Scotoma
Encyclopedia
A scotoma is an area of partial alteration in one's field of vision consisting of a partially diminished or entirely degenerated
visual acuity
which is surrounded by a field of normal - or relatively well-preserved - vision
.
Every normal mammal
ian eye has a scotoma in its field of vision, usually termed its blind spot
. This is a location with no photoreceptor cells, where the retina
l ganglion cell
axons that comprise the optic nerve
exit the retina. This location is called the optic disc
. When both eyes are open, visual signals that are absent in the blind spot of one eye are provided from the opposite visual cortex for the other eye, even when the other eye is closed. The absence of visual imagery from the blindspot does not intrude into consciousness
with one eye closed, because the corresponding visual field
locations of the optic discs in the two eyes differ.
The presence of the scotoma can be demonstrated subjectively by covering one eye, carefully holding fixation
with the open eye, and placing an object (such as your thumb) in the lateral and horizontal
visual field, about 15 degrees from fixation (see the blind spot
article). The size of the monocular
scotoma is surprisingly great: 5×7 degrees of visual angle
.
The term scotoma is also used metaphor
ically in psychology
to refer to an individual's inability to perceive personality traits in themselves that are obvious to others.
Scotoma is also a symptom of retinal damage from exposure to high powered lasers
scotomata may be due to a wide range of disease processes, affecting either the retina
(in particular its most sensitive portion, the macula
) or the optic nerve
itself. A pathological scotoma may involve any part of the visual 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 or macula
r vision will produce a severe visual handicap
, whereas a large scotoma in the more peripheral
part of a visual field may go unnoticed by the bearer because of the normal reduced optical resolution
in the peripheral visual field.
(retrobulbar neuritis), damage to nerve fiber layer in retina (seen as cotton wool spots) due to hypertension, toxic substances such as methyl alcohol, ethambutol
and quinine
, nutritional deficiencies, and vascular
blockages either in the retina or in the optic nerve. Scintillating scotoma
is a common visual aura
in migraine
. Less common, but important because sometimes reversible or curable by surgery
, are scotomata due to tumor
s such as those arising from the pituitary gland
, which may compress the optic nerve or interfere with its blood supply.
Rarely, scotomata are bilateral. One important variety of bilateral scotoma may occur when a pituitary tumour begins to compress the optic chiasm
(as distinct from a single optic nerve) and produces a bitemporal paracentral scotoma and later when the tumor enlarges, the scotomas extend out to the periphery to cause the characteristic Bitemporal hemianopsia
. This type of visual field defect tends to be obvious to the person experiencing it, but often evades early objective diagnosis
, as it is more difficult to detect by cursory clinical examination than the classical or text-book bi-temporal peripheral hemianopia and may even elude sophisticated electronic modes of visual field assessment.
In a pregnant woman, scotomata can present as a symptom of severe preeclampsia, a form of pregnancy-induced hypertension
. Similarly, scotomata may develop as a result of the increased intracranial pressure
that occurs in malignant hypertension
.
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...
visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
which is surrounded by a field of normal - or relatively well-preserved - vision
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
.
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian eye has a scotoma in its field of vision, usually termed its blind spot
Blind spot (vision)
A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
. This is a location with no photoreceptor cells, where the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
l ganglion cell
Ganglion cell
A retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
axons that comprise the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
exit the retina. This location is called the optic disc
Optic disc
The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...
. When both eyes are open, visual signals that are absent in the blind spot of one eye are provided from the opposite visual cortex for the other eye, even when the other eye is closed. The absence of visual imagery from the blindspot does not intrude into consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...
with one eye closed, because the corresponding visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
locations of the optic discs in the two eyes differ.
The presence of the scotoma can be demonstrated subjectively by covering one eye, carefully holding fixation
Fixation (visual)
Fixation or visual fixation is the maintaining of the visual gaze on a single location. Humans typically alternate saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appear to have developed for hunting prey...
with the open eye, and placing an object (such as your thumb) in the lateral and horizontal
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
visual field, about 15 degrees from fixation (see the blind spot
Blind spot (vision)
A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
article). The size of the monocular
Monocular
A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and sometimes prisms; the use of prisms results in a lightweight telescope. Volume and weight are less than half those of binoculars of similar optical...
scotoma is surprisingly great: 5×7 degrees of visual angle
Visual angle
The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size....
.
The term scotoma is also used metaphor
Metaphor
A metaphor is a literary figure of speech that uses an image, story or tangible thing to represent a less tangible thing or some intangible quality or idea; e.g., "Her eyes were glistening jewels." Metaphor may also be used for any rhetorical figures of speech that achieve their effects via...
ically in psychology
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...
to refer to an individual's inability to perceive personality traits in themselves that are obvious to others.
