Vitreous Block (Malignant Glaucoma, Aqueous Misdirection)

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Article initiated by:

Manishi Desai, MD, Meenakshi Chaku, MD, Melissa Mei Yiu Wong, MD

All contributors:

Monica K. Ertel, MD, PhD, Ahmad A. Aref, MD, MBA, Dr John Davis Akkara (MBBS, MS, FAEH, FMRF, FAICO), Manishi Desai, MD, Leonard K. Seibold, MD, Harry Quigley

Assigned editor:

Leonard K. Seibold, MD

Review:

Assigned status Update Pending

 by Leonard K. Seibold, MD on May 12, 2023.

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Contributing Editors:	add

Vitreous Block (Malignant Glaucoma, Aqueous Misdirection)

ICD-10

H40.8

Summary

Vitreous block, also referred to as malignant glaucoma and aqueous misdirection, is a form of secondary angle closure glaucoma characterized by elevated intraocular pressure (IOP) and flat central and peripheral anterior chamber, despite the presence of a peripheral iridotomy. There is agreement that it involves abnormal behavior of the vitreous humor, with increasing evidence that the inciting factors include lowering of IOP during surgery, smaller eye size, and choroidal expansion. While medical therapy with cycloplegics and YAG laser treatment to visible vitreous are used, definitive vitrectomy to produce a unicameral eye is most effective.

Disease Description

The term “malignant” was first applied to the immediate post-operative course in eyes with elevation of IOP and axial flattening of the anterior chamber by von Graefe. It was recognized as a rare complication in eyes with acute angle closure glaucoma unresponsive to iridectomy. In 1950, Chandler reported that this uncommon condition was more frequent in smaller eyes, but could occur during or after a variety of surgical procedures for glaucoma. Shaffer noted that the vitreous was involved in the process, advocating incisions of vitreous with needle or knife. This was confirmed by observations of Simmons , who noted that while ciliary processes were sometimes in contact with the lens, complete closure of the space between ciliary body and lens did not occur. It was recognized that bilateral or sequential occurrence in the two eyes was an inherent tendency. Shaffer and Hoskins proposed that the force moving the iris and lens forward to flatten the anterior chamber was due to “misdirected aqueous”, and this term has persisted in both naming the condition and in consideration of its pathogenesis. Surgical vitrectomy to eliminate vitreous block has become the standard of care and its definitive treatment.

Epidemiology

Vitreous block has been reported throughout the world, with an incidence intraoperatively or post-operatively of 0.6 to 4 %. The condition most commonly develops unilaterally, but the fellow eye is often involved at a later time or after surgery on the second eye.  It is more common in females, which may coincide with its greater prevalence in angle closure. It has been reported in both phakic and pseudophakic eyes,  and can occur spontaneously. Miotic eye drops are known to precipitate the condition.  It has been reported after many ocular surgical procedures and in a variety of ocular conditions.

Etiology

The precise underlying mechanism of aqueous misdirection is unknown. Generally, it is felt to result from diversion of aqueous flow into the posterior segment. This diversion of fluid results from an abnormal relationship between the ciliary body processes, lens, and anterior vitreous. Several theories for the pathogenesis of aqueous misdirection have been proposed. One theory is that overfiltration in the immediate post op period leads to shallowing of the anterior chamber and anterior rotation of the ciliary body as a consequence of posterior pressure (i.e. from pressure differential between anterior and posterior chamber) . Whatever the cause, a malignant cycle is created whereby pressure incrementally rises in the posterior chamber but the fluid build-up cannot decompress or exit to balance anterior chamber pressure resulting in forward movement of the lens-iris diaphragm.

Pathophysiology

The clinical features of this condition include the following: 1) The vitreous appears relatively opaque and either disruption of the gel or its removal can break the acute cycle. 2) Elimination of pupil block by iridectomy is not effective. 3) The iris and lens are so far anteriorly that the chamber is typically emptied of aqueous humor, leading to high IOP. 4) Expansion (thickening) of the choroid is now recognized as a feature identified by optical coherence tomography in both acutely involved and fellow eyes, compared to eyes with angle closure disease or open angles.

