Ophthalmology – Hypotony

Basics

Description

Ocular hypotony refers to abnormally low intraocular pressure (IOP). A commonly used statistical definition is an IOP below approximately 6–6.5 mmHg.

However, the clinically important concept is clinically significant hypotony, in which the pressure is low enough to produce structural changes in the eye or reduced vision.

Transient hypotony is relatively common after glaucoma surgery, procedures involving the pars plana, and ocular trauma. Chronic hypotony is less common but may produce serious complications such as hypotony maculopathy, choroidal detachment, corneal decompensation, and eventually phthisis bulbi.

Epidemiology

Transient postoperative hypotony is relatively common following trabeculectomy and other glaucoma operations, particularly when antifibrotic agents such as mitomycin-C are used.

It may also occur after vitreoretinal surgery, trauma, inflammatory ocular disease, or procedures that reduce aqueous production.

Hypotony following uncomplicated cataract surgery is uncommon.

Chronic hypotony severe enough to produce phthisis is relatively rare and generally occurs in eyes with substantial underlying damage.

Risk Factors

Risk factors include:

• Young age
• Male sex
• Myopia
• Low scleral rigidity
• Uveitis
• Previous glaucoma surgery
• Use of potent antifibrotic agents
• Ocular trauma
• Cyclodialysis
• Ciliary body dysfunction

Highly myopic eyes may be particularly susceptible to hypotony maculopathy because their sclera is more deformable.

General Prevention

Prevention is particularly important after glaucoma surgery.

During trabeculectomy, excessive filtration can be reduced by:

• Limiting the concentration and duration of antimetabolite exposure
• Using adequate scleral-flap sutures
• Employing releasable or adjustable sutures where appropriate
• Avoiding excessive filtration in high-risk eyes

With glaucoma drainage devices, flow may be reduced by using a valved implant or temporarily ligating a nonvalved tube.

Leaving the anterior chamber formed with viscoelastic at the end of surgery can also decrease immediate postoperative chamber shallowing.

Prompt control of postoperative inflammation helps prevent ciliary body shutdown and persistent hypotony.

Pathophysiology

Hypotony occurs when aqueous humor production is insufficient relative to aqueous outflow.

The problem can therefore result from either:

1. Excessive outflow, or
2. Reduced aqueous production.

Excessive Aqueous Outflow

Excessive outflow occurs with conditions such as:

• Wound leak
• Overfiltering trabeculectomy bleb
• Excessive drainage through a tube shunt
• Cyclodialysis cleft
• Occult scleral or corneal perforation

A cyclodialysis cleft creates an abnormal communication between the anterior chamber and the suprachoroidal space, allowing aqueous to bypass the normal outflow pathways.

Reduced Aqueous Production

Aqueous production decreases when the ciliary body is dysfunctional.

Causes include:

• Uveitis or iridocyclitis
• Ciliary body detachment
• Traction on the ciliary body
• Ocular ischemia
• Retinal detachment
• Cyclodestructive procedures
• Severe trauma
• Pharmacologic aqueous suppression

Inflammation increases permeability of the blood-aqueous barrier and can directly suppress ciliary body function.

Choroidal Effusion Cycle

At very low IOP, conventional aqueous outflow decreases because IOP approaches or falls below episcleral venous pressure.

Uveoscleral flow then becomes relatively more important.

Fluid may accumulate in the suprachoroidal space, producing choroidal effusion.

Anterior choroidal detachment can rotate the ciliary body forward and further impair aqueous production, creating a self-perpetuating cycle:

Hypotony → choroidal detachment → ciliary body dysfunction → more hypotony.

Etiology

Unilateral Hypotony

Common unilateral causes include:

• Postoperative wound leak
• Overfiltering bleb
• Tube-shunt overdrainage
• Cyclodialysis cleft
• Uveitis
• Ocular trauma
• Retinal detachment
• Ocular ischemia
• Scleral perforation
• Ciliary body destruction
• Excessive aqueous suppressant medication

Bilateral Hypotony

Bilateral hypotony is much less common.

