Perivenous encephalomyelitis – Perivenous demyelination is the pathological hallmark of acute disseminated encephalomyelitis. Acute disseminated encephalomyelitis (ADEM) is characterized by a brief but widespread attack of inflammation in the brain and spinal cord that damages myelin – the protective covering of nerve fibers. ADEM is thought to be an autoimmune disorder and often follows viral or bacterial infections, or less often, vaccination for measles, mumps, or rubella. The symptoms of ADEM appear rapidly, beginning with encephalitis-like symptoms such as fever, fatigue, headache, nausea and vomiting, and in the most severe cases, seizures and coma. ADEM typically damages white matter (brain tissue that takes its name from the white color of myelin), leading to neurological symptoms such as visual loss (due to inflammation of the optic nerve) in one or both eyes, weakness even to the point of paralysis, and difficulty coordinating voluntary muscle movements (such as those used in walking). ADEM is sometimes misdiagnosed as a severe first attack of multiple sclerosis (MS), since the symptoms and the appearance of the white matter injury on brain imaging may be similar. However, ADEM has several features which differentiate it from MS. Children are more likely than adults to have ADEM, whereas MS is a rare diagnosis in children. In addition, ADEM usually consists of a single episode or attack of widespread myelin damage, while MS features many attacks over the course of time. The long-term prognosis for individuals with ADEM is generally favorable. For most individuals, recovery begins within days, and within six months the majority of ADEM patients will have total or near total recoveries. Others may have mild to moderate lifelong impairment ranging from cognitive difficulties, weakness, loss of vision, or numbness. Severe cases of ADEM can be fatal but this is a very rare occurrence.