Glaucomas associated with raised episcleral pressure are difficult to diagnosis and treat. Clinicians must thoroughly understand the pathophysiology of these diseases in order to help affected patients without incurring vision-threatening complications.

MICROANATOMICAL CONSIDERATIONS

Figure 1 illustrates the drainage of aqueous humor from the anterior chamber to the venous blood system. In brief, from the anterior chamber, aqueous enters Schlemm canal through the trabecular meshwork and traverses the intrascleral emissary channels (aqueous veins of Ascher) before finally reaching the episcleral venous plexus and then the long ciliary venous vessels. The long ciliary veins and the vortex veins empty into the ophthalmic vein and then the cavernous sinus.

IOP is theoretically determined by the Goldmann equation: IOP = (F/C) + Pv. F represents aqueous flow rate, C represents tonographic outflow, and Pv represents episcleral venous pressure (EVP; ie, the pressure against which aqueous must flow). Based on this equation, one would expect a 1:1 relationship between IOP and any elevation in episcleral pressure.

Conditions that raise EVP above the usual value of approximately 8 mm Hg can result in increased IOP that is difficult to treat. Furthermore, the chronic elevation in EVP likely causes structural damage to both Schlemm canal and the trabecular meshwork, which then leads to chronically elevated IOP, even after the reversal of venous outflow abnormalities. Many of these pathological syndromes involve direct communication between the arterial or high-pressure circulation and the venous or low-pressure circulation. Another mechanism that raises EVP involves changes in orbital tissue that disturb venous outflow, thus preventing aqueous from exiting the intrascleral veins.

These glaucomas are not frequently seen in clinical practice, yet their recognition by physicians is crucial to doctors' formulation of a safe, successful treatment strategy. Surgical intervention in these cases is fraught with the potential for vision-threatening complications that are directly caused by the disturbed physiology of these eyes. A firm understanding of this pathophysiology can help ophthalmologists avoid catastrophes.

CLINICAL FINDINGS

Blood in Schlemm canal is the pathognomonic finding in this patient population. It is usually accompanied by prominent episcleral/conjunctival vessel engorgement (Figure 2). Heavily pigmented angles, however, may exhibit a brick red or reddish brown line instead of the more obvious blood red coloration one might expect. Findings may be unilateral, but they are not uncommonly bilateral, although asymmetrically so. In severe cases—particularly in high-flow arteriovenous (AV) shunting—retinal hemorrhages, ischemia, choroidal effusion with forward rotation of the ciliary body, and angle closure can be seen.

AV SHUNTS: HIGH FLOW AND LOW FLOW

Communication between the arterial and venous systems in and around the eye, orbit, or brain will result in the shunting of blood into veins, which, unlike arteries, are too thinly walled to withstand such pressure elevations.1 In some cases, the fistula is located between the carotid arterial system and the cavernous sinus, an arrangement that allows the transmission of arterial pressure into the periocular venous system. The location and degree of shunting dictate the clinical pictures.2 For example, involvement of the vortex veins can result in choroidal detachments and ciliary body rotation with an angle-closure component (Figure 3).3 An eye with an AV shunt or malformation located more anteriorly may present with only dilated episcleral vessels, blood in Schlemm canal, and elevated IOP. The chronic elevation of EVP in any of these syndromes can be expected to permanently damage the ocular outflow channels.

Truly high-flow shunts are usually traumatic in origin. They are accompanied by pain, proptosis, exposure, and diplopia. Retinal flow can be so severely disturbed as to cause an ocular ischemic syndrome with neovascularization of the angle and subsequent neovascular glaucoma.4,5 In some cases, high-flow AV shunts can be localized radiographically. Embolization or surgical ligation, if performed early in the disease process, may return the aqueous outflow channels to normal. If it also compromises ocular blood flow, however, such treatment may itself cause ocular hypoxia that can result in neovascular sequelae.

Low-flow shunts are more common and are thought to occur when vestigial congenital shunts reopen as a result of some obstruction in the transverse and sagittal venous drainage systems in the brain. In some cases, the presumed AV shunt(s) cannot be localized radiographically, because the AV malformation is tiny and/or located in the sclera. Referred to as idiopathic or presumed ocular AV shunts, they can require treatment to prevent an increase in IOP and subsequent damage to the optic nerve.6

STURGE-WEBER SYNDROME

Sturge-Weber syndrome is a congenitally occurring phakomatosis with the pathognomonic finding of unilateral, diffuse, dermal angioma. Other frequent findings include meningeal angioma with tram-track calcifications of the brain, choroidal hemangioma, and a unilateral glaucoma on the affected side that more often occurs when the angioma involves the upper lid.

