What causes cnvm

Overview

Choroidal Neovascularization (CNV) refers to the abnormal growth of new blood vessels originating from the choroid layer, located beneath the retina, that penetrate Bruch's membrane and extend into the subretinal space or the retinal pigment epithelium (RPE). This neovascularization is a hallmark of the 'wet' form of Age-Related Macular Degeneration (AMD), a leading cause of irreversible vision loss in individuals over 50. While AMD is the most frequent culprit, CNV can also arise from other ocular conditions, including pathological myopia, angioid streaks, choroidal osteomas, inflammatory diseases, and traumatic injuries to the eye.

The development of CNV is a complex process driven by an imbalance of growth factors, particularly Vascular Endothelial Growth Factor (VEGF). In healthy eyes, the RPE acts as a barrier, preventing blood vessels from growing into the retina. However, in conditions like AMD, the RPE cells may become damaged or dysfunctional, leading to an overproduction of VEGF and other pro-angiogenic factors. These factors stimulate the choroidal capillaries to proliferate and invade the retinal layers, forming fragile, leaky new blood vessels.

Causes of Choroidal Neovascularization (CNV)

Age-Related Macular Degeneration (AMD)

The most common cause of CNV is Age-Related Macular Degeneration (AMD), specifically the 'wet' or neovascular form. AMD affects the macula, the central part of the retina responsible for sharp, detailed vision. As people age, drusen (yellow deposits) can accumulate under the retina, and the retinal pigment epithelium (RPE) may begin to break down. In wet AMD, these changes trigger the growth of abnormal blood vessels from the choroid. These new vessels are often fragile and can leak fluid or blood into the retina, causing rapid and significant damage to photoreceptor cells and leading to blurred or distorted central vision.

Pathological Myopia

High myopia, or nearsightedness, particularly pathological myopia, is another significant risk factor for CNV. In pathological myopia, the eyeball elongates excessively, stretching and thinning the retina and choroid. This stretching can damage Bruch's membrane and the RPE, creating an environment conducive to neovascularization. CNV in myopic eyes can lead to sudden vision loss and is often associated with specific patterns of choroidal neovascularization, sometimes referred to as 'myopic CNV'.

Inflammatory and Infectious Conditions

Certain inflammatory diseases of the eye can also lead to CNV. Conditions such as uveitis (inflammation of the middle layer of the eye), ocular histoplasmosis (an infection caused by a fungus), and sarcoidosis can damage the RPE and Bruch's membrane, promoting neovascularization. The inflammatory process itself can stimulate the release of growth factors that encourage abnormal blood vessel growth.

Trauma and Other Ocular Conditions

Direct trauma to the eye, such as a penetrating injury or blunt force, can damage the delicate structures of the posterior segment, including the RPE and Bruch's membrane, potentially leading to CNV. Additionally, less common conditions like angioid streaks (abnormal breaks in Bruch's membrane), choroidal osteomas (bone formation in the choroid), and choroidal melanomas (tumors of the choroid) can also be associated with the development of CNV.

Genetic Factors and Environmental Influences

While aging is a primary risk factor, genetic predisposition plays a crucial role in the development of AMD and, consequently, CNV. Certain gene variations have been identified that increase an individual's susceptibility to developing AMD. Environmental factors, such as smoking, diet, and exposure to sunlight, may also interact with genetic factors to influence the risk and progression of these conditions.

Mechanism of CNV Development

The formation of CNV is driven by a complex interplay of molecular signals. Key players include:

• Vascular Endothelial Growth Factor (VEGF): This is the most critical factor in promoting angiogenesis (the formation of new blood vessels). In response to cellular stress, hypoxia, or inflammation, the RPE and other cells release VEGF.
• Fibroblast Growth Factors (FGFs): These factors also play a role in stimulating endothelial cell proliferation and migration.
• Platelet-Derived Growth Factor (PDGF): Involved in the recruitment of smooth muscle cells that support new blood vessels.
• Cytokines and Inflammatory Mediators: Pro-inflammatory molecules can further promote neovascularization.

When Bruch's membrane is compromised, these growth factors signal choroidal endothelial cells to proliferate, migrate, and form new, often tortuous and leaky, vessels. These vessels can penetrate the RPE and grow into the retina, where they are not meant to be. The leakage of fluid and blood from these abnormal vessels causes swelling (edema) and scarring in the macula, leading to the characteristic vision loss associated with wet CNV.

Symptoms and Diagnosis

Symptoms of CNV can include sudden onset of blurred or distorted vision, blind spots (scotomas) in the central visual field, and altered perception of straight lines appearing wavy (metamorphopsia). Diagnosis typically involves a comprehensive eye examination, including visual acuity testing, ophthalmoscopy, and specialized imaging techniques such as fluorescein angiography (FA) and optical coherence tomography (OCT). These imaging methods allow ophthalmologists to visualize the abnormal blood vessels and assess the extent of leakage and damage.

Treatment and Management

Treatment for CNV has advanced significantly, with anti-VEGF therapies being the mainstay. Injections of medications that block VEGF (e.g., ranibizumab, aflibercept, bevacizumab) into the eye can inhibit the growth and leakage of abnormal blood vessels, often leading to vision stabilization or improvement. Other treatments, such as photodynamic therapy (PDT) and laser photocoagulation, may be used in specific cases, although anti-VEGF injections are generally preferred due to their efficacy and safety profile.