Elevated Episcleral Venous Pressure: Pathophysiology, Diagnosis, and Management

Physiology of Aqueous Humor Drainage and Episcleral Venous Pressure

• Aqueous Drainage Pathway: The vast majority of aqueous humor drains through the trabecular meshwork, enters Schlemm's canal, moves into the collector veins, and ultimately enters the episcleral venous system.
• Normal Values: The normal episcleral venous pressure ($ESVP$) is typically between $8\,mm\,Hg$ and $10\,mm\,Hg$.
• Theoretical Minimum IOP: Because aqueous humor must drain into the episcleral venous system, the $ESVP$ represents the theoretical minimum intraocular pressure ($IOP$) that can be achieved by increasing aqueous outflow through the trabecular meshwork alone.
• Goldmann Equation Correlation: According to the Goldmann equation, the episcleral venous pressure ($PV$) is an additive component to the pressure generated by aqueous formation and outflow resistance.
• Pressure Ratio: In theory, each increase in the episcleral venous pressure by $1\,mm\,Hg$ results in a corresponding increase in the intraocular pressure by $1\,mm\,Hg$.

Etiology of Elevated Episcleral Venous Pressure

• Mechanism: Elevated $ESVP$ is caused by any condition that impedes venous egress from the orbit.
• Arteriovenous (AV) Fistulas and Malformations:
  • Dural Sinus Fistula:
    • Characterized as "low flow" fistulas.
    • Often present subtly.
    • Frequently seen in elderly patients with pre-existing vascular disease.
  • Carotid Cavernous Fistula:
    • Characterized as "high flow" fistulas.
    • Presentation is typically dramatic and not subtle.
    • Often traumatic in origin, resulting from severe closed-head injuries (e.g., a patient whose boat motor struck his head after hitting a log).
• Venous Outflow Obstruction/Compression:
  • Orbital Masses: Physical compression of the veins.
  • Thyroid Eye Disease: Orbital congestion impeding drainage.
• Vascular Abnormalities:
  • Orbital Varices.
  • Sturge-Weber Syndrome.
• Idiopathic: In some cases, the elevation of episcleral venous pressure occurs without a known underlying cause.

Clinical Presentation and Physical Exam Findings

• Laterality: Presentations are usually unilateral but can occasionally be bilateral.
• External Appearance:
  • Red eye caused by injected episcleral veins.
  • Proptosis (bulging of the eye).
  • Marked engorgement of the episcleral veins.
• Subjective Symptoms: Some patients report hearing a "swishing sound" (pulsatile tinnitus) in their head.
• Gonioscopy Findings:
  • Blood in Schlemm's Canal: A hallmark sign of elevated $ESVP$. This can range from subtle (difficult to see, especially for those with color vision deficiencies) to striking and dramatic engorgement where the canal is filled with blood.
• Tonometry Findings:
  • Goldmann Applanation Tonometry: During the measurement of $IOP$, there is often a significant "swing" of the mires against each other, reflecting an increase in pulse pressure.
• Auscultation: Some patients may exhibit an audible bruit when a stethoscope is placed over the globe or orbit.

Management and Treatment Considerations

• Address Primary Cause: The definitive treatment is to identify and treat the underlying etiology, such as the fistula, malformation, or thyroid eye disease.
• Pharmacological Management:
  • Aqueous Suppressants: Primarily used to decrease the rate of aqueous formation.
  • Prostaglandin Analogs: These have a theoretical advantage because they facilitate outflow through the uveoscleral pathway into the suprachoroidal space, bypassing the obstructed trabecular/episcleral system.
  • Ineffective Medications: Drugs or procedures that increase access to Schlemm's canal, such as cholinergic agonists (miotics) or trabeculoplasty, are generally not helpful because the resistance is distal to the canal.
• Surgical Risks:
  • Surgery in eyes with elevated $ESVP$ is very risky.
  • Potential for severe complications: Suprachoroidal hemorrhage or hemorrhage into the anterior chamber.
  • Case Example: A patient with a dural sinus fistula and an $IOP$ in the $50\,mm\,Hg$ range underwent a trabeculectomy because radiologic intervention was considered too dangerous at the time; the procedure resulted in the eye filling with blood.

Differential Diagnosis for Blood in Schlemm's Canal

• While elevated $ESVP$ is a major cause, blood in Schlemm's canal can occur due to other factors:
  • Hypotony: If the $IOP$ is lowered significantly below the normal episcleral venous pressure, blood will naturally reflux into Schlemm's canal.
  • Iatrogenic Reflux: During gonioscopy, applying excessive pressure with a lens that has a large area of contact (such as a Goldmann-style lens) can artificially cause blood to reflux into the canal.