Incidence and Management of Post-surgical Hypotony after Glaucoma Surgery
RN Bhatnagar, Sachin Walia, Deepak Sharma, Meenu Babber, Rajesh Garg, Pawan Prasher

INTRODUCTION
When the term hypotony is used in ophthalmology, a low intraocular pressure (IOP) is generally implied.  The statistical definition of hypotony refers to an IOP of less than 9 mm Hg, representing two standard deviations above or below.  The causes of post-trabeculectomy hypotony are over-filtering bleb, wound leak, overfiltration, cyclodialysis cleft, choroidal effusion, retinal detachment, prolonged use of aqueous suppressants.

The structural and functional changes associated with low IOP are collectively called Low Pressure Syndrome.

Structural changes are shallow anterior chamber, corneal astigmatism, corneal oedema, macular oedema, disc oedema, hypotonous maculopathy, choroidal effusion, haemorrhage, breakdown of blood-aqueous barrier, exudative retinal detachment.

Functional changes are reduced visual acuity, ocular discomfort, changing refractive errors, blurring of vision, hyperopic shift, myopic shift & pain.

The interventions advocated to reverse hypotony are of two types: Non invasive methods are observation, cynoacrylate glue, Simmons shell tamponade. Invasive methods are surgical revision, compression sutures, closure of the bleb leak, cyotherapy of the bleb, autologous blood injection, laser grid technique, amniotic membrane, TCA application, drainage of choroidal fluid.

METERIAL AND METHODS
The present study was conducted in Rajindra Hospital Patiala on 75 eyes of 70 patients who underwent trabeculectomy and developed early post- trabeculectomy hypotony. No anti-fibrotic agents were used. Out of total 75 patients, 22 developed hypotony.  Initially all of these 22 eyes were put on conservative treatement, which included paradoxical use of aqueous suppressants, minimizing use of steroids, patching or use of bandage soft contact lens.  Out of these 8 responded well to this modality of treatment while the rest of 14 eyes had to undergo surgical intervention with in 4 to 5 days. Three types of surgical interventions were used: Closure of leaking blebs, compression sultures & drainage of choroidal effusion.

The indications for surgical interventions were: Persistent bleb leak that would have placed the patient at high risk of infection, persistent ocular pain, choroidal effusion (kissing choroidals), large diffuse bleb with over filtration, persistent shallow anterior chamber with iridocorneal touch.

Closure of leaking blebs was done by using 10-0 nylon on tapered non cutting needle.

Compression sutures were used in cases, which showed large diffuse painful blebs with over filtration.  A 9-0 nylon suture was passed through 1 to 2 mm of peripheral cornea in a direction parallel to the limbus.  Then the suture is draped upwards over the bleb and passed through 2-4 mm of conjunctiva and Tenon's capsule posterior to the bleb in a direction parallel to the limbus.  A trapezoid pattern was formed when the suture was draped downward over the bleb and was tied tightly.  The knot was buried in peripheral cornea.  The suture was removed when the desired effect was achieved in the follow up period.

Choroidal fluid drainage was used in cases with kissing choroidals with flat AC. The surgical technique consisted of a conjunctival incision 35mm posterior to the limbus.  After cautery of the superficial sclera a radial 2-mm incision was made through the sclera, using cautery on the edges to pout the wound open.  When the suprachoroidal space was entered, straw colored fluid came spontaneously thus sclerotomy can be either be closed or left to drain spontaneously and the conjunctiva was closed with a single mattress or interrupted suture.

OBSERVATIONS AND RESULTS
Patient Distribution: Total cases - 75 Eyes of 70 Patients (Males-32, Females-38), Hypotony-22 (29.33%), Conservatively managed-8 936.36%), Surgical Intervention-14 (63.63%), Closure of leaking blebs-9 (40.9%), Compression Sutures-4 ((18.18%), Drainage of choroidal effusion-1 (4.54%).

DISCUSSION
Hypotony following glaucoma surgery is an important complication, which can occur both in early as well as late stages.  Shallowing of the chamber can lead to endothelial damage from inadvertent lens-cornea touch. Low IOP in the early postoperative period may be associated with a shallow anterior chamber.  In this scenario, the etiology may be overfiltration, ciliochoroidal detachment with reduced aqueous production, a cyclodialysis cleft, or a wound leak1.

Overfiltration usually is caused by loose scleral flap sutures and is less common today with the trend toward tighter scleral flap sutures and early laser suture lysis. The best management usually is external compression with a contact lens that compresses the bleb and/or scleral support from a Simmons Shell.  The use of these devices must be weighed against the potential for bleb failure. Consideration can be given to decreasing the level of postoperative anti-inflammatory medication in an attempt to promote wound healing, which may increase the resistance to flow, but this too has potential for increasing the likelihood of long-term failure of the procedure.  External compression sutures have been used successfully.  This technique involves a nylon suture placed over a portion of the bleb in an attempt to cause localized scarring and an overall decrease in the effective size of the filtering belb.

