Glaucoma Associated With Extraocular Venous Congestion
(Increased Episcleral Venous Pressure)
Ian Conner, MD, PhD; Joel S. Schuman, MD, FACS; and David L. Epstein, MD, MMM
Increased episcleral venous pressure may be seen in the following clinical situations:
- Carotid-cavernous sinus fistulas and dural shunts
- Sturge-Weber syndrome
- Iatrogenic secondary to encircling scleral band
- Idiopathic type1–5
Glaucoma due to increased episcleral venous pressure is likely more common than diagnosis rates suggest. It is routine for clinicians to focus on the slit-lamp examination of the cornea, anterior chamber, or posterior segment and to overlook the presence of prominent episcleral veins. Routinely evaluating eyes with a penlight (see Chapter 5) can greatly assist in making the diagnosis in these “red-eye” glaucomas. Both the idiopathic type and that due to a subclinical dural shunt are underdiagnosed.
When glaucoma therapy does not decrease the intraocular pressure (IOP), in addition to repeating gonioscopy, it is important to carefully reexamine the episcleral vessels. Recall that the IOP cannot be lowered below the episcleral venous pressure (see Chapter 3) without surgically bypassing the trabecular outflow system; therefore, the IOP remains elevated after initiation of medical therapy in this condition.
Any time blood in Schlemm’s canal is observed spontaneously in an eye with elevated IOP, elevated episcleral venous pressure should also be entertained. Recall the case with an overfiltering bleb and associated hypotony—it is not uncommon to observe blood reflux into Schlemm’s canal in this circumstance. This occurs because the episcleral venous pressure is higher than the IOP.
In many instances of elevated episcleral venous pressure, especially the idiopathic and Sturge-Weber types, filtering surgery is often performed because of the persistent elevation of IOP despite maximal tolerated medical therapy. The increased episcleral venous pressure causing the elevated IOP is an indicator of an increase in orbital venous pressure. Thus, when such eyes are decompressed through the usual limbal incision for the filtration fistula, the transmitted elevation in choroidal venous pressure may result in transudation of fluid into the suprachoroidal space,2 which then forces intraocular contents forward through the fistula. Prophylactic posterior sclerotomies performed before entry into the anterior chamber can diminish the severity of this occurrence and also the postoperative sequelae.2 Smaller incision/excision trabeculectomy techniques with use of anterior chamber viscoelastics or maintainers should also be used.
INCREASED ORBITAL VENOUS PRESSURE
Increasing the venous pressure in the head and orbit causes the IOP to increase. Some of the pathophysiology of this relationship was described experimentally by Comberg and Pilz.6 Clinically, the causes of elevation of venous pressure that are of particular significance to the eye include carotid-cavernous sinus fistula, thrombosis of orbital veins or cavernous sinus, dural arteriovenous shunts, endocrine exophthalmos,7 pulmonary venous obstruction by tumors or cardiopulmonary disease, and obstruction of the superior vena cava. Congestive heart failure causes only a slight increase in IOP.8
Figure 47-1. (A) Prominent episcleral veins, increased episcleral venous pressure, and glaucoma due to dural shunt. (B) The same eye appears normal with resolution of IOP elevation 4 weeks after neurosurgical closure of the shunt.
The venous abnormalities of the head and orbit are often unilateral and affect only one eye, whereas compression of veins in the neck or chest usually affects both eyes. There are other conditions in which both the extraocular venous pressure and the IOP are elevated, but their exact mechanism for this elevation of venous pressure is as yet unclear. This includes certain cases of hemangioma with glaucoma (Sturge-Weber syndrome) and the particular combination of conspicuously dilated episcleral vessels with open-angle glaucoma, but without other demonstrated extraocular vasculopathy (that we have termed idiopathic; Figure 47-1).
Signs and Symptoms
The signs and symptoms of thrombosis of orbital veins or arteriovenous fistula may have rapid onset, sometimes with pain on the same side of the head.9 The episcleral veins appear dilated on the affected side. Chemosis and exophthalmos may develop within hours or days. The exophthalmos may be pulsating, and an orbital bruit may be appreciated if an arteriovenous fistula is present. The retinal veins may appear dilated on the same side. The IOP becomes elevated, most commonly to the mid-20s to mid-30s, and the ocular pulse is increased. These changes are most pronounced in cases of arteriovenous communication. If pressure is measured in the episcleral or orbital veins, the increase of IOP is about the same as the increase of venous pressure above its ordinary baseline of approximately 8 to 10 mm Hg.
A rare complication of elevated orbital venous pressure is shallowing of the anterior chamber on the affected side10–12; angle-closure glaucoma may result. The shallowing is attributable to increased venous volume leading to effusion in the posterior segment, in some cases visible as choroidal detachment.10,11
In rare cases of carotid-cavernous sinus fistula, not only may there be elevated orbital venous pressure, but the arterial blood supply to the eye may be partially obstructed and the rate of aqueous formation may be reduced. Then, the IOP may remain low despite elevated venous pressure. Anterior segment ischemic necrosis has occurred under these circumstances.13,14 This danger appears to be greatest when the arteriovenous fistula has been treated by occluding the feeding artery.
A further complication in cases of long-standing ischemia associated with elevated venous pressure and reduced arterial supply is growth of new blood vessels on the iris and in the angle, leading potentially to neovascular glaucoma.12,15
In dural shunts (Figure 47-1A),16 which are lower-pressure arteriovenous shunts than the carotid-cavernous fistula described previously, the signs and symptoms may be much more subtle. Patients may actually be unaware of their red eye. This is also true in the idiopathic variety of increased episcleral venous pressure. Also, many patients with Sturge-Weber syndrome, although fully aware of their facial hemangioma, do not appreciate well the episcleral involvement, which, as will be discussed, may not become prominent until the teenage years.
