that geometric form of the eye is a sphere, the volume of its uveal tract will be approximately 560-600 mm3 (excluding the volume of the iris and the volume of ciliary muscle ). On the diagram on pic.5 (page 14) we can see the following: an average increase of volumetric blood flow 2 minutes after cornea dissection is approximately 20% of the initial, that is approximately equal to increase in the volume of the uvea by 115 mm3. Replacing the values received in formula № 1 we define that:

Vnd = 67+28+115=210 (mm3)

In real conditions of cavitary operations the volume of the humor lost by the eye after its decompression (Vh) as a rule is more than the volume of natural deformation (Vnd) at the expense of influence in this or that degree of factors of occasional deformation. In some cases Vh is even more than the volume of ocular chambers (to 450mm3 [92]) that is the reason of their complete emptying and spontaneous prolapse of the iris and the vitreous into the wound.

The difference between the volume of humor lost by the eye during the operation (Vh) and the volume of natural deformation (Vnd) constitute the volume of occasional deformation (Vod=Vh-Vnd=Vh-(Vp+Vfc+Vut) and may be considered as index of adequacy of the patient's preparation to the character of forthcoming operation. In the broad sense Vod is the index of degree of working out of technologies of any cavitary depressive operation and the level of its technical equipment. Technology of operation may be considered as theoretically ideal if Vod is equal to zero. However in real conditions to neutralize the influence of the factors of occasional deformation is impossible.

In the process of clinical investigations we have set the task to define an average volume of occasional deformation (Vod) during the operation of intracapsular cataract extraction under the local anaesthesia and to analyse relations between Vnd, Vod and general Vh. The researches were made during 20 operations in 20 patients not having serious somatic pathologies.

Before the operations all the patients passed through measuring of anterior-posterior size of the eye (1) and initial true intraocular pressure (Po). Then making puncture of anterior chamber with a special needle and aspiration of intraocular humor by microsyringe for 10-12 seconds, that is with speed approximately 2gPa/sec. decompression of the eye to the level of hydrostatic pressure was made and then the volume of the humor extracted (Vh) was measured. Vod was defined by the formula:

Vod = Vh - Vnp where Vnp=Vp+Vut+Vfc.

Because Vfc for the moment of decompression is equal to zero, the formula for Vod calculation was the following:

Vod = Vh (Vp+Vut) (2)

Vut was approximately defined on the basis of the diagram on picture 5 (page 14) - 15% or approximately 85 mm3. The results of the investigations are shown in table 6.

As it is seen from table 6, when usual technique of intraocular cataract extraction is used under the local anaesthesia the eye as a result of decompression looses at an average of 218.5mm of humor. From this volume loss of 144.4 mm is connected with natural processes developing in the tissues as a result of disorders of ocular hydrostatic and 74,2 mm of humor goes out of the eye under the influence of different occasional factors. So, we have received correlation of Vnd and Vod equal to 2:1.

However while making some operations the volume of occasional deformations was near to or even exceeded (!) the level of natural deformation. It points out to the fact that in some cases in formation of non-favourable reaction of the eye content to decompression the factors of occasional deformation are of decisive importance.

In many cases the surgeon can achieve decrease of Vh not only at the expense of minimising the Vod but also by means of some decrease of Vnp, for example influencing on the vessels of the choroid in proper time by different means. It's indirectly proved by our clinical observations of 24 patients who were applied the method of controlled dosed eye decompression with speed 0,111 gPa/sec with usage of aspiration system (CDDAS) during intracapsular cataract extraction.

Table № 6

Initial anatomy-physiological showings and value of volumetric deformation of the eye after drop of ophthalmotonus with speed of 2,0 gPa/sec

№	Parameter under the examination	Value of parameter
1	Average anterior-posterior size of the eye (1) mm	23,9 0,1
2	Average level of ophthalmotonus (Po) mm m.c.	15,2 0,2
3	Average volume of humor extracted (Vh) mm	218,5 15,9
4	Average volume of natural deformation (Vnd) mm	144,4 2,6
5	Average volume of occasional deformation (Vod) mm	74,2 13,0

As far as technology and conditions of operations of this group of patients, with the exception of method of decompression, were similar to the group of 20 patients considered above, it would be logical to suppose that an average Vod in both groups is approximately equal (74,2mm)*

Knowing the quantity of the humor in the aspiration system at the moment of decompression we (with known degree of approach) could define Vnd for the group of patients with CDDAS.

The results of investigations are shown in table № 7.

Table № 7

Initial anatomy-physiologic showings and value of volumetric deformation

of the eye after drop of ophthalmotonus with speed of 0,111hPa/sec

№	Parameter under examination	Value of parameter
1	Average anterior-posterior size of the eye ( )mm	23,7 0,1
2	Average level of ophthalmotonus (Po) mm m.c.	15,1 0,2
3	Average volume of humor extracted (Vh) mm	179,0 12,3
4	Average volume of natural deformation (Vnd) mm	104,8
5	Average volume of occasional deformation (Vod)mm (taken from table № 6)	74,2
6	Difference between Vnd of the patients operated with controlled dosed eye decompression and without it	39,6

* - Because of lack of precise data of dynamics of changes in volumetric blood flow in the choroid vessels during ICCE with the usage of CDDAS we could not calculate Vod for this group of patients by formula 2.

