Alexander disease is a progressive neurological disease that is genetically transmitted. It almost exclusively affects infants and children. Because of a faulty gene, the tissue of the cerebellum becomes replaced by eosinophilic fibres. It is very rare, with about 500 reported cases.
Patients with the condition start by losing motor function and, eventually, even the ability to speak. It can be diagnosed by genetic tests and, in addition, the decaying tissue will often show up on an MRI.
However, the condition is untreatable. It generally kills within ten years of onset, but those who develop it earlier in life have much shorter lifespans.