Choroidal neovascularization
Introduction
Choroidal neovascularization (CNV) is a pathological process characterized by the formation of new blood vessels originating from the choroid, the vascular layer of the eye located between the retina and the sclera. This condition is often associated with severe vision loss and is a hallmark of various ocular diseases, most notably age-related macular degeneration (AMD). CNV can also occur in other conditions such as high myopia, ocular histoplasmosis syndrome, and angioid streaks.
Pathophysiology
The pathophysiology of CNV involves the disruption of the Bruch's membrane, a critical barrier between the retina and the choroid. This disruption allows for the invasion of choroidal blood vessels into the subretinal space. The neovascularization process is driven by various growth factors, particularly vascular endothelial growth factor (VEGF), which promotes the proliferation and migration of endothelial cells.
Role of VEGF
VEGF is a potent angiogenic factor that plays a central role in the development of CNV. It is upregulated in response to hypoxia and other stimuli, leading to increased vascular permeability and the formation of new blood vessels. Anti-VEGF therapies, such as ranibizumab and aflibercept, have been developed to inhibit VEGF activity and are widely used in the treatment of CNV.
Clinical Presentation
Patients with CNV typically present with symptoms of visual distortion (metamorphopsia), decreased visual acuity, and central vision loss. The severity and progression of symptoms can vary depending on the underlying cause and the extent of neovascularization.
Diagnostic Imaging
Several imaging modalities are employed to diagnose and monitor CNV, including:
- **Fluorescein Angiography (FA):** This technique involves the injection of a fluorescent dye into the bloodstream, allowing for the visualization of retinal and choroidal circulation. CNV appears as areas of hyperfluorescence due to leakage of the dye from the neovascular membranes.
- **Optical Coherence Tomography (OCT):** OCT provides high-resolution cross-sectional images of the retina and choroid. It is particularly useful for detecting fluid accumulation, retinal thickening, and the presence of subretinal or intraretinal neovascular membranes.
- **Indocyanine Green Angiography (ICGA):** ICGA uses indocyanine green dye to visualize the choroidal vasculature. It is often used in conjunction with FA to provide complementary information about the extent and location of CNV.
Etiology
CNV can arise from various etiologies, each with distinct clinical and pathological features:
AMD is the most common cause of CNV in the elderly population. It is classified into two forms: dry (atrophic) and wet (neovascular). The wet form is characterized by the presence of CNV, leading to rapid and severe vision loss.
Pathologic Myopia
High myopia is associated with excessive elongation of the eyeball, resulting in thinning of the retina and choroid. This predisposes individuals to the development of CNV, often referred to as myopic CNV.
Ocular Histoplasmosis Syndrome
This condition is caused by a fungal infection with Histoplasma capsulatum, leading to chorioretinal scars and subsequent CNV formation. It is more prevalent in certain geographic regions, such as the Ohio and Mississippi River valleys.
Angioid Streaks
Angioid streaks are linear breaks in Bruch's membrane, often associated with systemic conditions like pseudoxanthoma elasticum. These breaks serve as a conduit for choroidal vessels to invade the subretinal space, resulting in CNV.
Treatment
The management of CNV has evolved significantly with the advent of anti-VEGF therapies. These treatments have become the mainstay for controlling neovascular growth and preserving vision.
Anti-VEGF Therapy
Anti-VEGF agents, such as ranibizumab, aflibercept, and bevacizumab, are administered via intravitreal injections. These drugs bind to VEGF, inhibiting its activity and reducing vascular permeability and neovascularization. Regular monitoring and repeated injections are often necessary to maintain therapeutic efficacy.
Photodynamic Therapy
Photodynamic therapy (PDT) involves the intravenous administration of a photosensitizing agent, followed by laser activation. This process selectively targets and destroys neovascular membranes while sparing surrounding healthy tissue. PDT is less commonly used today due to the effectiveness of anti-VEGF treatments.
Laser Photocoagulation
Laser photocoagulation was one of the earliest treatments for CNV. It involves the application of thermal laser energy to coagulate and obliterate neovascular membranes. However, this approach is limited by its potential to cause collateral damage to the retina and is now largely reserved for specific cases.
Prognosis
The prognosis for patients with CNV varies depending on the underlying cause and the timeliness of treatment. Early detection and intervention with anti-VEGF therapy have significantly improved visual outcomes for many patients. However, chronic and recurrent CNV can lead to permanent retinal damage and irreversible vision loss.
Research and Future Directions
Ongoing research aims to further understand the molecular mechanisms underlying CNV and to develop novel therapeutic strategies. Emerging treatments include gene therapy, which targets the genetic basis of CNV, and combination therapies that utilize multiple agents to enhance efficacy and reduce treatment burden.
Conclusion
Choroidal neovascularization is a complex and sight-threatening condition associated with various ocular diseases. Advances in diagnostic imaging and the development of anti-VEGF therapies have revolutionized the management of CNV, offering hope for improved visual outcomes. Continued research and innovation are essential to further enhance our understanding and treatment of this challenging condition.