| Scotoma | |
|---|---|
| Other names | Scotomas, scotomata |
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| Animation of ascintillating scotoma that is almost spiral-shaped, with distortion of shapes but otherwise melting into the background similarly to thephysiological blind spot, as may be caused bycortical spreading depression | |
| Specialty | Ophthalmology |
Ascotoma is an area of partial alteration in thefield of vision consisting of a partially diminished or entirelydegeneratedvisual acuity that is surrounded by a field of normal – or relatively well-preserved –vision.
Every normalmammalian eye has a scotoma in its field of vision, usually termed itsblind spot. This is a location with nophotoreceptor cells, where theretinal ganglion cellaxons that compose theoptic nerve exit the retina. This location is called theoptic disc. There is no direct conscious awareness of visual scotomas. They are simply regions of reduced information within the visual field. Rather than recognizing an incomplete image, patients with scotomas report that things "disappear" on them.[1]
The presence of the blind spot scotoma can be demonstrated subjectively by covering one eye, carefully holdingfixation with the open eye, and placing an object (such as one's thumb) in thelateral andhorizontal visual field, about 15 degrees from fixation. The size of themonocular scotoma is 5×7 degrees ofvisual angle.
A scotoma can be a symptom of damage to any part of the visual system, such as retinal damage from exposure to high-powered lasers,macular degeneration, and brain damage.
The termscotoma is also usedmetaphorically in several fields. The common theme of all the figurativesenses is of a gap not in visual function but in the mind'sperception,cognition, orworld view. The term is from Greek σκότος (skótos) 'darkness'.
Symptom-producing, orpathological, scotomata may be due to a wide range of disease processes, affecting any part of the visual system, including theretina (in particular its most sensitive portion, themacula), theoptic nerve and even the visual cortex.[2] A pathological scotoma may involve any part of thevisual field and may be of any shape or size. A scotoma may include and enlarge the normal blind spot. Even a small scotoma that happens to affect central ormacular vision will produce a severe visualdisability, whereas a large scotoma in the moreperipheral part of a visual field may go unnoticed by the bearer because of the normal reducedoptical resolution in the peripheral visual field.[citation needed]
Common causes of scotomas includedemyelinating disease such asmultiple sclerosis (retrobulbar neuritis), damage to nerve fiber layer in the retina (seen ascotton wool spots[3]) due to hypertension, toxic substances such as methyl alcohol, ethambutol and quinine, nutritional deficiencies, vascular blockages either in the retina or in the optic nerve, stroke or other brain injury, and macular degeneration, often associated with aging. Scotomata can also occur indecompression sickness, where gas bubble formation and associated vascular or neurological effects may produce transient or persistent visual field defects.Scintillating scotoma is a common visualaura inmigraine.[4] Less common, but important because they are sometimes reversible or curable bysurgery, are scotomata due totumors such as those arising from thepituitary gland, which may compress the optic nerve or interfere with its blood supply.[citation needed]
Rarely, scotomata arebilateral. One important variety of bilateral scotoma may occur when a pituitarytumour begins to compress theoptic chiasm (as distinct from a single optic nerve) and produces a bitemporal paracentral scotoma, and later, when the tumor enlarges, the scotomata extend out to the periphery to cause the characteristicbitemporal hemianopsia. This type of visual-field defect tends to be obvious to the person experiencing it but often evades early objectivediagnosis, as it is more difficult to detect by cursory clinical examination than the classical or textbook bitemporal peripheral hemianopia and may even elude sophisticated electronic modes of visual-field assessment.[citation needed]
In a pregnant woman, scotomata can present as a symptom of severepre-eclampsia, a form of pregnancy-inducedhypertension. Similarly, scotomata may develop as a result of the increasedintracranial pressure that occurs inmalignant hypertension.[citation needed]
Aminoglycoside antibiotics, particularlystreptomycin, may also cause a scotoma.[citation needed]
Beyond its literalsense concerning the visual system, the termscotoma is also usedmetaphorically in several fields, includingneurology,neuropsychology,psychology,philosophy, andpolitics. The common theme of all the figurativesenses is of a gap not in visual function but in the mind'sperception,cognition, orworld view. Their concrete connection to the literal sense, however, is by the connection between thenervous system and the mind, via the chain of links fromsensory input, to nerve conduction, to the brain, toperception (the processing and interpreting of that input) via the brain-mind correlation, to psychological function. Thus there is not only (or not necessarily) a visual inability to see an aspect of reality but also (or instead) a mental inability to conceive even the possibility of seeing that aspect, due to a cognitiveschema that lacks any provision for it.[citation needed]
At the mostconcrete level, there isneuropsychological scotoma. One example is thehemispatial neglect that is sometimes experienced by people who have hadstrokes. Another type is the phenomenon of reverse or negative phantom limb, in whichnerve injuries to the limbs, such as trauma in which a limb's nerves are severed but the limb is spared fromamputation, can affect the mind'sbody schema in such a way that an existing limb seems to its owner like it should not exist, and its presence thus seemsuncanny. NeurologistOliver Sacks, who experienced a reverse phantom leg that later resolved,[5] considered it a form of spatial neglect in thebody schema analogous to hemispatial neglect in that the mind could notconceive of the leg as self because it could not conceive that there was any space for the leg to exist in. Sacks and others agreed that the leg thus seemed like someone else's leg, including sometimes acadaverous one, which was part of the reason for thedysphoria but not the sole explanation. Even for people who intellectually understood that the leg or hand was supposed to be theirs simply could not believe itemotionally and could not completely reconcile reality with schema, prompting great unease. Given how hard this is to comprehend for a person who has not experienced it, people recently experiencing it for the first time consider it bothuncanny andineffable (as Sacks self-reported and found in others[5]). Sacks also explored the regular type ofphantom limb (a positive phantom), which does not produce a neuropsychological scotoma but shares with reverse phantoms the trait that thebody schema resists revision despite a person's perfect intellectual awareness and acceptance of the current physical reality (that is, that the amputated limb is gone or that the spared limb is still present). This suggests that aspects of schema in the mind (body schema, world schema) have neurologic bases that cannot be revised by mere intellectual understanding—at least not quickly. Sacks does explore the topic of how people adapt to phantoms over the years and how positive phantom limbs often gradually foreshorten and sometimes disappear; but some remain for the rest of life.[5]
At a higher level of abstraction are what have been calledpsychological scotomas, in which a person's self-perception of his or her ownpersonality is judged by others to have a gap in perceptive ability. Thus, inpsychology,scotoma can refer to a person's inability to perceivepersonality traits in themselves that are obvious to others. And at the highest abstraction level are what have been calledintellectual scotomas, in which a person cannot perceive distortions in theirworld view that are obvious to others. Thus, in philosophy or politics, a person's thoughts or beliefs might be shaped by an inability to appreciate aspects of social interaction orinstitutional structure.
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