When present, elevated IOP develops in infancy or the late teens and can be severe. In newborns, the glaucoma mechanism is more likely to be caused by abnormal trabecular microanatomy similar to that seen in congenital glaucoma, whereas the later-onset form can result from elevated EVP.7 Phelps reported an elevation in EVP related to the extent of the episcleral angioma.8 Sometimes, this episcleral angioma was not obvious until the conjunctiva and thick Tenon capsule were reflected during surgery. As with other causes of episcleral glaucoma, standard filtration methods carry a high risk of expulsive choroidal hemorrhage, but this complication may be even more likely in eyes with Sturge-Weber syndrome that have a choroidal hemangioma.9 Unusual serous nonrhegmatogenous retinal detachments have also been observed.10

OBSTRUCTION OF VENOUS OUTFLOW (SUPERIOR VENA CAVA SYNDROME)

Obstruction or compression of the superior vena cava that drains blood into the heart can raise pressure throughout the venous system, including the cavernous sinus. As would be expected, elevated EVP may signal an obstructed superior vena cava.11 Among the lesions that can cause this problem are mediastinal tumors of many types, aortic aneurysms, goiters, and enlarged hilar lymph nodes. The generalized increase in venous pressure can obstruct a central retinal vein and cause engorgement of the retinal veins and peripapillary retinal edema. Elevated IOP can worsen in the supine position. Although IO-Plowering medication or surgery may be somewhat effective, correction of the underlying obstruction is usually required.

GRAVES DISEASE

When infiltrated by inflammatory cells and fluid, orbital tissues—especially fat—can compromise venous drainage, elevate EVP, and dramatically increase IOP.12-14 In addition, severe muscle infiltration can compress the ophthalmic veins and cause a restriction in gaze that will place tension on the globe in some or all directions of gaze.13 Many eyes are affected by both mechanisms. Patients with these “congested” orbits require a referral to the appropriate specialists, who can address both the orbital pathology and the systemic abnormalities. The optic nerve is at risk because of the IOP and the compression from an orbital compartment syndrome. As with any secondary glaucoma, treatment of the underlying disorder, when possible, is the preferred strategy. In thyroid ophthalmopathy, this will usually involve systemic steroids to reduce inflammation, but topical and oral glaucoma medication can temporize the IOP. In severe cases, however, orbital decompression is ultimately required.

TREATMENT

As noted earlier, although medical therapy and laser trabeculoplasty sometimes lower IOP, the best way to treat these secondary glaucomas is to address the underlying pathological condition when possible. Unfortunately, the underlying cause is not always amenable to treatment, and sometimes, the ocular outflow channels have been permanently damaged. Those patients require glaucoma surgery. Surgical intervention that even temporarily decreases the IOP intraoperatively can trigger massive choroidal effusions and/or hemorrhage due to the now unopposed venous back pressure.

To avoid these complications, ophthalmologists must make several modifications to their surgical technique in all cases of elevated EVP. Surgeons in the author's group have been able to avoid or at least reduce the occurrence of these complications by prophylactically creating scleral windows or sclerostomies inferotemporally and inferonasally that are left open. In addition, surgeons in the author's group use tight flap sutures in trabeculectomy/filtration surgeries and ligature(s) in aqueous shunt procedures. Another potentially helpful technique that can be combined with tight outflow restriction is to use viscoelastics in the anterior chamber to maintain perioperative IOP levels.

Surgeons may wish to consider needling the bleb if the IOP rises after successful trabeculectomy/filtration surgery. In such cases, it is prudent to maintain the IOP with viscoelastic and/or reopening of the sclerostomies. Severe choroidal detachment or hemorrhage can otherwise occur, even if the original glaucoma intervention took place many years earlier.

CONCLUSION

Glaucomas associated with elevated EVP are uncommon, but they require careful diagnosis and modifications in surgical technique. A consultation with retinal, orbital, and/or radiology specialists may be required to manage these patients.

George R. Reiss, MD, is in private practice, with offices in Glendale and Scottsdale, Arizona. Dr. Reiss may be reached at drreiss@reissmd.com.

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