Hashimoto's encephalopathy : epidemiology, pathogenesis and management.

Hashimoto's encephalopathy is a term used to describe an encephalopathy of presumed autoimmune origin characterised by high titres of antithyroid peroxidase antibodies. In a similar fashion to autoimmune thyroid disease, Hashimoto's encephalopathy is more common in women than in men. It has been reported in paediatric, adult and elderly populations throughout the world. The clinical presentation may involve a relapsing and remitting course and include seizures, stroke-like episodes, cognitive decline, neuropsychiatric symptoms and myoclonus. Thyroid function is usually clinically and biochemically normal.Hashimoto's encephalopathy appears to be a rare disorder, but, as it is responsive to treatment with corticosteroids, it must be considered in cases of 'investigation negative encephalopathies'. Diagnosis is made in the first instance by excluding other toxic, metabolic and infectious causes of encephalopathy with neuroimaging and CSF examination. Neuroimaging findings are often not helpful in clarifying the diagnosis. Common differential diagnoses when these conditions are excluded are Creutzfeldt-Jakob disease, rapidly progressive dementias, and paraneoplastic and nonparaneoplastic limbic encephalitis. In the context of the typical clinical picture, high titres of antithyroid antibodies, in particular antithyroid peroxidase antibodies, are diagnostic. These antibodies, however, can be detected in elevated titres in the healthy general population. Treatment with corticosteroids is almost always successful, although relapse may occur if this treatment is ceased abruptly. Other forms of immunomodulation, such as intravenous immune-globulin and plasma exchange, may also be effective. Despite the link to autoimmune thyroid disease, the aetiology of Hashimoto's encephalopathy is unknown. It is likely that antithyroid antibodies are not pathogenic, but titres can be a marker of treatment response. Pathological findings can suggest an inflammatory process, but features of a severe vasculitis are often absent. The links between the clinical pictures, thyroid disease, auto-antibody pattern and brain pathology await further clarification through research. It may be that Hashimoto's encephalopathy will be subsumed into a group of nonvasculitic autoimmune inflammatory meningoencephalopathies. This group may include disorders such as limbic encephalitis associated with voltage-gated potassium channel antibodies. Some authors have suggested abandoning any link to Hashimoto and renaming the condition 'steroid responsive encephalopathy associated with autoimmune thyroiditis' to better reflect current, if limited, understanding of this condition.