Toxic and Nutritional Optic Neuropathy
Encyclopedia
Toxic and Nutritional Optic Neuropathy is a group of medical disorders defined by visual impairment due to optic nerve
damage secondary to a toxic substance and/or nutritional deficiency. The causes of these disorders are various, but they are linked by shared signs and symptoms, which this article will describe. In several of these disorders, both toxic and nutritional factors play a role, acting synergistically.
(wood alcohol), ethylene glycol
(automotive antifreeze), disulfiram
(used to treat chronic alcoholism), halogenated hydroquinolones (amebicidal medications), ethambutol
and isoniazid
(tuberculosis treatment
), and antibiotics such as linezolid
and chloramphenicol
. Tobacco
is also a major cause of toxic optic neuropathy.
(vitamin B1) and cyanocobalamin
(vitamin B12). Deficiency of pyridoxine
(vitamin B6), niacin
(vitamin B3), riboflavin
(vitamin B2), and/or folic acid
also seems to play a role. Those individuals who abuse alcohol and tobacco are at greater risk because they tend to be malnourished. Those with pernicious anemia
are also at risk due to an impaired ability to absorb vitamin B12 from the intestinal tract.
and Kjer's optic neuropathy.
may start with a blur or haze at the point of fixation, followed by a progressive decline. The degree of vision loss can extend to total blindness, but a loss beyond 20/400 is rare, except in the case of methanol ingestion. Peripheral vision is usually spared since the pattern of loss typically involves a central or cecocentral scotoma
, a visual field defect at or surrounding the point of fixation. This pattern can be revealed via visual field testing.
Upon examination, the pupils usually demonstrate a normal response to light and near stimulation. In those who are practically blind, the pupils will be dilated with a weak or absent response to light. The optic disc
may appear normal, swollen, or hyperemic in early stages. With hyperemia, disc hemorrhages may also be present. Continued damage to the optic nerve results in the development of optic atrophy, classically seen as temporal pallor of the optic disc.
and careful eye examination
. If the medical history clearly points to a cause, neuroimaging
to rule out a compressive or infiltrative lesion is optional. However, if the medical history is atypical or does not clearly point to a cause, neuroimaging is required to rule out other causes and confirm the diagnosis. In most cases of suspected toxic or nutritional optic neuropathy that require neuroimaging, an MRI scan is obtained. Further testing, guided by the medical history and physical examination, can be performed to elucidate a specific toxin or nutritional deficiency as a cause of the optic neuropathy. Examples include blood testing for methanol levels or vitamin B12 levels.
In both toxic and nutritional neuropathy, vision generally recovers to normal over several days to weeks, though it may take months for full restoration and there is always the risk of permanent vision loss. Visual acuity usually recovers before color vision.
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...
damage secondary to a toxic substance and/or nutritional deficiency. The causes of these disorders are various, but they are linked by shared signs and symptoms, which this article will describe. In several of these disorders, both toxic and nutritional factors play a role, acting synergistically.
Epidemiology
In industrialized nations, toxic and nutritional optic neuropathy is relatively uncommon and is primarily associated with specific medications, occupational exposures, or tobacco and alcohol abuse. However, in developing nations, nutritional optic neuropathy is much more common, especially in regions afflicted by famine. Both genders and all races are equally affected, and all ages are susceptible.Toxic optic neuropathy
There are several causes of toxic optic neuropathy. Among these are: ingestion of methanolMethanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
(wood alcohol), ethylene glycol
Ethylene glycol
Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid...
(automotive antifreeze), disulfiram
Disulfiram
Disulfiram is a drug discovered in the 1920s and used to support the treatment of chronic alcoholism by producing an acute sensitivity to alcohol. Trade names for disulfiram in different countries are Antabuse and Antabus manufactured by Odyssey Pharmaceuticals...
(used to treat chronic alcoholism), halogenated hydroquinolones (amebicidal medications), 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 isoniazid
Isoniazid
Isoniazid , also known as isonicotinylhydrazine , is an organic compound that is the first-line antituberculosis medication in prevention and treatment. It was first discovered in 1912, and later in 1951 it was found to be effective against tuberculosis by inhibiting its mycolic acid...
(tuberculosis treatment
Tuberculosis treatment
Tuberculosis treatment refers to the medical treatment of the infectious disease tuberculosis .The standard "short" course treatment for TB is isoniazid, rifampicin , pyrazinamide, and ethambutol for two months, then isoniazid and rifampicin alone for a further four months...
), and antibiotics such as linezolid
Linezolid
Linezolid is a synthetic antibiotic used for the treatment of serious infections caused by Gram-positive bacteria that are resistant to several other antibiotics...
and chloramphenicol
Chloramphenicol
Chloramphenicol is a bacteriostatic antimicrobial that became available in 1949. It is considered a prototypical broad-spectrum antibiotic, alongside the tetracyclines, and as it is both cheap and easy to manufacture it is frequently found as a drug of choice in the third world.Chloramphenicol is...
. Tobacco
Tobacco
Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...
is also a major cause of toxic optic neuropathy.
Nutritional optic neuropathy
The predominant cause of nutritional optic neuropathy is thought to be deficiency of B-complex vitamins, particularly thiamineThiamine
Thiamine or thiamin or vitamin B1 , named as the "thio-vitamine" is a water-soluble vitamin of the B complex. First named aneurin for the detrimental neurological effects if not present in the diet, it was eventually assigned the generic descriptor name vitamin B1. Its phosphate derivatives are...