Wound leak is one of the most common causes of hypotony in the early postoperative period.  Leakage may be from a conjunctival flap perforation not recognized at the time of surgery, an area of inadvertent postcautery conjunctival necrosis, a wound dehiscence, or a traumatized, thin filtration bleb.  Although the site of leakage often can be determined with a Seidel test, 2,3 gentle external pressure on the globe may be required to increase flow sufficiently to demonstrate a hole during times of decreased aqueous production.  Early wound dehiscence when limbus-based conjunctival flaps are used and retraction of the flap in fornix-based cases are the most serious wound-related complications and generally require immediate surgical repair, particularly if the edge of the scleral flap becomes exposed. The administration of aqueous suppressants can enhance the closure of some leaks by decreasing flow across the leak.  Nevertheless, a further reduction in IOP can occur, which may cause an increased shallowing of the anterior chamber.  Definitive therapy for all wound leaks is surgical closure.  This procedure is often performed in a minor surgical suite or even at the slit lamp.  A tapered, noncutting cardiovascular needle should be used at all times, if possible, to avoid additional leakage at the suture tracks. Tissu adhesive also has been used.4

Visual Acuity of the cases prior to and following intervention of hypotony

Visual Acuity Conservative treatment Surgical closure of Bleb Compression Suture Drainage of choroidal effusion
6/6 - 6/12 1/4 0/3 0/2 0/0
6/12 - 6/36 5/4 4/5 3/2 1/0
6/36 or less 2/0 5/1 1/0 0/0

Mean IOP Score At Different Time Intervals

Weeks 1 2 3 4 5 6
Conservative 6.5+2.72 6.87+2.94 8.0+3.12 8.50+3.62 9.87+4.12 11.50+5.07
Surgical closure 5.44+2.79 6.89+3.33 7.70+3.19 8.0+3.46 9.89+3.69 11.11+4.66
Compression sutures 3.75+2.98 5.25+3.86 6.50+4.50 7.5+5.25 8.55+5.91 9.75+6.84

Mean Difference Score from Baseline

Weeks 1 2 3 4 5 6
Conservative 2.08+0.16 2.95+0.28 3.55+1.08 4.65+1.22 5.62+1.48 6.72+1.82
Surgical closure 2.92+0.26 3.05+0.48 3.22+0.62 4.08+1.02 4.78+1.11 5.56+1.67
Compression sutures 2.01+1.07 2.52+1.57 2.75+2.01 3.98+2.63 4.46+3.12 5.75+3.53

Choroidal effusions can occur in any situation in which there is decreased IOP.  This process further enhances hypotony by decreasing aqueous production due to abnormal positioning of ciliary body and to transudation of fluid into the potential space between the sclera and uveal tissues.5 Most cases of effusion resolve spontaneously and surgical drainage is usually indicated only for cases of kissing choroidals in which retinal apposition may lead to retinal tears upon separation.  The presence of supra choroidal fluid contributes to reduced aqueous production, in turn aggravating hypotony and the tendency to more choroidal effusion.  Thus any surgical procedure aimed at reversing hypotony should give consideration to drainage of chroidal fluid. Chronic hypotony occurs more frequently when full-thickness procedures or trabeculectomy with 5-FU or MMC are used than in guarded procedures without antifibrosis agents. Although IOPs of 4 mm Hg to 10 mm Hg have not been shown to increase the incidence of visual compromise,6 IOPs of less than 4 mm Hg more commonly have been associated with the development of vision threatening maculopathy.7,8 The study conducted in National Survey of Trabeculectomy. III. Early and late complications by Edmunds B. et al,9 showed the percentage of occurrence of hypotony to be 24.3% in their study on 1240 patients.  The study by Benezra et al10 showed the percentage of occurrence of hypotomy to be 10% in their study of 80 cases.  In our study shallow anterior chamber was observed in 27.4% cases, which is higher than that reported in the same study.  In contrast Gamal11 in his study had shown the percentage occurence of hypotony to be 0.5%.  In the present study the same was 29.33%.  Bellows et al.12 in their study had shown the IOP rises to 11.0+4.4 mm Hg from preoperative level of 3.7+2.6 mm Hg after successful closure of bleb leaks.  In our study the pressure change was from 5.44+2.79 to 11.11+4.66.  Conservative treatment has been proved to be successful in early cases of hypotomy in the ophthalmic literature from time to time. Desai and Krishna13 had used compression sutures in overfiltring large diffuse blebs and reported that visual acuity and intraocular pressure were maintained with the resolution of the symptoms.

In our study bleb leak was observed in 12% cases while Edmunds et al.9 reported an incidence of 17.8% in their study.  Early post operative bleb leaks are most often related to surgical trauma to the conjunctiva and can be avoided by careful surgical technique14 Surgical bleb closure has a high  success rate of closing belb leaks, maintaining glaucoma control and preserving vision.15 In a study conducted by La Borwit et al.16 visual acuity increased from6/24 to 6/9 following surgical closure of the bleb, in comparison to ur study which has also shown similar results. Picht et al17 have successfully employed conservative treatment in 33.6% of cases in their study of 113 eyes, while the same is employed in 36.3% of our cases.  Overfilteration occured in 5.33% of our cases while the same study showed it to be 4.4%.

CONCLUSIONS
Current study clearly reveals that surgical closure of bleb leaks, compression sutures for overfiltering blebs with due consideration to drainage of choroidal fluid, when aimed at reversing hyptony are quite  effective methods in the management of cases not responding to conservative management following glaucoma surgery.

REFERENCES

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Address for Correspondence
Dr. RN Bhatnagar, Deptt. of Ophthalmology,
Govt. Medical College, Patiala