Possible causes include:

• Severe dehydration
• Systemic acidosis
• Uremia
• Poorly controlled metabolic disease
• Hyperosmotic therapy
• Certain neuromuscular disorders such as myotonic dystrophy

Commonly Associated Conditions

A particularly important association is ocular trauma with occult globe-wall injury or cyclodialysis.

Hypotony also commonly occurs in association with:

• Uveitis
• Glaucoma surgery
• Retinal detachment
• Ciliary body damage
• Choroidal effusion
• Suprachoroidal hemorrhage

Diagnosis

History

Ask specifically about:

• Recent ocular surgery
• Glaucoma surgery
• Use of mitomycin-C or other antifibrotic agents
• Recent trauma
• Ocular inflammation
• Previous retinal surgery
• Glaucoma medications
• Symptoms suggestive of retinal detachment

Patients may complain of:

• Blurred vision
• Distorted vision
• Deep ocular aching
• Photophobia
• Reduced vision after surgery

Pain is particularly concerning when associated with choroidal hemorrhage, which may cause severe ocular pain.

Physical Examination

The examination should include visual acuity, IOP, slit-lamp examination, anterior chamber depth, fundus examination, and evaluation for wound leakage.

Important findings include:

• Low IOP
• Positive Seidel test
• Large or excessively elevated filtering bleb
• Shallow or flat anterior chamber
• Corneal folds
• Corneal edema
• Peripheral anterior synechiae
• Anterior chamber inflammation
• Cyclodialysis cleft
• Choroidal detachment
• Retinal folds
• Optic disc edema
• Retinal detachment

Seidel Test

A positive Seidel test demonstrates aqueous leakage through a corneal, limbal, or conjunctival wound.

Fluorescein is placed over the suspected leakage site. Clear aqueous diluting the fluorescein produces a visible streaming pattern.

A positive Seidel test after ocular surgery strongly suggests a wound or bleb leak.

Shallow Anterior Chamber

Hypotony may cause the anterior chamber to become shallow because the globe loses internal support.

A very shallow or flat chamber increases the risk of:

• Iris-cornea touch
• Lens-cornea touch
• Peripheral anterior synechiae
• Corneal endothelial damage
• Cataract formation

A flat chamber therefore requires prompt attention.

Ciliochoroidal Detachment

Low IOP can lead to accumulation of fluid in the suprachoroidal space.

Choroidal detachments may be:

• Serous
• Hemorrhagic

Serous detachments are often smooth and dome shaped.

Large detachments may approach or touch centrally and are sometimes referred to as kissing choroidals.

Hypotony Maculopathy

Hypotony maculopathy is one of the most important causes of visual loss in chronic hypotony.

Reduced pressure allows inward collapse and deformation of the posterior scleral wall.

This produces:

• Chorioretinal folds
• Macular striae
• Retinal vascular tortuosity
• Optic disc swelling
• Distortion of the foveal architecture

Patients may complain of reduced or distorted central vision.

Young, myopic patients are particularly susceptible because of greater scleral elasticity.

Diagnostic Tests and Interpretation

Laboratory Testing

Hypotony is usually diagnosed primarily through history and examination, rather than laboratory testing.

When recurrent or unexplained uveitis is present, systemic inflammatory evaluation may be indicated.

In bilateral hypotony, laboratory tests may include:

• Blood glucose
• BUN
• Creatinine
• Electrolytes
• Assessment of hydration and acid-base status

B-Scan Ultrasonography

B-scan ultrasonography is particularly useful when the fundus cannot be adequately visualized.

It can identify:

• Choroidal detachment
• Suprachoroidal hemorrhage
• Retinal detachment
• Vitreous abnormalities

Ultrasound Biomicroscopy

UBM is especially useful for evaluating the anterior segment and ciliary body.

