Indian Journal of Ophthalmology

: 1981  |  Volume : 29  |  Issue : 3  |  Page : 173--176

An investigation of the mechanism of function of trabeculectomy (operation)

JD Batra, B Patnaik, G Singh, R Kalsi, BS Jain, DP Agarwal 
 Guru Nanak Eye Centre, Maulana Azad Medical College, New Delhi, India

Correspondence Address:
J D Batra
Guru Nanak Eye Centre Maulana Azad Medical College New Delhi

How to cite this article:
Batra J D, Patnaik B, Singh G, Kalsi R, Jain B S, Agarwal D P. An investigation of the mechanism of function of trabeculectomy (operation).Indian J Ophthalmol 1981;29:173-176

How to cite this URL:
Batra J D, Patnaik B, Singh G, Kalsi R, Jain B S, Agarwal D P. An investigation of the mechanism of function of trabeculectomy (operation). Indian J Ophthalmol [serial online] 1981 [cited 2024 Mar 2 ];29:173-176
Available from:

Full Text

Since the introduction of Trabeculectomy a number of mechanisms of its function have been claimed to be operative by different authors e.g., (i) a simple fistulising operation, (ii) to provide a direct communication between the anterior chamber and Schlemm's canal by pas�sing the diseased trabecular meshwork, (iii) by directly draining into the collector channels and aqueous veins by passing even the Schlemm's canal, (iv) acts by a localized cyclodialysis, (v) phenomenon of hyposecre�tion, (vi) the drainage is believed to be taking place through newly developed aqueous veins, flowing out through lymphatic channels and lastly drainage is said to be occuring through normal aqueous veins.

Therefore in this paper it is proposed to present some hard evidence on the mecha�nism of action of this operation.


Total of 64 glaucomatous eyes of 54 patients operated and followed up at this Centre is the subject material for this study.

These were placed in 2 groups

I. Group : (Cases operated for trabeculectomy)

This group was divided into two sub�groups

(A) Eyes operated during the period of study

Twenty six eyes with difierent types of glaucoma (chronic simple glaucoma : 13 eyes; acute congestive glaucoma : 9 eyes; absolute glaucoma : 4 eyes) requiring glaucoma surgery were investigated preoperatively. Trabeculec�tomy was performed in all types of glaucoma by modified Watson technique, after control�ling the tension.

(B) Eyes operated before the period of this study

This group comprised of 34 eyes with diffe�rent types of glaucoma (chronic simple glau�coma : 26 eyes; acute congestive glaucoma : 6 eyes; secondary glaucoma : 2 eyes) which underwent trabeculectomy (by modified Wat�son technique) during the period of last 2 years.

II. Group (Eyes operated for glaucoma by other techniques)

Four eyes, out of which two had under�gone iridencleisis and rest punch sclerectomy were also included in the present study.

Preoperatively in all the patients a brief history and visual acuity was recorded. Exa�mination of anterior and posterior segments, intraocular tension by Applanation and Schiotz tonometer, tonography with a Mueller electro�nic tonometer, gonioscopy with Goldmann's single mirror contact lens were performed. Efforts were made to procure the retrospective detailed data of group II

Postoperatively in all the patients of Groups I & II, visual acuity, examination of anterior segment (especially the site of opera�tion), ophtbalmoscopy, intraocular tension by Applanation and Schiotz tonometer, tono�graphy were performed.

The site of operation was studied meticu�lously on the slit lamp. A note was made of the following points

Whether obvious bleb was present or absent.If bleb was present, whether it was raised or flat.Whether bleb was localised or diffuse.Condition of the conjunctiva over the site of operation was noticed in terms of vascularity, avascularity and thinning (devi�talisation). Fibrosis of the bleb or any fibro�sis in the vicinity of bleb was noticed.

Insufficiently pressure regulated eyes were those where intraocular pressure was 21 mmHg or more.

An eye was said to be hypotonic when intraocular pressure was less than 6.5 mmHg. Out flow factlity was determined by conduc�ting tonography using a Mullers electronic tonometer. C value was taken as normal when it was more than 0.17% suspicious when bet�when 0.13% to 0.17% pathological when less than 0.13 %.