As it is seen from table № 7, an average Vnd of the patients with CDDAS is less by 39,6mm. Our rheographic investigations on the rabbit's eyes let us suppose that decrease of Vnd was made at the expense of more valuable adaptation of choroid vessels to slow drop of ophthalmotonus and decrease of level of its hyperemia.

Theory of linear deformation of ocular tissues in conditions of acute hypotonia

Analysing mechanisms of volume deformation of the eye and in particular changes in the cavity volume of its fibrous capsule we have made a conclusion that volume deformation must go with linear deformation, that is by disharmony arising between surfaces of conjugated tissues.

In the literature we did not find any works containing analysis of role and place of linear deformation in the pathogenesis of complications of acute hypotonia of the eye. Nevertheless significance of linear deformation seems to be very important for us.

As our experimental and clinical investigations showed, more expressed volumetric changes after eye decompression occur in its uveal tract. That is why we would like to pay special attention to the mechanism of linear choroid deformation.

Unique feature of structure of vascular tract of the brain and the eye is the following: vessels in it are placed by several floors and go to different directions, crossing many times with each other at different levels. [21]. Analysing specificity of anatomic structure of the choroid, we came to a conclusion that simultaneous widening or narrowing of all its vessels must cause not only change of thickness of membrane but also corresponding change of area (!) of its surface. (Mechanism of volume and linear changes of the choroid is shown on picture 11 in a simple way).

It leads to disharmony between the area of the surface of the uvea and the area of sclera surface (from the outside) and posterior hyaloid membrane (from the inside) that is linear deformation of the membranes (pic. 12).

Consequently, unique feature of separate (autonomous) vessels to keep unchanged length in spite of degree of contraction of their muscular wall (because of presence of radial and absence of longitudinal muscular fibres) in relation to all vascular tissue does not work.

Evidently new more perfect type of blood supply of neuroepithelium of retina and cerebral cortex which is provided by the presence of specific choroidal tissue, phylogenetically became possible only in conditions of elevated hydraulic pressure in the isolated space (eyeball, scalp cavity).

Insignificant tensile strength of the walls of such space (osteal or fibrous tissue, availability of natural hydraulic backwater and incompressibility of fluid content (the vitreous, cerebral tissue, intraorbital or cerebrospinal fluid) prevent the uvea from essential volumetric changes, because any changes in the volume of blood deposited in the uveal tract are conjugated with necessity of correspondent changes in the volume of scull or ocular cavity and that is impossible.

In connection with it changes in the volume of the blood circulated (VBC) in the choroid (especially if they take place within short period of time) in natural conditions are possible only at the expense of changes in the linear speed of the blood-stream, but not at the expense of changes in the vascular lumen [88,118].

In other words the volume of the blood deposited in the vascular tract of the brain (or the eye) is kept unchanged in any changes of VBC because otherwise widening of the vessels and enlargement in the volume of the choroid would demand the presence of additional space in the cranium or scleral cavity, and diminution in the volume of the choroid vice versa should be filled in by the correspondent enlargement of the volume of the brain + cerebrospinal humor of humor or the vitreous + intraocular humor. (In the present case we don't take into consideration important regulating function of or humor, which they in fact perform in balance of volumes of the choroid and other tissues of the scull and the eye, because mechanisms providing its circulation work in temporal amplitude which many times exceeds fulminant pathophisiologic reaction typical for syndrome of acute hypotonia).

Hence it appears that active regulation of blood circulation in the uvea by change in tonus of the vessels if choroidal tissue in conditions of its physiologic hydraulic compression is impossible. Such regulation can be performed only by corresponding change in tonus of afferent vessels (for example posterior long ciliary arteries), placed autonomously from choroidal tissue.

In our opinion position of posterior long ciliary arteries (PLCA) isolated from choroidal tissue is very important for autoregulation of blood stream and humor secretion in ciliary body. On the one hand arterial vessel is situated just in the cavity of the eyeball in conditions of IOP and any changes of ophthalmotonus are perceived with its baroreceptal apparatus. On the other hand changes of tonus and lumen of PLCA as a result of vasovasal reflex on any physiological irritant do not influence on topographic changes between the uvea and conjugated tissues can not be blocked by hydraulic backwater.

So, on conditions of normal intraocular pressure potential feature of choroidal tissue to change the area of its surface under this or that changes of volumetric blood stream is blocked by the presence of hydraulic backwater. In conditions of traumatic depressurization and decompression of the eyeball its vessels loose natural hydraulic “buffer”, phylogenetically more ancient type of vasculomotor reactions switches on, lumen of choroidal vessels changes, and it leads finally to changes in not only volume but also in the area of the whole uveal tract.

Linear deformation of the uvea, to our minds, is the main starting moment of development of CCD. “Vacuum-effect” of arisen disharmony between the areas of conjugated tissues leading to intensified ultrafiltration of the humor into suprachoroidal space on the background of hyperemia, atony and elevated permeability of the vessels also promotes it.