|Classification and external resources|
A boy affected by konzo displaying the typical gait. The upper motor neuron is the suspected neurodamage site.
Konzo  is an epidemic paralytic disease occurring in outbreaks in remote rural areas of low income African countries. The people of these regions have been associated with several weeks of almost exclusive consumption of insufficiently processed "bitter" (cyanide yielding) cassava (Manihot esculenta) — an annual crop native to South America, but widely cultivated in tropical regions worldwide. African farmers often prefer the bitter cassava varieties because they deter pests, animals and thieves. The more toxic varieties are a fall back resource (a "food security crop") in times of famine in some places. Konzo was first described by Giovanni Trolli in 1938 who compiled the observations from eight doctors working in the Kwango area of the Belgian Congo (now Democratic Republic of the Congo).
Konzo has mainly been reported in outbreaks among women and children in remote rural populations in East and Central Africa such as DR Congo, Mozambique (where it is known as mantakassa), Tanzania and Central African Republic.
The first reported outbreak occurred in Bandundu Province in present day DR Congo in 1936-1937 and the second in Nampula Province of Northern Mozambique in 1981. Each of these outbreaks numbered more than 1000 cases. Familial clustering is common. Outbreaks typically occur in the dry season in households living in absolute poverty that have sustained themselves for weeks or months on insufficiently processed bitter cassava. Both smaller outbreaks and sporadic cases have been reported from all the countries above.
The onset of paralysis (spastic paraparesis) is sudden and symmetrical and affects the legs more than the arms. The resulting disability is permanent but does not progress. Typically a patient is standing and walking on the balls of the feet with rigid legs and often with ankle clonus.
Initially, most patients experience generalized weakness during the first days and are bedridden for some days or weeks before trying to walk. Occasional blurred vision and /or speech difficulties typically clear during the first month, except in severely affected patients. Spasticity is present from the first day, without any initial phase of flaccidity. After the initial weeks of functional improvement, the spastic paraparesis remains stable for the rest of life, as confirmed by longterm follow up studies. Some patients may suffer an abrupt aggravating episode, e.g. a sudden and permanent worsening of the spastic paraparesis. Such episodes are identical to the initial onset and can therefore be interpreted as a second onset.
The severity of konzo varies from hyperreflexia in the lower limbs to a severely disabled, bedridden patient with spastic paraparesis, associated weakness of the trunk and arms, impaired eye movements, dysarthria and possibly visual impairment. Although the severity varies from patient to patient the longest upper motor neurons are invariably more affected than the shorter ones. Thus a konzo patient with dysarthria always shows severe symptoms in the legs and arms.
Recently, neuropsychological effects of konzo have been described from DR Congo.
The WHO has recommended three criteria for the diagnosis of konzo:
- a visible symmetric spastic abnormality of gait while walking or running;
- a history of onset of less than one week followed by a non-progressive course in a formerly healthy person;
- bilaterally exaggerated knee or ankle jerk reflexes without signs of disease of the spine.
Depending on its severity, konzo is divided into three categories: mild when individuals are able to walk without support, moderate when individuals need one or two sticks to walk, and severe when the affected person is unable to walk unsupported.
The full etiology and the character of the neurological imjury remains unclear. The disease onset is associated to high dietary exposure from cyanide liberated from the naturally occurring glucosides, such as linamarin and lotaustralin that normally are removed by processing before consumption of bitter cassava roots. Cassava, Manihot esculenta, is a high-yielding root crop which is the main staple food for 400 million inhabitants in the tropics. Bitter varieties are preferred in many areas with low agricultural suitability. Although these varieties contain large amounts of toxic cyanogenic glucosides, they can be rendered safe by mechanical grating or fermentation followed by drying or heating. Most of these processing methods takes several days to complete. Cassava is not the only plant to contain cyangogenic glucosides; for instance linseed and almonds, especially the bitter almonds contain similar cyanogenic substances.
Konzo has only been reported from cassava-growing areas, but affected populations constitute only about 1% of all cassava-eating populations in the tropics and consumption of cassava alone is not sufficient to cause konzo. However, during food shortage, war and other severe disruptions of life in poor rural cassava growing communities, the population is forced to make short-cuts in normal processing. A number of epidemiological studies implicates the combination of high intake of cyanogen glucosides and simultaneous low intake of sulfur amino acids needed to detoxify cyanide as the main etiological factor. It is noteworthy that not one single case of similar type of upper motor neuron damage has been reported from cyanide exposure without simultaneous protein malnutrition and even more that no case of konzo has yet been reported from poor cassava eating populations in South America.
Treatment and prognosis
Although no treatment has been found it has been shown that affected individuals benefit considerably from rehabilitation and use of adequate walking aids. In the Central African Republic some children have been operated with an elongation of the Achilles tendon which improved the position of the foot but the long term consequence remains uncertain.
It has been suggested that the disease may be prevented by advising population at risk to grow non-toxic ("sweet") cultivars of cassava. However, their reason for preferential use of toxic ("bitter") cassava varieties is that the toxin and the bitterness associated to the toxin protect the crop from attacks by wild animals and thieves. Prevention of konzo depends on swift restoration of food security that enables higher protein intake and normal cassava processing practices. High awareness of the non-infectious character of the disease is needed to avoid actions that may aggravate the dietary situation of the poor populations that are affected by this disorder.
"Konzo" means "tied legs" in the Yaka language in Southwestern DR Congo and was the designation by the first affected population in DR Congo as reported by Dr G. Trolli in 1938. The name, taken up by Hans Rosling and colleagues, aptly describes the typical spastic gait of those afflicted.
The clinical symptoms are strikingly similar to those of lathyrism and also similar to tropical spastic paraparesis and hereditary spastic paraparesis, only that the latter two disorders have a slow onset. Konzo is distinct from polio which is a flaccid paralysis and most often affects a person asymmetrically.
A disease that is not similar, but also has been associated to cyanogen intake from cassava, is tropical ataxic neuropathy (TAN) first described in parts of Nigeria by Prof. Osuntokun half a century ago. The disease is still occurring in the same areas. 
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