Ocular Hypotony 

Hypotony is usually defined as an intraocular pressure (IOP) of 5 mm Hg or less. Low IOP can adversely impact the eye in many ways, including corneal decompensation, accelerated cataract formation, maculopathy, and discomfort. Clinically significant changes occur more frequently as the IOP approaches 0 mm Hg.

The usual rate of aqueous humor production and outflow is 2.5 µL/min. According to traditional teaching, in healthy human eyes, about 90% of aqueous humor exits through the conventional trabecular meshwork/juxtacanalicular/Schlemm canal/episcleral venous route. The remaining 10% exits via the uveoscleral outflow, where it crosses the ciliary body, sclera, or scleral openings to reach the suprachoroidal space. Flow through the trabecular route ceases when IOP declines below the episcleral venous pressure, usually 9 mm Hg. Therefore, uveoscleral outflow predominates at low IOPs.

Hypotony occurs when aqueous humor production does not keep pace with outflow. Outflow may be greater than usual, as seen with wound leak, overfiltering bleb, or cyclodialysis cleft. Conditions that decrease ciliary body function, such as iridocyclitis, hypoperfusion, or tractional ciliary body detachment, may cause inadequate aqueous humor production. Hypotony is also seen in association with rhegmatogenous retinal detachments and some altered osmotic states.

Inflammation plays a key role in the evolution of hypotony. It causes increased permeability of the blood-aqueous barrier and impairs ciliary body aqueous production. Choroidal fluid is believed to accumulate in its potential space as a result of a relative increase in uveoscleral outflow and the lack of sufficient IOP to maintain closure of the space. This cycle is often perpetuated once choroidal effusions develop. An anterior ring of choroidal fluid can rotate the ciliary body forward, potentiating its inability to produce aqueous humor.

Frequency

United States

Hypotony following glaucoma surgery is common but is often not clinically significant. Transient hypotony can develop after other types of ocular surgery, especially if a pars plana approach has been used, or following trauma. The rate of hypotony following uncomplicated cataract surgery is extremely low. The incidence of hypotony associated with trabeculectomy increases with the use of antifibrinolytic agents. Chronic hypotony leading to phthisis is rare and occurs only in eyes with severe damage or complex problems.

Mortality/Morbidity

Hypotony usually occurs as a complication of an underlying ocular disorder, trauma, or surgery.

Transient or permanent visual impairment may result from corneal changes, accelerated cataract formation, choroidal fluid, choroidal folds, maculopathy with disturbance of the retinal pigment epithelium (RPE), cystoid macular edema, or optic disc edema. Hypotony increases the risk of suprachoroidal hemorrhage, which can result in severe vision loss.

Hypotony in the setting of an incompetent corneal or limbal wound can predispose the patient to epithelial ingrowth.

Severe chronic hypotony can ultimately lead to phthisis.

Sex

Females may be more predisposed to hypotony following antimetabolite-enhanced trabeculectomy. Males may be more prone to hypotony maculopathy.

Age

Young patients with myopia may be more predisposed to hypotony following trabeculectomy.