A scotoma is a spot which causes partial alteration in one’s field of vision. It results in partially diminshed visual acuity surrounded by a field of normal vision. It is usually found normally in every mammalian eye. Scotoma is also known as blind spots in mammals. Blind spots do not have any photo receptors, where the retinal ganglion cell axons exit the retina. It is known as optic disc.
Visual signals absent in the blind spot of one eye are provided from the opposite visual cortex. This is even possible when the other eye is closed. The absence of visual imagery from the blindspot does not interfere with the consciousness when one eye is closed. This is due to the differences found in the visual field locations of the optic discs in the two eyes.
To demonstrate the presence of the scotoma,
- cover one eye
- hold a fixation with the open eye carefully
- place an object (like your thumb) in the horizontal and lateral visual field (about 15 degrees from fixation)
Monocular scotoma are large in size. For example, 5 × 7 degrees of visual angle.
How Pathological Scotoma Are Presented?
Pathological scotomata may occur due to wide range of disesases. They may either affect the optic nerve itself or the retina (mostly the sensitive part, the macula). It might involve any part of the visual field. It can be of any size or shape.
A scotoma may enlarge and include the normal blind spot. A small scotoma will produce a severe visual impairment by affecting the macular or central vision.
Causes Of Scotomata
- Toxic substances like quinine, ethambutol and methyl alcohol
- Vascular blockages found in the optic nerve or in the retina
- Nutritional deficiencies
- Demyelinating disease such as multiple sclerosis