(vitamin B1) and cyanocobalamin
Cyanocobalamin
Cyanocobalamin is an especially common vitamer of the vitamin B12 family. It is the most famous vitamer of the family, because it is, in chemical terms, the most air-stable...
(vitamin B12). Deficiency of pyridoxine
Pyridoxine
Pyridoxine is one of the compounds that can be called vitamin B6, along with pyridoxal and pyridoxamine. It differs from pyridoxamine by the substituent at the '4' position. It is often used as 'pyridoxine hydrochloride'.-Chemistry:...
(vitamin B6), niacin
Niacin
"Niacin" redirects here. For the neo-fusion band, see Niacin .Niacin is an organic compound with the formula and, depending on the definition used, one of the forty to eighty essential human nutrients.Niacin is one of five vitamins associated with a pandemic deficiency disease: niacin deficiency...
(vitamin B3), riboflavin
Riboflavin
Riboflavin, also known as vitamin B2 or additive E101, is an easily absorbed micronutrient with a key role in maintaining health in humans and animals. It is the central component of the cofactors FAD and FMN, and is therefore required by all flavoproteins. As such, vitamin B2 is required for a...
(vitamin B2), and/or folic acid
Folic acid
Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...
also seems to play a role. Those individuals who abuse alcohol and tobacco are at greater risk because they tend to be malnourished. Those with pernicious anemia
Pernicious anemia
Pernicious anemia is one of many types of the larger family of megaloblastic anemias...
are also at risk due to an impaired ability to absorb vitamin B12 from the intestinal tract.
Pathophysiology
All of the above risk factors impact mitochondrial oxidative phosphorylation. Thus, the toxic and nutritional optic neuropathies are actually acquired mitochondrial optic neuropathies. The clinical picture that they produce is akin to that of the congenital mitochondrial optic neuropathies, e.g., Leber's hereditary optic neuropathyLeber's hereditary optic neuropathy
Leber’s hereditary optic neuropathy or Leber optic atrophy is a mitochondrially inherited degeneration of retinal ganglion cells and their axons that leads to an acute or subacute loss of central vision; this affects predominantly young adult males...
and Kjer's optic neuropathy.
Signs and symptoms
Vision loss in toxic and nutritional optic neuropathy is bilateral, symmetric, painless, gradual, and progressive. Dyschromatopsia, a change in color vision, is often the first symptom. Some patients notice that certain colors, particularly red, are less bright or vivid; others have a general loss of color perception. Loss of visual acuityVisual 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....
may start with a blur or haze at the point of fixation, followed by a progressive decline. The degree of vision loss can extend to total blindness, but a loss beyond 20/400 is rare, except in the case of methanol ingestion. Peripheral vision is usually spared since the pattern of loss typically involves a central or cecocentral scotoma
Scotoma
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 mammalian eye has a scotoma in its field of vision, usually...
, a visual field defect at or surrounding the point of fixation. This pattern can be revealed via visual field testing.
Upon examination, the pupils usually demonstrate a normal response to light and near stimulation. In those who are practically blind, the pupils will be dilated with a weak or absent response to light. 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"...
may appear normal, swollen, or hyperemic in early stages. With hyperemia, disc hemorrhages may also be present. Continued damage to the optic nerve results in the development of optic atrophy, classically seen as temporal pallor of the optic disc.
Diagnosis
The diagnosis of toxic or nutritional optic neuropathy is usually established by a detailed medical historyMedical history
The medical history or anamnesis of a patient is information gained by a physician by asking specific questions, either of the patient or of other people who know the person and can give suitable information , with the aim of obtaining information useful in formulating a diagnosis and providing...
and careful eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....
. If the medical history clearly points to a cause, neuroimaging
Neuroimaging
Neuroimaging includes the use of various techniques to either directly or indirectly image the structure, function/pharmacology of the brain...
to rule out a compressive or infiltrative lesion is optional. However, if the medical history is atypical or does not clearly point to a cause, neuroimaging is required to rule out other causes and confirm the diagnosis. In most cases of suspected toxic or nutritional optic neuropathy that require neuroimaging, an MRI scan is obtained. Further testing, guided by the medical history and physical examination, can be performed to elucidate a specific toxin or nutritional deficiency as a cause of the optic neuropathy. Examples include blood testing for methanol levels or vitamin B12 levels.
Treatment and prognosis
Treatment of toxic and nutritional optic neuropathy is dictated by the cause of the disorder.- Toxic optic neuropathy is treated by identification and removal of the offending agent. Depending upon the individual affected, the nature of the agent, total exposure prior to removal, and degree of vision loss at the time of diagnosis, the prognosis is variable.
- Nutritional optic neuropathy is treated with improved nutrition. A well-balanced diet with plenty of protein and green leafy vegetables, vitamin supplementation (thiamine, vitamin B12, folic acid, multivitamins), and reduction of smoking and/or drinking are the mainstay of treatment. Again, prognosis is variable and dependent upon the affected individual, treatment compliance, and degree of vision loss at diagnosis.
In both toxic and nutritional neuropathy, vision generally recovers to normal over several days to weeks, though it may take months for full restoration and there is always the risk of permanent vision loss. Visual acuity usually recovers before color vision.