Study of Aqueous Veins

Aqueous veins and drainage at the opera�tion site were studied by the Fluorescein in�jection method.

Seidel Test

This test was done in all the 64 eyes of Groups I & 11. In this test a drop of sterile fluorescein dye (2%) was instilled over the area of operation and slight pressure was applied on the globe, whereupon the flow of coloured fluid could be seen on slit lamp examination under Cobalt blue filter.


The facility of outflow in all the 51 pres�sure regulated eyes was normal (C> 0.18). The majority (43=84.31%) of the these eyes were showing a well formed bleb. The remaining 8 eyes had either flat or no bleb.

In the 20 eyes having well vascularised conjunctiva over the area of operation (bleb area) the average I.O.P. was 13.1 mmHg and C value 0.28.

The 31 eyes with areas of avascularity over the area of operation (bleb area) were having an average I.O.P. of 11.3 mmHg and C value 0.29

In the 37 eyes where there was no trans�conjunctival seepage of aqueous the average I.O.P. was 12.4 mm Hg. and C value 0.29. On the other hand the average I.O.P. was 10.9 mmHg. or C value 0.30 in 14 eyes where there was transconjunctival seepage.

The 4 eyes with hypotony the bleb had avascularity and thinning.

Most (8 out of 9) of the eyes with insuffic�iently regulated I.O.P. there was either flat or no bleb. There was only one eye in this group with a seemingly well formed bleb. However, the C value was low (C=0.09).

Out of the 60 eyes, well formed blebs were seen in 44 eyes (73.33%). Of these all but one were pressure regulated.

The remaining 16 eyes eyes where there was either flat or no bleb-50% (8 eyes) were pressure regulated and other 50% were not.

C Value : 32 eyes were tested for C-value pre-and post-operatively. Of these the average C-value of 27 pressure regulated eyes pre�and post-operatively were 0.06 and 0.28 res�pectively. The remaining 5 cases which were insufficiently pressure regulated the average pre-and post-operative C-values were 0.07 and 0.11 respectively.

Aqueous Veins (26 eyes) : Twenty six eyes were studied for aqueous veins both pre-and postoperatively. The number of aqueous veins in 22 pressure regulated eyes preoperatively was 28. Postoperatively they were reduced to 12. Aqueous channels carry aqueous more often in eyes with flat (4 out of 6; 66.5%) or no bleb (2 out of 2; 100%) than with well formed bleb (15 out of 43; 35%). On the other hand there were 11 aqueous veins seen preoperatively in 4 eyes which were insufficiently pressure regulated. The number did not change postoperatively. Study by Fluorescein Injection (Pre-and post�operative group)

Thirteen eyes with trabeculectomy were studied by intravenous fluorescein injection technique both pre and post operatively. Out of these 9 were pressure regulated (R.P.) and 4 were not (I.P.R.).

The number of channels draining aqueous before operation in 9 pressure regulated eyes were 23. After the operation these were re�duced to 7. In remaining 4 eyes which were insufficiently pressure regulated the number of aqueous veins both pre and postoperatively remained at 11.

In one eye of P.R. group there was a rela�tively flat bleb draining aqueous well which had developed a new aqueous draining channel.

All these eyes were also studied by intraca�meral injection technique. Precisely the same was the observation. There after the eyes were studied by either technique.

(a) Pressure regulated 51 eyes (chronic simple glaucoma : 34, Absolute glaucoma : 4, acute congestive glaucoma : 13).

In 30 out of 51 eyes no aqueous veins could be demonstrated. However in all the 54 eyes with controlled pressure the dye was demonstrated in the subconjunctival tissue in the area of operation (bleb area). The value varied between 0.18 to 0.54.

Fourteen of these eyes also showed trans�conjunctival seepage of the dye as demon�strated by Seidel test.

In 2 eyes of this group the bleb was appa�rently flat (considered to have had no bleb). Both of them showed normal aqueous channels. But there were new aqueous channels draining aqueous from from the bleb area was seen one in each eye. The C-value in them was 0.20 and 0.27 respectively.

