Scotoma

Scotoma
Other names	Scotomas, scotomata
Animation of ascintillating scotomathat 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

Ascotomais an area of partial alteration in thefield of visionconsisting of a partially diminished or entirelydegeneratedvisual acuitythat is surrounded by a field of normal – or relatively well-preserved –vision.

Every normalmammalianeye has a scotoma in its field of vision, usually termed itsblind spot.This is a location with nophotoreceptor cells,where theretinal ganglion cellaxonsthat compose theoptic nerveexit 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.

The presence of the blind spot scotoma can be demonstrated subjectively by covering one eye, carefully holdingfixationwith the open eye, and placing an object (such as one's thumb) in thelateralandhorizontalvisual field, about 15 degrees from fixation (see theblind spotarticle). The size of themonocularscotoma 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 termscotomais also usedmetaphoricallyin several fields. The common theme of all the figurativesensesis of a gap not in visual function but in the mind'sperception,cognition,orworld view.The term is fromGreekσκότος/skótos,"darkness."

Signs and symptoms

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 nerveand even the visual cortex.A pathological scotoma may involve any part of thevisual fieldand 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 ormacularvision will produce a severe visualdisability,whereas a large scotoma in the moreperipheralpart of a visual field may go unnoticed by the bearer because of the normal reducedoptical resolutionin the peripheral visual field.

Causes

Common causes of scotomas includedemyelinatingdisease such asmultiple sclerosis(retrobulbar neuritis), damage to nerve fiber layer in the retina (seen ascotton wool spots) due to hypertension, toxic substances such asmethyl alcohol,ethambutolandquinine,nutritional deficiencies,vascularblockages either in the retina or in the optic nerve,strokeor other brain injury, andmacular degeneration,often associated with aging.Scintillating scotomais a common visualaurainmigraine.Less common, but important because they are sometimes reversible or curable bysurgery,are scotomata due totumorssuch as those arising from thepituitary gland,which may compress the optic nerve or interfere with its blood supply.

Rarely, scotomata arebilateral.One important variety of bilateral scotoma may occur when a pituitarytumourbegins to compress theoptic chiasm(as distinct from a single optic nerve) and produces abitemporal paracentral scotoma,and later, when the tumor enlarges, the scotomas 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.

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 pressurethat occurs inmalignant hypertension.

The scotoma is also caused by theaminoglycosideantibiotics mainly bystreptomycin.

Terminology

Beyond its literalsenseconcerning the visual system, the termscotomais also usedmetaphoricallyin several fields, includingneurology,neuropsychology,psychology,philosophy,andpolitics.The common theme of all the figurativesensesis 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 systemand the mind, via the chain of links fromsensoryinput, 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 cognitiveschemathat lacks any provision for it.

At the mostconcretelevel, there isneuropsychological scotoma.One example is thehemispatial neglectthat is sometimes experienced by people who have hadstrokes.Another type is the phenomenon of reverse or negative phantom limb, in whichnerveinjuries 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 schemain 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,considered it a form of spatial neglect in thebody schemaanalogous to hemispatial neglect in that the mind could notconceive of the leg as selfbecause 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 acadaverousone, which was part of the reason for thedysphoriabut not the sole explanation. Even for people who intellectually understood that the leg or hand was supposed to be theirs simply could not believe itemotionallyand 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 bothuncannyandineffable(as Sacks self-reported and found in others). 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 schemaresists 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.

At a higher level of abstraction are what have been calledpsychological scotomas,in which a person's self-perception of his or her ownpersonalityis judged by others to have a gap in perceptive ability. Thus, inpsychology,scotomacan refer to a person's inability to perceivepersonality traitsin 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 viewthat 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 orinstitutionalstructure.

See also

• Floater
• Scotomization

Detection

• Amsler grid
• Horizontal eccentricity
• Visual field test

Types

• Binasal hemianopsia
• Bitemporal hemianopsia
• Cortical spreading depression
• Scintillating scotoma