Optic nerve head drusen (ONHD) are small, calcified deposits that form within the optic nerve head, the part of the optic nerve visible inside the eye. These deposits are often discovered incidentally during a routine eye examination. Optical Coherence Tomography (OCT) is a non-invasive imaging technique that plays a significant role in detecting and evaluating ONHD. This article explores the nature of ONHD, how OCT aids in its detection and differentiation from other conditions, and its potential implications for vision.
What is Optic Nerve Head Drusen?
Optic nerve head drusen are calcified deposits composed of proteins and calcium salts within the optic nerve head. They are thought to be remnants of degenerated retinal ganglion cells. Present from birth, they become more noticeable with age as they calcify further. ONHD are found in approximately 1% to 2% of the general population and commonly affect both eyes, though they can sometimes appear in just one.
These drusen are categorized into two main types based on their visibility. Superficial drusen are directly visible on the surface of the optic nerve head, appearing as yellowish-white, refractile bodies. Buried drusen are located deeper within the optic nerve head tissue and are not directly observable with standard ophthalmoscopy. They may cause the optic nerve head to appear elevated or crowded, mimicking conditions like optic disc swelling.
How OCT Reveals Optic Nerve Head Drusen
Optical Coherence Tomography (OCT) utilizes light waves to generate detailed cross-sectional images of the retina and optic nerve head. This technology provides a high-resolution view of internal structures, allowing for precise visualization of subtle changes. OCT is considered a gold standard for diagnosing both superficial and buried ONHD due to its ability to image deeper layers.
OCT scans reveal specific features of ONHD that aid in diagnosis. Drusen appear as hyperreflective lesions with signal attenuation, or shadowing, within the optic nerve head. Enhanced Depth Imaging OCT (EDI-OCT) can visualize these structures up to 500-800 micrometers deeper than conventional OCT, making it effective for detecting buried drusen and assessing their full extent. It can also show collections of smaller drusen coalescing into larger conglomerates, often with a signal-poor core encased in hyperreflective bands.
Beyond detection, OCT is valuable in differentiating ONHD from other conditions, particularly optic disc edema, which is swelling of the optic nerve head. In optic disc edema, OCT shows a smooth internal contour of the optic nerve head and a thickened retinal nerve fiber layer (RNFL) with surrounding subretinal fluid. Conversely, ONHD presents with an irregular, “lumpy-bumpy” internal optic nerve contour and may exhibit a normal or thinned RNFL due to compression from the drusen. Quantitative measurements of RNFL thickness can further aid in distinguishing between these two conditions.
Potential Effects of Optic Nerve Head Drusen
Many individuals with optic nerve head drusen experience no symptoms and maintain normal vision. However, ONHD can sometimes lead to visual impairments, though these occurrences are rare. The most common potential effect is visual field defects, involving a loss of peripheral or side vision. These defects can include an enlarged blind spot, generalized constriction of the visual field, or specific nerve fiber bundle defects.
Progression of visual field loss is often related to the direct compression of optic nerve fibers and their blood supply by the enlarging drusen. ONHD has also been associated with rare complications, such as optic neuropathy, which is damage to the optic nerve. This can manifest as non-arteritic anterior ischemic optic neuropathy (NAION), where blood flow to the optic nerve is disrupted. Additionally, vascular occlusions, blockages of blood vessels in the retina or optic nerve, and hemorrhages can occur.
There is no specific treatment for the drusen themselves. Management primarily focuses on regular monitoring for associated complications and addressing them if they arise. While some studies have explored intraocular pressure-lowering medications to potentially slow visual field defects, there is no strong evidence to support this as a standard treatment. Routine follow-up with an eye care professional is important to detect and manage any changes in vision or the optic nerve head.