A logical sequence of events that would explain these observations begins with expansion of the choroid. The choroid occupies about 2% of the internal volume of the normal eye and over 4% of smaller eyes (prone to angle closure) due to thicker choroid and smaller overall volume. Based on the known pressure—volume relationship of the living eye, even a tripling of the normal choroid thickness of ~300 µm would increase IOP to over 60 mm Hg. Yoga head-standing doubles IOP and the Valsalva effect of trumpet playing increases IOP to 40 mm Hg with only a 20% increase in choroidal thickness measured by ultrasonic biomicroscopy. These increases in the posterior 2/3 of the eye would increase aqueous outflow from the posterior and anterior chambers as a response to the pressure differential.

If fluid from the vitreous cavity moves to replace the departing aqueous, the anterior chamber would remain formed and IOP would return to normal. This compensation to reduce IOP fails in eyes with vitreous block. Studies of normal vitreous humor show its ability to transmit water across the gel is finite and decreases when the pressure differential across it is higher. Studies in post mortem human eyes found that “differences in composition or structure can have an important influence on the permeability of the vitreous body”. This suggests that eyes with vitreous block have either a low water conductivity or a greater tendency to compress under pressure, leading to a vicious cycle that carries the vitreous, lens and iris forward until the anterior chamber completely empties.

Choroidal detachment is thought of as typical for low IOP, since in most eyes, the increased posterior volume is accommodated by loss of vitreous water and aqueous humor through the trabecular meshwork in eyes with normal fluid conductivity. In vitreous block, ciliary processes may be more visible clinically and “annular ciliary body detachment” is present, though there is not complete obstruction of the narrow, donut-shaped space between ciliary body and native lens or lens implant. The ciliary body, being contiguous with the choroid, would rotate anteriorly with choroidal expansion, but earlier observations were made with UBM which does not include the posterior ocular structures and cannot show posterior choroidal thickening.

Choroidal volume could increase by an increase in either its intravascular or extravascular spaces. Intravascular choroidal expansion would be very short term (e.g. Valsalva, tight necktie) and not sufficiently sustained to cause vitreous block. It is thus more likely that abnormal permeability of the choriocapillaries or larger vessels allows large proteins into its extravascular stroma. Angle closure eyes have greater propensity for choroidal expansion, likely contributing to their propensity to vitreous block. The pressure difference from posterior to anterior eye induced by anterior segment surgical incisions potentiates choroidal expansion intra-operatively. Once expansion of the choroidal stroma by serous fluid occurs, its exit through the sclera is slow, since the normal osmotic pressure to return fluid to the veins is gone. Fluid within the vitreous chamber must pass into the posterior chamber through the donut-shaped space between the ciliary body and lens. Vitreous that is compressed in this space blocks further fluid movement forward. Thus, sustained higher pressure in the vitreous cavity allows vitreous block in susceptible eyes.

Shaffer recognized that aqueous misdirection behind or through the vitreous required a one-way valve mechanism, but no evidence has been presented to support this mechanism. Epstein concluded from experiments with human eyes that “there must be factors other than, or in addition to, diversion of aqueous humor through the vitreous body to explain this condition. It seems in malignant glaucoma that the permeability of the vitreous humor…must be less than normal”.Thus, the terms aqueous misdirection and ciliary block are not compatible with present evidence and a recent review confirms vitreous block  as an appropriate designation for the condition.

Diagnosis

• Signs of vitreous block can present either intra-operatively or postoperatively after ocular surgery or they can occur without surgery, spontaneously.