It may demonstrate:

• Cyclodialysis cleft
• Ciliary body detachment
• Anterior choroidal effusion
• Abnormal ciliary body position

Anterior Segment OCT

Anterior-segment OCT can help evaluate:

• Anterior chamber depth
• Wound architecture
• Bleb anatomy
• Selected cyclodialysis clefts

Macular OCT

Posterior-segment OCT is very useful in suspected hypotony maculopathy.

It may show:

• Chorioretinal folds
• Macular thickening
• Intraretinal fluid
• Cystoid changes
• Distortion of retinal layers

Subtle folds may be much easier to recognize on OCT than by ophthalmoscopy alone.

Fluorescein Angiography

Fluorescein angiography may demonstrate characteristic alternating linear areas of hyperfluorescence and hypofluorescence associated with chorioretinal folds.

It is rarely essential but can help in subtle cases.

Pathological Findings

Chronic hypotony may cause changes involving multiple ocular structures.

These include:

• Corneal folds and decompensation
• Accelerated cataract formation
• Choroidal effusion
• Choroidal folds
• Retinal pigment epithelial disturbance
• Cystoid macular edema
• Optic disc swelling
• Progressive structural collapse of the globe in severe cases

Differential Diagnosis

Important differential diagnoses include:

• Idiopathic chorioretinal folds
• Choroidal detachment from another cause
• Postoperative endophthalmitis
• Posterior scleritis
• Uveitis
• Hyphema
• Corneoscleral laceration
• Rhegmatogenous retinal detachment
• Tractional retinal detachment
• Postoperative retinal detachment

Treatment

Treatment is directed primarily at the underlying cause of the low pressure.

The goal is not simply to increase IOP numerically, but to restore enough pressure and ocular anatomy to prevent structural and visual damage.

Medication

Corticosteroids

When hypotony results from ciliary body inflammation, corticosteroids may restore aqueous production by suppressing inflammation.

Depending on severity, treatment may include:

• Topical corticosteroids
• Periocular corticosteroids
• Systemic corticosteroids
• Other immunosuppressive therapy for chronic uveitis

Cycloplegics

Atropine or other cycloplegic agents can be useful.

Cycloplegia:

• Relaxes the ciliary muscle
• Deepens the anterior chamber
• Reduces iris-cornea contact
• Helps stabilize the blood-aqueous barrier
• Reduces discomfort from ciliary spasm

Viscoelastic Injection

Injection of ophthalmic viscoelastic into the anterior chamber can temporarily:

• Reform a shallow or flat chamber
• Increase IOP
• Protect the corneal endothelium
• Separate the iris and lens from the cornea

This is generally a temporizing measure, and the underlying cause must still be corrected.

General Measures

Patients should avoid:

• Heavy lifting
• Bending
• Straining
• Vigorous exercise
• Direct pressure on the eye

These precautions are particularly important when choroidal detachments are present because sudden changes in pressure may increase the risk of suprachoroidal hemorrhage.

An eye shield may be useful, especially during sleep.

Good hydration should be maintained unless medically contraindicated.

Treatment of Wound or Bleb Leak

Small leaks with a well-formed anterior chamber may sometimes be managed conservatively.

Options include:

• Reduced manipulation
• Protective measures
• Large-diameter bandage contact lens
• Modification of topical medications

A persistent focal leak may occasionally be sealed with tissue adhesive.

Large leaks associated with marked hypotony or a shallow chamber usually require surgical repair.

Overfiltering Trabeculectomy

When excessive filtration is responsible, management may include:

• Adjustment or placement of scleral-flap sutures
• Compression sutures
• Bleb revision
• Conjunctival advancement
• Autograft or patch graft when tissue is markedly thin

Persistent hypotony with maculopathy generally warrants more aggressive correction.

Tube-Shunt Overdrainage

When a tube shunt causes excessive drainage, options include:

• Temporary tube ligation
• Intraluminal stenting
• Tube revision

The objective is to reduce outflow while preserving long-term glaucoma control.

