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Glaucoma with High Episcleral Venous Pressure
Zinaria Y. Williams, MD
Episcleral venous pressure (EVP) plays an important role in IOP regulation. EVP venous pressure can be raised due to obstruction or shunting of blood through several pathways due to a variety of clinical disease processes. Elevated EVP can be a significant cause of secondary open-angle glaucoma. To appreciate the processes that cause high EVP, physicians must understand the aqueous-venous drainage system and how the system can determine IOP.
Aqueous-venous Drainage
Aqueous drains from the anterior chamber through two routes. The conventional route is through the trabecular meshwork and into the Schlemm's canal. Uveoscleral flow is an unconventional route. Aqueous travels through the spaces between the ciliary body muscle fibers and into the supraciliary spaces between the ciliary body and the sclera.
In the conventional route, aqueous enters the circular Schlemm's canal from the trabecular meshwork and then exits through collecting channels, deep scleral, and intrascleral venous plexi. These pathways drain into the aqueous veins of Ascher near the limbus and join the episcleral veins. Episcleral veins are drained by the vortex veins, which exit the sclera on the posterior aspect of the globe.
Venous blood can leave the vortex veins through three routes (Slide 1):
Elevated EVP and Glaucoma
Three factors contribute to IOP as illustrated in the Goldmann equation (Slide 2):
EPV is directly proportional to IOP. Therefore, if EVP is elevated due to a compromise in the aqueous-venous pathway, IOP will increase (given the rate of aqueous production and outflow facility remain stable). Normal EVP is 9 ± 1.5 mm Hg. EVP can be measured with a venomanometer.1 This is an applanation device that attaches to the slit lamp and uses a transparent silicone membrane tip against the vessel until the walls are compressed. The point at which the vein compresses is measured as the EVP.
The pathophysiology of glaucoma due to high EVP is likely related to the chronically elevated IOP that leads to glaucomatous optic nerve damage. A component of an increased venous pressure can lead to decreased optic head perfusion.
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The causes of raised EVP may be divided into four categories: entities that obstruct venous return to the heart, entities that shunt blood from the arterial to venous system, change in body position (transient), and idiopathic (Slide 3).
The clinical findings can be unilateral or bilateral and they include dilated, tortuous episcleral veins (Slide 4), an open angle, blood in Schlemm's canal, and ocular ischemia with or without neovascularization due to prolonged decreased venous outflow.
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Thyroid Ophthalmopathy
Thyroid ophthalmopathy involves orbital infiltration of lymphocytes directed against thyroid follicular cell antigens that are cross-reacting to orbital tissue antigens. This infiltration leads to hypertrophy of extraocular muscles and proliferation of orbital fat and connective tissue (Slide 5A and Slide 5B).
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Congestion within the orbit compromises the orbital venous system and can lead to elevated EVP and high IOP. Patients may also have lid retraction, unilateral or bilateral proptosis, eye movement limitations, and optic neuropathy. In a study to determine the prevalence of elevated IOP and progression to glaucomatous damage in patients with thyroid ophthalmopathy, investigators found that the duration of active orbital involvement with elevated IOP was associated with progression to glaucomatous damage.2
Sturge-Weber Syndrome
Sturge-Weber syndrome (encephalotrigeminal angiomatosis) is characterized by facial cutaneous hemangiomas (nevus flammeus) that usually follow the distribution of the trigeminal nerve. Intracranial involvement leads to ipsilateral angiomas of the meninges and brain and gyriform calcifications.
Ocular sequelae include choroidal hemangioma, retinal detachment, dilated and tortuous conjunctiva, episcleral and retinal vessels, and glaucoma. Glaucoma may occur in infancy, childhood, or adulthood. Mechanisms proposed are goniodysgenesis and elevated EVP. Gonioscopy reveals a high iris insertion.