In most cases of elevated orbital venous pressure with secondary modest elevation of IOP, the angle remains open, and in most cases, the tonographic facility of outflow remains normal. In these cases, if the orbital venous pressure subsequently becomes normal, either spontaneously or as a result of treatment, the IOP returns to normal. However, when the facility of outflow is abnormal (as it often is in long-standing cases of increased episcleral venous pressure), glaucoma may persist even after venous pressure is normalized as though permanent damage has been done to the aqueous outflow system by the chronic elevation of episcleral venous pressure.17 In cases of neovascularization and synechial closure of the angle, this persistent glaucoma is understandable, but the outflow system of eyes with open angles may also be affected as postulated by Weekers and Grieten.18 This is a consideration that seems to us to deserve more study: Why would chronic elevation of episcleral venous pressure cause permanent trabecular meshwork (TM) dysfunction?
IDIOPATHIC ELEVATION OF EPISCLERAL VENOUS PRESSURE
To this point, we have mainly discussed elevations of IOP that are apparently secondary to abnormally high extraocular venous pressure and that can usually be relieved if the venous pressure can be reduced. There is another group of unusual cases1–5,19 in which there is no recognized cause outside of the eye for elevation of venous pressure; these are thus termed idiopathic cases, but vessels on the globe are conspicuously large, and measurement shows the pressure in these vessels to be elevated to more than 30 mm Hg in some cases. Patients who have primary open-angle glaucoma have normal episcleral venous pressure.
This idiopathic variety of increased episcleral venous pressure is fascinating but poorly understood. It is likely underdiagnosed and needs to be considered in the differential diagnosis of refractory cases of open-angle glaucoma, as the external manifestations can be rather subtle. This condition can be either unilateral or bilateral. As this is a diagnosis of exclusion, these patients have intracranial imaging with no observed abnormality. Presumably, this condition must then be caused by some orbital4 or periocular venous abnormality with increased pressure transmitted to both the episcleral and choroidal veins. Therefore, prophylactic posterior sclerotomies,2 smaller incision/excision trabeculectomies, and anterior chamber viscoelastics or maintainers should be strongly considered at the time of fistulizing surgery in such patients, as in other glaucoma patients with elevated venous pressure.
STURGE-WEBER SYNDROME
A similar set of conditions can be found unilaterally or occasionally bilaterally in cases of hemangioma (Sturge-Weber syndrome) with open-angle glaucoma.20–23 Patients with Sturge-Weber syndrome and glaucoma fall into 1 of the following 2 categories:
- Those with congenital mesodermal angle abnormalities
- Those with elevated episcleral (and orbital) venous pressure presumably due to the presence of orbital and periocular hemangiomas
In the first category, the onset of glaucoma is typically early in life with frequent buphthalmos, anisometropia with amblyopia, and advanced disc cupping when first detected. These patients are usually quite refractory to medical glaucoma therapy. Sometimes, the angle abnormality24–26 may be accompanied (perhaps later in life) by the presence of dilated episcleral vessels with increased venous pressure.
In the second category, there is no buphthalmos, etc, and, in fact, there is often a history of onset of a red eye and accompanying glaucoma in the teenage years, sometimes at menarche. If examined at that time, there is often high IOP but only early disc cupping, implying a later onset to the glaucoma (and perhaps also the episcleral venous prominence), as if something new were happening hemodynamically in the teenage years. Regardless, the glaucoma associated with Sturge-Weber syndrome, once observed, is commonly characterized by persisting high IOP, requiring filtration surgery.
AQUEOUS HUMOR DYNAMICS: A TRICK QUESTION
David L. Epstein, MD, MMM
A frequent trick question given to ophthalmology residents and students is, “What is the tonographic outflow facility in a patient with elevated IOP due to increased episcleral venous pressure?” There is a natural tendency to answer that outflow facility is abnormal because, obviously, the IOP is elevated. This is incorrect. Tonographic outflow facility, at least early in the disease process, can be, and is usually, normal despite the elevated IOP.
Consider the following: With a normal IOP of 16 mm Hg, there is a resistance decrease across the TM (the inverse of which is mathematically termed the outflow facility or C value) to the downstream collector channel (episcleral venous) pressure of 8 mm Hg. Assume that episcleral venous pressure is now suddenly elevated an additional 10 mm Hg to a value of 18 mm Hg, then the IOP would also increase 10 mm Hg to 26 mm Hg, even though the resistance across the TM is unchanged (ie, the C value remains normal).
We have studied this phenomenon in preliminary research using ophthalmic resident volunteers who, in performing a Valsalva maneuver, raised both measured episcleral venous pressure and IOP similarly millimeter for millimeter. For some residents, despite facial congestion during the attempted Valsalva maneuver, their episcleral venous pressure and IOP did not increase at all, whereas others did as described.
For completeness sake, the concept of pseudofacility1 will also be described. In normal patients who had a blood pressure cuff placed around their neck and inflated, episcleral venous pressure became elevated but IOP increased to a slightly smaller degree. IOP usually increased within 1 mm Hg of the induced episcleral venous pressure elevation. This small difference between episcleral venous and IOP elevation was termed pseudofacility and indicated what was believed to be an induced decrease in aqueous humor formation in response to this elevation of eye pressure. The effect is small, and the overall phenomenon actually indicated that aqueous humor formation does not, in fact, self-regulate in response to the level of IOP. There are also no known feedback loops or anatomical connections that could allow such a regulatory process (other than a direct pressure effect on the fluid-secreting cells in the ciliary body).
REFERENCE
1. Kupfer C. Clinical significance of pseudofacility. Sanford R. Gifford Memorial Lecture. Am J Ophthalmol. 1973;75:193-204.
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