• While IOP is typically high, it can be within the normal range.
• The anterior chamber is flat or nearly so, often with lens to corneal contact. Most often there is asymmetry between the involved chamber depth and the fellow eye.
• Ocular pain and decreased visual acuity result from IOP elevation and forward lens movement.
• A history of vitreous block in the first eye is important, since occurrence in the second eye is likely.
• The condition is more likely in those with angle closure disease, but can occur in any eye.
• If an existing iridotomy is not present, it should be performed, when possible, to rule out pupil block alone.
• Wound leak should be ruled out.
• B-scan ultrasound examination or posterior OCT images often show thickening of the choroid, though this can be only 3 times normal thickness without clear separation of choroid from sclera.

Differential diagnosis

• Pupil block glaucoma (primary angle closure) is ruled out by a patent iridotomy. Further, primary angle closure eyes have symmetrical chamber depth, while vitreous block leads to flat chamber and depth asymmetry.
• Choroidal detachment, either spontaneous or post-operative with shallow or flat anterior chamber most often has low IOP. The choroid is most often easily seen as separated by ophthalmoscopy or has large lucent separation from sclera by B-scan ultrasound. A wound leak or cyclodialysis cleft may be identifiable.
• Suprachoroidal hemorrhage most often occurs intra- or post-operatively with sudden loss of anterior chamber, high IOP, and severe pain requiring narcotic relief. The presence of blood in detached choroid is diagnostic by ultrasound.
• Secondary angle closure simulating vitreous block occurs in uveal effusion, in Sturge-Weber syndrome, and after pan-retinal photocoagulation, scleral buckling surgery, central retinal vein occlusion, and carotid-cavernous fistula.

Management

Medical Management

• Maximum IOP-lowering should be delivered topically (without pilocarpine) and with acetazolamide orally.
• Cycloplegic drops are recommended to widen the ciliary body ring of space to the lens, allowing more fluid transfer from vitreous to posterior chamber.
• Osmotic agent intravenously or orally decrease fluid from the choroid and vitreous, especially as part of intraoperative management.
• Anti-inflammatory eye drops may be given.
• Oral pain relief by narcotic may be needed.
• If IOP is not relieved nor the chamber reformed within days, surgery should be planned.
• If the acute condition resolves medically, long-term cycloplegia is continued.

Laser Management

• If an iridotomy is not present nor clearly patent, it should be made, though the flat anterior chamber makes this difficult.
• YAG laser delivery to disrupt the vitreous gel has occasionally aborted vitreous block. The energy is placed ideally in the zonular area through an existing iridectomy, or posterior to the lens or IOL-capsule area. While the vitreous may be compressed anteriorly, YAG treatment often cannot produce a channel adequate to allow freer fluid passage.
• Slit lamp delivery of diode (“argon”) laser to ciliary processes was reported to alleviate vitreous block prior to the development of YAG lasers. It is uncomfortable and not preferred to YAG laser.

Surgical Management

Surgical therapy is frequently necessary to definitively manage aqueous misdirection. In a retrospective review of 26 eyes from 24 patients with aqueous misdirection from a single center, all but one case required surgery eventually within 3 months of presentation. If the patient is refractory to both medical and laser management of aqueous misdirection, surgery is necessary to disrupt the anterior vitreous face or remove vitreous, thereby increasing aqueous flow to the anterior chamber.

• Surgical vitrectomy is often needed when medical and laser treatments fail.
• Either a pars plana or anterior vitrectomy approach may be used.
• It is critical to remove vitreous at the space between ciliary body and lens/IOL, most often by performing irido-zonulo-hyaloidectomy (IZH), resulting in a “unicameral” eye.
• The anterior approach for IZH is favored in the phakic eye and lens extraction may be necessary.
• When surgery is planned on the fellow eye of a person with vitreous block in the first eye, vitrectomy should be planned or available.
• With long-standing vitreous block or advanced pre-existing glaucoma, permanent angle closure may be present, necessitating either combined or subsequent glaucoma surgery. Tube shunt placed in the pars plana has been advocated in this situation.