A scotoma is a localized area of partial or complete vision loss within an otherwise normal visual field. It is not a disease itself but a symptom indicating an underlying issue affecting the visual pathway, including the retina, optic nerve, or visual cortex. Recognizing a scotoma is important because its characteristics provide clues about its origin and severity. The visual disturbance may be temporary, such as during a migraine, or permanent, indicating structural damage to the eye or nervous system.
Defining the Visual Phenomenon
The subjective experience of a scotoma varies widely, but it generally involves seeing a blind or dark spot, a blurry patch, or a gap in sight. A positive scotoma is perceived as flickering, shimmering, or sparkling light, often appearing as zigzagging lines or a luminous arc. The patient actively sees this visual disturbance.
In contrast, a negative scotoma is an area where vision is absent or deficient, but the patient may not consciously notice the gap. This invisibility occurs because the brain uses “perceptual completion” or “filling-in,” masking the blind spot with surrounding visual information. The brain essentially guesses what should be in the missing area. This mechanism allows negative scotomas, such as those caused by early glaucoma, to progress significantly before the patient notices the vision loss.
Categorizing Types of Scotomas
Clinical practice classifies scotomas based on perception and location within the visual field. Positive scotomas, such as the aura of a migraine, are transient events generated by abnormal electrical activity in the brain’s visual cortex. Negative scotomas are typically associated with structural damage to the retina or optic nerve and represent a true, often permanent, loss of visual function.
Scotomas are also categorized by their position relative to the central point of fixation.
Location-Based Classification
A central scotoma is located directly in the center of the visual field. Because it affects the macula, it is disruptive to activities like reading and recognizing faces.
Peripheral scotomas are located toward the edges of the visual field. They may go unnoticed longer but compromise side vision, affecting mobility and driving.
A paracentral scotoma is positioned close to the center, often within ten degrees of fixation, and is frequently seen in conditions affecting the nerve fibers near the optic disc.
The duration of the visual disturbance is another factor. Transient scotomas, like those from a migraine, resolve quickly, typically within minutes to an hour. Permanent scotomas result from irreversible damage to the neural structures of the eye or brain and require ongoing management.
Underlying Causes and Related Conditions
Scotomas arise from conditions that damage or disrupt the function of the retina, optic nerve, or visual processing centers in the brain.
Common Causes of Scotomas
The most common cause of a transient positive scotoma is a migraine with aura. This visual aura, or scintillating scotoma, is caused by a wave of abnormal electrical activity (cortical spreading depression) moving across the visual cortex. The effect is an expanding area of flickering, jagged lines that temporarily impairs vision in both eyes.
Glaucoma is a significant cause of permanent negative scotomas, resulting from increased intraocular pressure damaging the optic nerve head. This damage typically results in arcuate scotomas, which are arc-shaped defects starting near the natural blind spot and curving into the peripheral vision. Since these defects begin peripherally, they often remain unnoticed until the disease is advanced.
Age-related macular degeneration (AMD) is a leading cause of permanent central scotomas. AMD targets the macula, the central area of the retina responsible for sharp vision. Damage here leads to a blind spot in the middle of the visual field, severely impacting reading or driving.
Diabetic retinopathy, caused by high blood sugar damaging retinal blood vessels, can lead to paracentral or central scotomas if the macula is affected by swelling.
Other neurological conditions affect the optic nerve. Optic neuritis, often associated with multiple sclerosis, involves inflammation of the optic nerve, causing a centrocecal scotoma that encompasses both the center of vision and the natural blind spot. Acute vascular events like a stroke or transient ischemic attack (TIA) can cause sudden-onset scotomas by interrupting blood flow to the visual pathways.
Diagnosis and Clinical Management
Diagnosis begins with a comprehensive eye examination and a detailed history of symptoms.
Diagnostic Tools
A simple tool used to detect and monitor central scotomas is the Amsler grid. This test involves a square grid pattern with a central dot; the patient covers one eye and reports any missing, blurry, or distorted areas. It is often used for at-home monitoring for people with macular degeneration.
More precise diagnosis relies on automated visual field testing, also known as perimetry. This procedure uses specialized equipment to map the entire scope of vision by presenting lights of varying brightness in different locations. The results create a detailed map that identifies the exact size, shape, and location of any scotomas, helping the physician pinpoint the underlying cause.
Management focuses on treating the underlying disease process. Scotomas caused by glaucoma require treatment to lower intraocular pressure and prevent further optic nerve damage. Migraine management involves identifying and avoiding triggers or using prophylactic medications to reduce event frequency. For permanent scotomas, such as those from advanced macular damage, treatment shifts toward maximizing remaining vision through visual rehabilitation and the use of low-vision aids.