Introduction
Background
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, and maculopathy. Clinically significant changes occur more frequently as the IOP approaches 0 mm Hg.
Pathophysiology
The usual rate of aqueous humor production 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. Studies suggest that the percentage of uveoscleral outflow may actually be much higher. 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. Choroidal fluid is believed to accumulate as a result of enhanced uveoscleral outflow and decreased aqueous humor production, a cycle that is often perpetuated once choroidal effusions develop. An anterior ring of choroidal fluid can rotate the ciliary body forward, impairing its ability to produce aqueous humor.
Frequency
United States
Hypotony following glaucoma surgery is common but often not clinically significant. Transient hypotony can be seen following 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 following 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.
- 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 disckedema.
- 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.
Clinical
History
- Recent trauma or surgery, especially the following:
- Glaucoma surgery with antimetabolites, a loosely sutured trabeculectomy flap, or a large track created to introduce a tube shunt
- 25-gauge vitrectomy surgery
- Surgery requiring partial thickness scleral sutures, particularly in patients with myopia or abnormally thin tissue
- History of iridocyclitis
- Blurred vision
- Eye pain (usually a deep ache), especially with choroidal detachment (Hemorrhagic choroidal detachment can cause extreme pain.)
- Signs and symptoms associated with retinal detachment
- History of eye inflammation or systemic illnesses predisposing to uveitis
- History of trauma
- Use of IOP-lowering medications, including recent exposure to anesthesia
Physical
- Seidel positive wound leak
- Large bleb following trabeculectomy or tube shunt
- Inadvertent postoperative filtering bleb
- Hyperopic shift/reduced axial length
- Suspected traumatic globe rupture, especially if the following are present:
- 360° of subconjunctival hemorrhagic chemosis
- Peaked pupil
- Hyphema
- Intraocular foreign body
- Preexisting weakness in the globe integrity, such as staphyloma, coloboma, or an old incision
- Inflammatory cells and flare in the anterior chamber
- Shallowing of the anterior chamber
- Corneal edema and decompensation, especially in areas of corneal-iris touch
- Synechiae formation
- Corneal astigmatism
- Accelerated cataract formation
- Cyclodialysis cleft seen on gonioscopy
- Ciliochoroidal detachment - Serous or hemorrhagic
- Hypotony maculopathy
- Retinal folds
- Vascular engorgement and tortuosity
- Optic disc swelling
- Retinal detachment
- The anterior chamber is often deeper than usual when a retinal detachment is present.
- Vitreous hemorrhage or needle tract is visible on funduscopic examination.
Causes
- Unilateral hypotony
- Wound leak
- Overfiltering or inadvertent bleb
- Ciliary body detachment – Serous, hemorrhagic, or tractional
- Cyclodialysis cleft
- Inflammation - Iridocyclitis or blunt trauma
- Retinal detachment or retinotomy
- Ocular ischemia
- Scleral perforation with needle or suture, or scleral rupture following trauma
- Chemical cyclodestruction from antimetabolites
- Photocoagulation or cryoablation of the ciliary body
- Pharmacologic aqueous humor suppression
- Bilateral hypotony
- Systemic hypertonicity or acidosis - Dehydration, uremia, uncontrolled diabetes, or use of hyperosmotic agents
- Myotonic dystrophy
More on Ocular Hypotony |
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| Differential Diagnoses & Workup: Ocular Hypotony |
| Treatment & Medication: Ocular Hypotony |
| Follow-up: Ocular Hypotony |
| Multimedia: Ocular Hypotony |
| References |
| Next Page » |
References
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Further Reading
Keywords
low intraocular pressure, choroidal detachment, wound leak, overfiltration, complication of glaucoma surgery, flat anterior chamber, cyclodialysis cleft, uveitis
