Hypotony is defined as low intraocular pressure (IOP) and occasionally is associated with decreased vision. Normal IOP is usually between 12 and 22 mm Hg. Similar to using inches as a way to report length, mm Hg refers to millimeters of mercury and is a way to report pressure. An eye is considered hypotonous when the IOP drops below 10 mm Hg. However, hypotony may not be a problem unless the IOP drops below 5 mm Hg.
Low pressures are generally below five or so, but everyone is different. Just as some eyes are more sensitive to high pressure, some eyes are more easily troubled by low pressures. When the eye pressure is too low it can cause several distortions of the retina, lens and cornea that can degrade vision.
What causes hypotony?
Hypotony has several causes. The most common are a post-surgical wound leak, chronic inflammation within the eye, or retinal detachment. It is also often associated with detachment of the choroid (the layer of the eye lying between the retina and sclera) and shallowing of the anterior chamber (the front inner part of the eye).
The surgical procedure most commonly associated with post-surgical hypotony is filtering surgery, with trabeculectomy being the most common type used to treat glaucoma.
Trabeculectomy involves the removal of part of the trabecular meshwork (the eye’s drainage canals) or peripheral cornea (the clear window in the front of the eye) to increase fluid flow from the eye. Depending on its duration and severity, post-surgical hypotony can lead to engorged retinal vessels, swollen optic discs, and folds in the choroid and retina.
Because of modern techniques, chronic hypotony rarely occurs today after a trabeculectomy. When it does occur, it may be caused by excess drainage through the scleral flap (the incision in the white part of the eye), or a leak through the external conjunctiva tissue (the delicate membrane covering the eyeball).
How is hypotony treated?
Treatment of hypotony is especially important when it is associated with visual loss. Hypotony may be treated with a variety of techniques, depending on the cause.
Post-surgical leaks may be repaired by a variety of ways. This can include patching, placement of an oversized contact lens, injection of blood to promote scarring, various suturing techniques, drainage of fluid external to the choroid, or using a special kind of substance (called viscoelastics) that helps reshape the front inner part of the eye (the anterior chamber) if it has become shallow. Chronic inflammation can be treated with topical and/or systemic corticosteroids. A retinal detachment can be surgically repaired.
Last reviewed on August 19, 2022