Scotoma is also a symptom of retinal damage from exposure to high powered lasers
Presentation of pathological scotoma
Symptom-producing or pathologicalPathology
Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....
scotomata may be due to a wide range of disease processes, affecting either the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
(in particular its most sensitive portion, the macula
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
) or the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
itself. A pathological scotoma may involve any part of the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
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 or macula
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
r vision will produce a severe visual handicap
Disability
A disability may be physical, cognitive, mental, sensory, emotional, developmental or some combination of these.Many people would rather be referred to as a person with a disability instead of handicapped...
, whereas a large scotoma in the more peripheral
Peripheral vision
Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
part of a visual field may go unnoticed by the bearer because of the normal reduced optical resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
in the peripheral visual field.
Causes
Common causes of scotomata include demyelinating disease such as multiple sclerosisMultiple sclerosis
Multiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
(retrobulbar neuritis), damage to nerve fiber layer in retina (seen as cotton wool spots) due to hypertension, toxic substances such as methyl alcohol, ethambutol
Ethambutol
Ethambutol is a bacteriostatic antimycobacterial drug prescribed to treat tuberculosis. It is usually given in combination with other tuberculosis drugs, such as isoniazid, rifampicin and pyrazinamide....
and quinine
Quinine
Quinine is a natural white crystalline alkaloid having antipyretic , antimalarial, analgesic , anti-inflammatory properties and a bitter taste. It is a stereoisomer of quinidine which, unlike quinine, is an anti-arrhythmic...
, nutritional deficiencies, and vascular
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
blockages either in the retina or in the optic nerve. Scintillating scotoma
Scintillating scotoma
Scintillating scotoma is the most common visual aura preceding migraine and was first described by 19th century physician Hubert Airy . It is often confused with ocular migraine which originates in the eyeball or socket.-Presentation:...
is a common visual aura
Aura (symptom)
An aura is a perceptual disturbance experienced by some migraine sufferers before a migraine headache, and the telltale sensation experienced by some people with epilepsy before a seizure. It often manifests as the perception of a strange light, an unpleasant smell or confusing thoughts or...
in migraine
Migraine
Migraine is a chronic neurological disorder characterized by moderate to severe headaches, and nausea...
. Less common, but important because sometimes reversible or curable by surgery
Surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...
, are scotomata due to tumor
Tumor
A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...
s such as those arising from the pituitary gland
Pituitary gland
In vertebrate anatomy the pituitary gland, or hypophysis, is an endocrine gland about the size of a pea and weighing 0.5 g , in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain, and rests in a small, bony cavity covered by a dural fold...
, which may compress the optic nerve or interfere with its blood supply.
Rarely, scotomata are bilateral. One important variety of bilateral scotoma may occur when a pituitary tumour begins to compress the optic chiasm
Optic chiasm
The optic chiasm or optic chiasma is the part of the brain where the optic nerves partially cross...
(as distinct from a single optic nerve) and produces a bitemporal paracentral scotoma and later when the tumor enlarges, the scotomas extend out to the periphery to cause the characteristic Bitemporal hemianopsia
Bitemporal hemianopsia
Bitemporal hemianopsia is the medical description of a type of partial blindness where vision is missing in the outer half of both the right and left visual field...
. This type of visual field defect tends to be obvious to the person experiencing it, but often evades early objective diagnosis
Medical diagnosis
Medical diagnosis refers both to the process of attempting to determine or identify a possible disease or disorder , and to the opinion reached by this process...
, as it is more difficult to detect by cursory clinical examination than the classical or text-book bi-temporal peripheral hemianopia and may even elude sophisticated electronic modes of visual field assessment.
In a pregnant woman, scotomata can present as a symptom of severe preeclampsia, a form of pregnancy-induced hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
. Similarly, scotomata may develop as a result of the increased intracranial pressure
Intracranial pressure
Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
that occurs in malignant hypertension
Malignant hypertension
Malignant hypertension or hypertensive emergency is severe hypertension with acute impairment of an organ system and the possibility of irreversible organ-damage...
.
Types
- Binasal hemianopsiaBinasal hemianopsiathumb|Paris as seen with full visual fieldsthumb|Binasal hemianopsia is the medical description of a type of partial blindness where vision is missing in the inner half of both the right and left visual field...
- Bitemporal hemianopsiaBitemporal hemianopsiaBitemporal hemianopsia is the medical description of a type of partial blindness where vision is missing in the outer half of both the right and left visual field...
- Blind spotBlind spot (vision)A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
- Scintillating scotomaScintillating scotomaScintillating scotoma is the most common visual aura preceding migraine and was first described by 19th century physician Hubert Airy . It is often confused with ocular migraine which originates in the eyeball or socket.-Presentation:...
- Cortical spreading depressionCortical spreading depressionCortical spreading depression is a wave of electrophysiological hyperactivity followed by a wave of inhibition, usually in the visual cortex.The term is used by neuroscientists to represent at least one of the following cortical processes:...