HYPOTONIC MACULOPATHY

Ocular hypotony may occur after ocular inflammation, trauma or surgery, especially glaucoma filtration surgery (GFS) with antifibrosis drugs. Postoperative hypotony may develop following retina, cataract, cornea and strabismus surgeries in addition to filtering surgery. 

Several definitions are used for describing ocular hypotony. A statistical or numerical definition is IOP below 6.5 mmHg, which is more than three standard deviations below the mean. An alternative clinical definition is IOP low enough to result in vision loss, although low IOP alone may not result in vision loss. Vision loss associated with low IOP is commonly caused by corneal edema, astigmatism, cystoid macular edema or maculopathy. The World Glaucoma Association considers hypotonic IOP one which causes clinical complications with a potential of visual disturbance. This is regarded as IOP ≤5 mmHg.

Hypotony maculopathy is characterized by a low IOP with associated fundus changes, including chorioretinal folds, optic nerve swelling and vascular tortuosity. The chorioretinal folds are most likely secondary to the collapse of the scleral wall. Wrinkling in the retina or thickening in the choroid may cause axial shortening of the eye, leading to hyperopia.

Disc swelling results from restricted axoplasmic flow, presumably from anterior bowing of the lamina cribrosa in the optic nerve. These findings may be less pronounced in eyes with advanced glaucoma because these eyes have fewer remaining axons that can swell. Clinically, patients may experience metamorphopsia or central vision loss, or they may be asymptomatic.

Hypotony maculopathy can occur with increased outflow of aqueous humor or, less often, with decreased aqueous production. Outflow can increase because of a wound leak, a scleral rupture, a cyclodialysis cleft, a retinal detachment or an overfiltering bleb—or, rarely, from a ciliochoroidal detachment. Decreased aqueous humor production may be secondary to uveitis, hypoperfusion of the ciliary body in ocular ischemia, or a ciliochoroidal detachment.

One of the most important risk factors for hypotony maculopathy is the use of antifibrosis drugs, especially mitomycin C, during GFS. Intraoperative use of MMC can lead to overfiltration or bleb leaks in the late postoperative period which may be associated with hypotony maculopathy. Higher concentrations and increased application times of antifibrosis agents not only may lead to excessive filtration but also may have a toxic effect on the ciliary body, thus leading to decreased aqueous humor production.

A retrospective study of other risk factors for hypotony maculopathy showed that young age, male gender and myopia increased the risk, whereas a history of diabetes and the presence of choroidal effusions decreased the risk.

Diagnosis of the condition is made by performing a Seidel's test which demonstrates aqueous leak from the bleb. Fundus examination reveals the characteristic retino-choroidal changes. Investigations such as B-scan and OCT will demonstrate other findings associated with hypotonic maculopathy such as thickening of the posterior sclera or choroid, and choroidal detachments or macular changes. Ultrasound biomicroscopy and intravenous fluorescein angiography help in identifying choroidal folds.

In treating hypotony maculopathy, the goal is to normalize IOP as soon as possible to prevent permanent retinal dysfunction and associated vision loss. However, IOP correction is not guaranteed to improved visual acuity, especially in cases of long-standing hypotony maculopathy.

Treatment of hypotonic maculopathy is aimed at the cause. Bleb leaks can be managed by scleral or bandage contact lenses, symblepheron ring, aqueous suppression, topical antibiotics like gentamicin which facilitates fibrosis and broad spectrum antibiotics to prevent infection, or suturing if conservative approach fails. Other options are autologous blood injection into the bleb, fibrin tissue glue, cyanoacrylate glue, argon laser application or direct suturing of the bleb, although these treatments seldom produce a longlasting solution. 

Late-onset bleb leaks that do not resolve spontaneously or with conservative measures often require surgical intervention. Amniotic membrane or autologous conjunctival grafts can be considered.

Hypotony maculopathy secondary to overfiltration may be managed by adding compression sutures to the elevated, overfiltering bleb. Autologous blood injection, in or around the bleb, may be helpful in some patients. During the early postop period, with an overfiltering bleb, anti-inflammatory medications may be rapidly tapered to facilitate episcleral scarring.

Preventive measures can be taken intraoperatively during GFS to reduce the risk of post-op hypotony maculopathy. There are three critical areas created during GFS that ensure smooth passage of aqueous humor. These are:

1) sclerostomy; 

2) scleral flap; and 

3) subepiscleral/subconjunctival areas. 

The above-mentioned intraoperative areas should be controlled meticulously to prevent the development of post-op hypotony. 

Multiple flap sutures should be placed, with additional sutures if aqueous flow is excessive. The conjunctiva should be closed with the use of tapered (vascular) needles to reduce suture track leaks. If possible, the conjunctival closure should incorporate Tenon’s capsule, whether by a two-layer or a one-layer method. Postoperatively, properly timed removal of releasable sutures or laser lysis of other sutures can decrease the precipitous reduction of IOP and the potential for hypotony maculopathy.

In cases of glaucoma drainage devices, intraoperative tube ligation is performed using absorbable sutures. These sutures usually absorb after 5 weeks and controlled IOP is seen. However, early absorption or opening of the suture causes over-filtration. Techniques such as blocking the tube ab interno or ligating the tube have been described.

Some authors recommend viscoelastic substance or perfluoropropane gas injection into the anterior chamber in conjunction with tube ligation. In case of failure, ab interno stent, tube shunt plate truncation and implant 

explantation are other surgical methods that can be considered.