Cyclodialysis Cleft

Small cyclodialysis clefts may occasionally close spontaneously, especially with cycloplegic therapy.

Persistent clefts can be treated with:

• Laser photocoagulation
• Cryotherapy
• Transscleral treatment
• Direct surgical cyclopexy

Definitive closure reconnects the ciliary body to the scleral spur and reduces abnormal uveoscleral outflow.

Choroidal Detachment

Small serous choroidal detachments often improve when the underlying hypotony is corrected.

Management may include:

• Cycloplegia
• Corticosteroids
• Correction of wound leak or overfiltration

Surgical drainage is considered when detachments are:

• Large
• Persistent
• Associated with a flat anterior chamber
• Kissing centrally
• Accompanied by severe pain or hemorrhage
• Preventing recovery of ocular anatomy

Flat Anterior Chamber

A flat chamber associated with iris or lens-cornea touch can rapidly damage the corneal endothelium.

Prompt management may include:

• Cycloplegia
• Viscoelastic chamber reformation
• Correction of wound leak
• Tightening of the filtration site
• Drainage of significant choroidal effusion when necessary

Chronic Uveitic Hypotony

Chronic inflammatory hypotony can be particularly difficult to treat.

Aggressive anti-inflammatory therapy is essential.

When a cyclitic membrane is pulling on the ciliary body, surgical removal of the membrane may restore ciliary body anatomy and aqueous production.

Selected refractory eyes may benefit from:

• Pars plana vitrectomy
• Membrane removal
• Silicone oil tamponade

Inpatient Considerations

Most cases can be treated as an outpatient.

Hospitalization may become appropriate if severe associated disease is present, such as:

• Major ocular trauma
• Suprachoroidal hemorrhage
• Severe postoperative complications
• Systemic disease requiring inpatient management

Follow-Up

Follow-up frequency depends on severity.

Patients with:

• Wound leakage
• Flat or markedly shallow anterior chamber
• Large choroidal detachments
• Rapid visual decline

may need examination every few days or more frequently until stable.

Patients wearing a therapeutic bandage contact lens require monitoring for infectious keratitis and are commonly given prophylactic topical antibiotic coverage.

Patient Monitoring

Monitoring should include:

• Visual acuity
• IOP
• Anterior chamber depth
• Seidel testing
• Corneal condition
• Choroidal detachment
• Macular anatomy on OCT
• Optic nerve appearance

Visual recovery may lag behind normalization of IOP because macular folds and retinal distortion can take time to resolve.

Diet and Hydration

Patients should maintain adequate oral hydration unless restricted for another medical reason.

Avoidance of constipation and Valsalva may also be useful in patients with severe hypotony and choroidal detachment.

Patient Education

Patients should understand that low eye pressure is not always harmless.

They should be warned to report:

• Increasing pain
• Sudden decrease in vision
• New distortion
• Increasing redness
• New flashes or floaters

Patients should also understand that restoration of normal IOP does not always produce immediate visual recovery.

Prognosis

The prognosis depends primarily on:

• Cause
• Duration
• Severity
• Presence of hypotony maculopathy
• Associated retinal or choroidal disease
• Speed of correction

Transient postoperative hypotony often resolves without permanent damage.

Chronic hypotony can produce progressive structural changes and irreversible visual loss.

Earlier correction of hypotony maculopathy generally provides a better chance of restoring macular anatomy and vision.

Complications

Potential complications include:

• Hypotony maculopathy
• Chorioretinal folds
• Corneal edema and decompensation
• Peripheral anterior synechiae
• Cataract
• Cystoid macular edema
• Choroidal detachment
• Retinal detachment
• Suprachoroidal hemorrhage
• Optic disc edema
• Permanent visual loss

Severe persistent hypotony may ultimately result in pre-phthisis or phthisis bulbi, with progressive shrinkage and loss of function of the eye.