Histopathologic examination has shown a compact trabecular meshwork with thickened trabecular beams, anterior insertion of the iris root, and thickened uveoscleral meshwork. Hemangiomas are an abnormal proliferation of blood vessels.3,4 Within them are arteriovenous shunts that can increase the orbital venous system. They can involve the lid, orbit, conjunctiva, episclera, ciliary body, and iris. Hemangiomas that involve the eyelid are more commonly associated with elevated EVP, which can lead to glaucoma.
Arteriovenous Fistula
Arteriovenous fistulas are abnormal communications between an artery and vein that can be caused by degeneration (i.e., hypertension and atherosclerosis) or trauma (i.e., basal skull fracture). Carotid-cavernous sinus fistula connects the internal carotid artery and cavernous sinus. Clinical signs are tortuous, "corkscrew" episcleral vessels, audible bruit, pulsating exophthalmos, proptosis with exposure, diplopia, pain, and orbital compression.
Elevated IOP can result from high EVP, angle closure, and neovasular glaucoma. Several diagnostic imaging studies can be performed. An orbital computed tomography scan with intravenous contrast may demonstrate a dilated superior ophthalmic vein.
Color Doppler imaging can illustrate impedance and reversal of flow in the superior ophthalmic vein. Angiography may reveal filling of both the internal carotid artery and the cavernous sinus together. Magnetic resonance imaging and magnetic resonance angiography studies may also be helpful in securing a diagnosis.
Dural-sinus fistulas arise from the dural arteries to the cavernous sinus. There is female preponderance and an insidious clinical course. Clinical presentation includes dilated episcleral veins, blood in Schlemm's canal, and high IOP. These fistulas spontaneously resolve in 20% to 50% of patients and IOP can normalize.
Nonsurgical options for patients with an arteriovenous fistula are observation or medical therapy with aqueous suppressants. Surgical treatments include embolization of fistula or balloon occlusion.
Idiopathic
Idiopathic elevation of EVP can be familial or sporadic. Patients have dilated episcleral veins and increased IOP, but there are no angiographic or radiographic evidence of an underlying cause. This is likely the most common and least recognized etiology for the phenomenon of elevated EVP.
Treatment for high IOP due to elevated EVP should be tailored to the underlying cause. Medical treatment of elevated IOP in the setting of thyroid orbitopathy may provide some reduction. However, in severe cases, systemic corticosteroids are given to reduce orbital congestion. Radiation therapy is also used to help decrease orbital soft tissue volume.
Surgical management is performed for severe or progressive cases, as well as for emergent intervention. Orbital decompression increases the orbital volume by removing part of the bony orbit and opening the periorbita. Indications include compressive optic neuropathy, severe proptosis, severe orbital congestion, and marked cosmetic deformity. Orbital decompression may reduce the IOP by decreasing EVP.5
Medical therapy often fails in patients with Sturge-Weber syndrome and glaucoma may eventually require surgical treatment such as laser trabeculoplasty or trabeculectomy.
The serious complications of filtration surgery are intraoperative choroidal effusions and expulsive hemorrhage, and bleeding from dilated episcleral vessels. Techniques such as posterior sclerotomy, viscoelastic placement in the anterior chamber during the procedure, as well as placing tight scleral flap sutures can be used to reduce these risks.
Nonsurgical options for patients with an arteriovenous fistula are observation or medial therapy with aqueous suppressants. Surgical treatments include embolization of fistula or balloon occlusion.
In idiopathic patients, treatment focuses on reducing IOP with aqueous suppressants. Filtration surgery is also a consideration to control IOP. Reports exist of patients who underwent trabeculectomy with good reduction in IOP, but dilated episcleral veins were unchanged.6 Care must be taken to avoid choroidal effusion and suprachoroidal hemorrhage during and after filtering surgery.
Some surgeons have suggested prophylactic sclerotomies in the inferior two quadrants before entry of the anterior chamber during trabeculectomy, similar to those openings made during drainage of choroidal effusions. These sclerotomies are left open at the conclusion of surgery, but the conjunctiva over them is closed.7
