Tropical ataxic neuropathy (TAN, also known as Strachan-Scott Syndrome and prisoners of war neuropathy) is a disease or category of diseases that commonly causes disability and increases mortality. The causes of TAN are not understood; there is no generally accepted treatment, and the reported outcomes are inconsistent. The disease affects poor tropical populations; there are no good statistics on how many people are affected worldwide, but in some populations, more than a quarter of people are affected. Malnutrition may play a role.

Classification and signs and symptoms

TAN is one of many

tropical myeloneuropathies Tropical neuropathy is a class of illnesses with similar signs and symptoms, including konzo, tropical spastic paraparesis (TSP), and tropical ataxic neuropathy Tropical ataxic neuropathy (TAN, also known as Strachan-Scott Syndrome and prisoners ...

. It was first described in Jamaica in 1897, by postmortems of 510 cases; in 1959, it was dubbed "tropical ataxic neuropathy". The diagnostic criteria were defined in 1968. TAN is defined by "bilateral optic atrophy, bilateral sensory neural deafness, predominant

posterior column Posterior may refer to: * Posterior (anatomy), the end of an organism opposite to its head ** Buttocks, as a euphemism * Posterior horn (disambiguation) * Posterior probability, the conditional probability that is assigned when the relevant evidenc ...

involvement, and

pyramidal tract The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (''cort ...

myelopathy, with

ataxic Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...

polyneuropathy Polyneuropathy ( poly- + neuro- + -pathy) is damage or disease affecting peripheral nerves (peripheral neuropathy) in roughly the same areas on both sides of the body, featuring weakness, numbness, and burning pain. It usually begins in the hand ...

". The classification of TAN is still not settled, and researchers disagree about it. There are thought to be two neurological syndromes lumped together as TAN. One affects adolescents, appears with retrobulbar optic neuropathy and evidence of malnutrition, and improves with better nutrition. Half of these adolescents are seen to have spinal

ataxia Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...

. The other affects middle-aged and elderly people. They suffer sensory polyneuropathy, including weakness and paresthesic sensations.

Paresthesia Paresthesia is an abnormal sensation of the skin (tingling, pricking, chilling, burning, numbness) with no apparent physical cause. Paresthesia may be transient or chronic, and may have any of dozens of possible underlying causes. Paresthesias ar ...

s include sensations of numbness, heat, cold, tightness, crawling motion, tingling,

pins and needles ''Pins and Needles'' (1937) is a musical revue with a book by Arthur Arent, Marc Blitzstein, Emmanuel Eisenberg, Charles Friedman, David Gregory, Joseph Schrank, Arnold B. Horwitt, John Latouche, and Harold Rome, and music and lyrics by Rome. ...

, and a feeling of walking on cotton or pebbles. Weaknesses show as gait

(lack of co-ordination). Affected people also suffer

optic atrophy Optic neuropathy is damage to the optic nerve from any cause. The optic nerve is a bundle of millions of fibers in the retina that sends visual signals to the brain. Damage and death of these nerve cells, or neurons, leads to characteristic featu ...

and sensory neural deafness, on both sides of the body. There is neurological damage to the

of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

. For these older patients, evidence of malnutrition is rarer, and improving nutrition does not improve symptoms. Most of those with the older-onset form have symptoms in their legs, but a third to a half also have arm symptoms. Symptoms tend to worsen during the

rainy season The rainy season is the time of year when most of a region's average annual rainfall occurs. Rainy Season may also refer to: * ''Rainy Season'' (short story), a 1989 short horror story by Stephen King * "Rainy Season", a 2018 song by Monni * ''T ...

(see

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...

harmattan The Harmattan is a season in West Africa that occurs between the end of November and the middle of March. It is characterized by the dry and dusty northeasterly trade wind, of the same name, which blows from the Sahara over West Africa into the ...

), and are often worse at night. Symptoms associated with the lower cranial nerve are rarer; most patients do not show them. These symptoms include

dysarthria Dysarthria is a speech sound disorder resulting from neurological injury of the motor component of the motor–speech system and is characterized by poor articulation of phonemes. In other words, it is a condition in which problems effectively ...

(difficulty articulating words),

dysphagia Dysphagia is difficulty in swallowing. Although classified under "symptoms and signs" in ICD-10, in some contexts it is classified as a disease#Terminology, condition in its own right. It may be a sensation that suggests difficulty in the passag ...

(difficulty

swallowing Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. Swallowing i ...

shortness of breath Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing disc ...

, and

dysphonia A hoarse voice, also known as dysphonia or hoarseness, is when the voice involuntarily sounds breathy, raspy, or strained, or is softer in volume or lower in pitch. A hoarse voice, can be associated with a feeling of unease or scratchiness in the ...

(difficulty speaking); dysphonia is more common in women, and shows as hypophonia (lack of co-ordination in the vocal cords) and an inability to shout.

Treatment

Nutritional improvements; supplying nutritional yeast, teaching improved cassava preparation, and ceasing cassava consumption.

Outlook

Unclear, with contradictory reports from different studies.

Research directions

A 2016 review listed twenty-one open research questions. The value of international co-operation on TAN research has also been highlighted.

Epidemiology

TAN has only been described as developing in Africa, South-east Asia, and the Caribbean (if

Guiana The Guianas, sometimes called by the Spanish loan-word ''Guayanas'' (''Las Guayanas''), is a region in north-eastern South America which includes the following three territories: * French Guiana, an overseas department and region of France * ...

be included). It does not seem to occur in

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

countries. People affected by TAN tend to be poor and live in rural areas. In some areas, more than a quarter of the population are affected. In India and Africa, more women are affected; in Africa, the elderly are most likely to suffer, and in India, people in their thirties. Historical data suggests that, in the 1960s, TAN in Africa was most common in people in their 30s and 40s. While the areas affected roughly correspond to the areas in which

cassava ''Manihot esculenta'', common name, commonly called cassava (), manioc, or yuca (among numerous regional names), is a woody shrub of the spurge family, Euphorbiaceae, native to South America. Although a perennial plant, cassava is extensively ...

is grown, some people in non-cassava-growing populations get TAN, and some cassava-growing populations do not get TAN. It is possible that there are several diseases being categorized as TAN. It has been estimated that 5% of surviving World War II

prisoners of war A prisoner of war (POW) is a person who is held Captivity, captive by a belligerent power during or immediately after an armed conflict. The earliest recorded usage of the phrase "prisoner of war" dates back to 1610. Belligerents hold priso ...

held in the Far East acquired TAN; while they were held for 3.5 years or less, the TAN symptoms persisted chronically after they returned to temperate climates.

Other animals

The behaviour and neurology of malnourished and cassava-fed rats has been compared to that of humans with TAN.

References

{{reflist Neurological disorders Tropical diseases