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. TAN is one of many

tropical myeloneuropathies Tropical neuropathy is a class of illnesses with similar signs and symptoms, including konzo, tropical spastic paraparesis Tropical spastic paraparesis (TSP), is a medical condition that causes weakness, muscle spasms, and sensory disturbance by h ...

. It was first described in Jamaica in 1897, by postmortems of 510 cases; in 1959, it was dubbed "tropical ataxic neuropathy".

Epidemiology

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

Guiana The Guianas, also spelled Guyanas or Guayanas, are a geographical region in north-eastern South America. Strictly, the term refers to the three Guianas: Guyana, Suriname, and French Guiana, formerly British, Dutch, and French Guiana respectiv ...

be included). It does not seem to occur in

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

countries. People affected by TAN tend to be poor and live in rural areas. In some areas, more than a quarter of the population is 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 suggest 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, from Brazil, Paraguay and parts of the Andes. Although ...

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 held 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 prisoners of war for a ...

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.

Causes and symptoms

The causes of TAN are unclear; there may be several separate causes of similar symptoms. 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 (from Greek α- negative prefix+ -τάξις rder= "lack of order") is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in e ...

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

Paresthesia Paresthesia is a sensation of the skin that may feel like numbness (''hypoesthesia''), tingling, pricking, chilling, or burning. It can be temporary or Chronic condition, chronic and has many possible underlying causes. Paresthesia is usually p ...

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

pins and needles Paresthesia is a sensation of the skin that may feel like numbness (''hypoesthesia''), tingling, pricking, chilling, or burning. It can be temporary or chronic and has many possible underlying causes. Paresthesia is usually painless and can oc ...

, 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 Nerve injury is an injury to a nerve. There is no single classification system that can describe all the many variations of nerve injuries. In 1941, Herbert Seddon introduced a classification of nerve injuries based on three main types of nerve ...

to the

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 ...

of the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

. 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 * '' ...

(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 Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

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 th ...

), 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. It is a condition in which problems effectively occur with the ...

(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 condition in its own right. It may be a sensation that suggests difficulty in the passage of solids or l ...

(difficulty

swallowing Swallowing, also called deglutition or inglutition in scientific and medical contexts, is a physical process of an animal's digestive tract (e.g. that of a human body) that allows for an ingested substance (typically food) to pass from the mou ...

shortness of breath Shortness of breath (SOB), 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 discomfort that con ...

, 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 Hypophonia is soft speech, especially resulting from a lack of coordination in the vocal musculature. This condition is a common presentation in Parkinson's disease. This condition is generally treated with voice training programs, use of shorter s ...

(lack of co-ordination in the vocal cords) and an inability to shout.

Diagnosis

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 anterior ** Buttocks, as a euphemism * Posterior horn (disambiguation) * Posterior probability The posterior probability is a type of conditional probability that ...

involvement, and

myelopathy, with ataxic

polyneuropathy Polyneuropathy () 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 hands and feet and may prog ...

". The classification of TAN is still not settled, and researchers disagree about it.

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.

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