SRF is the accumulation of clear fluid in the space beneath the neurosensory retina. This area is normally a tight junction between the light-sensing retina and the underlying retinal pigment epithelium (RPE). When fluid collects, it physically lifts the neurosensory retina, separating it from its support layer. This detachment disrupts the function of the photoreceptor cells, which convert light into signals the brain interprets.
Defining Subretinal Fluid and Common Causes
SRF indicates a breakdown in the eye’s fluid management system. The fluid originates from the choroid, a vascular layer beneath the RPE, leaking due to a failure of the blood-retinal barrier. The RPE normally acts as a pump, moving fluid out of the subretinal space to keep the retina dry. SRF accumulates when the rate of leakage from the choroid exceeds the RPE’s pumping capacity, often due to structural defects or compromised RPE function.
The two most frequent causes of leakage are Wet Age-related Macular Degeneration (AMD) and Central Serous Chorioretinopathy (CSC). In Wet AMD, leakage is caused by choroidal neovascularization, the growth of new, abnormal blood vessels from the choroid beneath the retina. These vessels are fragile and readily leak fluid and blood into the subretinal space.
CSC is associated with dysfunction or hyperpermeability of the underlying choroid. This condition often involves a thickened choroid with engorged vessels, increasing pressure and fluid seepage. The pressure forces fluid through microscopic defects in the RPE, resulting in a localized fluid pocket. Less common causes of SRF include inflammatory disorders like Vogt-Koyanagi-Harada disease, choroidal tumors, and severe optic nerve swelling.
Recognizing Visual Symptoms
The physical elevation of the retina by the fluid pocket causes noticeable changes in central vision. A characteristic symptom is metamorphopsia, where straight lines appear wavy or distorted. This happens because the photoreceptor layer is displaced and stretched over the fluid dome. Patients also commonly experience blurred or diminished central vision, as the fluid interferes with the macula’s function. Additionally, objects may appear smaller (micropsia), or a dark spot may be perceived in the center of the visual field.
Tools Used for Diagnosis
Specialized imaging confirms the presence of SRF and identifies its underlying cause. Optical Coherence Tomography (OCT) is the standard technique, providing a high-resolution, cross-sectional view of the retina. The OCT scan visualizes the fluid as a dark, separated space between the retina and the RPE layer. This non-invasive tool allows clinicians to measure the fluid and track its response to treatment.
To determine the source of leakage, dye-based tests like Fluorescein Angiography (FA) or Indocyanine Green Angiography (ICG) may be performed. FA involves injecting a fluorescent dye and capturing images as it leaks from abnormal vessels. ICG is better for visualizing choroidal circulation and identifying the focal leak points seen in CSC. OCT Angiography (OCTA) provides non-invasive imaging of blood flow, helping detect leaky vessels in Wet AMD without dye.
Medical and Surgical Management
Management of SRF depends on the underlying cause. For Wet AMD, the primary goal is to stop the growth and leakage of abnormal blood vessels. This is achieved using anti-vascular endothelial growth factor (anti-VEGF) medications, administered via injections directly into the eye. These drugs, including Ranibizumab, Aflibercept, and Bevacizumab, block the VEGF protein that stimulates vessel growth and promotes leakage.
Anti-VEGF injections are effective at drying up SRF and stabilizing vision, but they often require long-term, repeated administration to prevent recurrence. Treatment intensity is guided by the presence of fluid on follow-up OCT scans. The goal is to achieve a fluid-free retina, as persistent fluid can lead to irreversible photoreceptor damage and permanent vision loss.
Management for Central Serous Chorioretinopathy (CSC) differs, as acute cases often resolve on their own within a few months. Initial strategy is observation, sometimes involving the discontinuation of corticosteroid medications that can worsen the condition. If the fluid persists or recurs, targeted treatment is necessary to prevent long-term damage. Targeted Photodynamic Therapy (PDT) is often used for chronic CSC, involving a light-activated drug injection followed by a low-intensity laser to treat the hyperpermeable choroid. Other options include oral medications or specific laser procedures to seal the leakage point.