(b) Insufficiently Pressure Regulated : 9 eyes (chronic simple glaucoma : 5, Acute con�gestive glaucoma : 2, Secondary glaucoma : 2).

In all the 9 eyes aqueous veins were demon�strable. Though the bleb area showed fluo�rescence it was of very small quantity.

(c) Control group of conventional surgery for chronic simple glaucoma : 4 eyes (Iridenc�leisis : 2 eyes, Punch sclerectomy : 2 eyes.)

The pressure was regulated in all these eyes. There were no demonstrable aqueous veins. No no new channels either were visible. The C value varied between 0.30 to 0.37.

The blebs were localised, markedly thinned out and avascular and showed transconjunc�tival dye seepage


After a study of the blebs in pressure regu�lated and insufficiently pressure regulated eyes, it is seen that by and large a well formed bleb means good drainage and introcular pressure control. However, no bleb or a flat bleb need not mean poor drainage or bad control. 50% of such eyes in this series, infact were well controlled. However, fluorescein study did show a good subconjunctival aqueous drai�nage in the pressure regulated eyes and a poor drainage in those where intraocular pressure was not well controlled.

What was even more interesting is that in one eye with apparantly well formed bleb but with insufficiently controlled intraocular ten�sion and poor C value showed a deficient drai�nage of aqueous, studied by fluorescein injec�tion technique.

Many authors in literature support the view of subconjunctival route of drainage of aqueous though their methods of study have been diffe�rent. Only Benedikt[1] has conducted the study in the manner in which the present study has been conducted i.e., colour marking of the aqueous with sodium fluorescein dye. In Benedikt's as well as in the present study, the subconjunctival route of drainage of aqueous was present in all the pressure regulated eyes.

Aqueous veins were also taking part in drainaging the aqueous after trabeculectomy in both the studies.

Benedikt[1] has detected lymphatic vessels in his cases in the area of bleb. No such vessels has been found in the present study.

He has reported the presence of new chan�nels in the area of apparent bleb in few eyes whereas no such channels were found in the present study.

The number of demonstrable aqueous veins either decrease or disappear after trabeculec�tomy in pressure regulated eyes. On the other hand in insufficiently pressure regulated eyes the number of aqueous veins do not decrease after operation. This is conformity with obser�vation of Benedikt.


It was noticed that a well formed bleb means goon drainage of aqueous and well con�trolled intraocular tension. However, no bleb or a flat bleb need not mean poor drainage of aqueous or bad control of intraocular tension. What was even more interesting was that the eye with apparently well formed bleb but with insufficiently controlled intraocular tension and poor C-value showed a deficient drainage of aqueous studied by fluorescein injection tech�nique. Hence, the look of the bleb can not be the sole basis of assessment of the functioning quality of trabeculectomy. The fluorescein method of examination does provide direct and satisfactory evidence on the state of drai�nage of aqueous through the trabeculectomy wound.

The avascular spots on the conjunctiva over the operated area (bleb) seems to be due to localised stripping of the tenon's capsule from the conjunctival flap during operation has little to do with nature (degree) of drainage of aqueous.

The transconjunctival seepage of aqueous through the avascular areas seem to depend on the size of these areas and degree of devitali�sation rather than nature of aqueous drainage.

Since controlled intraocular pressure (pres�sure regulated eyes) was associated with:

(a) Copious subconjunctival drainage of aqueous through the operated area (as demon�strated by fluorescein studies).

(b) A normal C-value.

(c) Decrease or disappearance of aqueous carrying channels (aqueous veins).

(d) Convincing evidence to prove that the cut ends of the Schlemm's canal at the edge of the operated area were obliterated. The mechanism of functioning of the trabeculec�tomy operation seems to be simple filtration through the operated area (points (a) and (b) ) and not a direct flow of aqueous through the cut ends of the Schlemm's canal and then the aqueous veins (points (c) and (d) ).


1Benedikt, 1975, Zur wirkungsweise den Trabe�kulektomine Min. Mbl. Augen Heilk. 167: 679.