A scotoma is the medical term for a blind spot or an area of diminished vision within the normal field of sight. It is not a disease in itself but a symptom indicating a disruption somewhere along the visual pathway, which runs from the retina to the visual cortex in the brain. The experience of a scotoma can range from a faint blur to a completely dark patch.
Defining the Visual Phenomenon
Scotomas are categorized into two primary types based on how the patient perceives the defect. A positive scotoma is a visual disturbance the patient actively sees, such as a dark spot, fog, or shimmering pattern. This type is frequently caused by a process within the retina or optic nerve, where damaged tissue generates an abnormal visual signal.
A negative scotoma is a missing visual field the patient is typically unaware of unless they specifically look for the defect. The brain attempts to “fill in” the missing information. A specific and often temporary form is the scintillating scotoma, which appears as a shimmering, expanding arc of zigzag lines. This phenomenon often precedes a migraine headache and is caused by a wave of electrical activity across the visual cortex.
Categorization of Scotomas
Ophthalmologists classify scotomas using distinctions that help localize the source of the problem. One classification is based on location relative to the central fixation point. A central scotoma is directly in the middle of the visual field, making it disruptive to reading, driving, and recognizing faces. Conversely, a peripheral scotoma is located near the outer edges of the visual field and is often less noticeable.
A separate classification describes the severity of vision loss. An absolute scotoma indicates a total loss of vision in that specific spot, meaning no light can be perceived. A relative scotoma describes an area where vision is only partially diminished; the patient may see large or bright objects but not smaller or dimmer ones.
Underlying Medical Causes
Scotomas arise from damage or dysfunction along the visual pathway, from the retina to the brain. Migraine is a common cause of scintillating scotomas, which are temporary visual auras resulting from a wave of cortical spreading depression in the occipital lobe. These visual disturbances typically resolve completely within 60 minutes.
Progressive conditions often lead to permanent scotomas with distinct patterns. Glaucoma, associated with elevated pressure inside the eye, damages the optic nerve head, commonly producing arcuate scotomas that arch over the central fixation point. Central scotomas are the hallmark of Age-Related Macular Degeneration (AMD), where the degeneration of light-sensing cells in the macula leads to a blind spot straight ahead.
Damage to the optic nerve, such as from Optic Neuritis associated with multiple sclerosis, typically results in a central scotoma due to inflammation and demyelination. A sudden peripheral scotoma, often described as a dark shadow or curtain, is a serious sign of retinal detachment, where the sensory retina separates from the layer beneath it. Vascular events, such as stroke or transient ischemic attack (TIA) affecting the visual cortex, can also cause scotomas, though they more frequently result in larger visual field defects.
Diagnosis and Management
A sudden onset scotoma, especially when accompanied by flashes of light or floaters, warrants immediate ophthalmologic evaluation to rule out acute, vision-threatening conditions like retinal detachment. For chronic or stable visual defects, diagnosis involves specialized tests to pinpoint the location and cause.
The most definitive test is automated visual field testing, or perimetry, which generates a detailed map of the patient’s entire field of vision by measuring sensitivity to light. A dilated fundus examination is also performed, where the eye care professional inspects the retina, optic disc, and blood vessels at the back of the eye. Advanced imaging tools like Optical Coherence Tomography (OCT) provide cross-sectional images to detect structural damage. Management focuses on treating the underlying cause, such as medication for optic neuritis, surgery for retinal detachment, or pressure-lowering drops for glaucoma. For permanent scotomas, low-vision rehabilitation techniques are employed to help the patient adapt and use their remaining peripheral vision effectively.