Neurologists are often consulted to evaluate patients with acute or subacute encephalopathy. The differential diagnosis for encephalopathy is wide, but the clinical features and findings on blood, cerebrospinal fluid (CSF), electroencephalography (EEG), and neuroimaging studies often (but not always) lead to an accurate diagnosis. Once infectious causes are excluded, an autoimmune or inflammatory process may be suspected on the basis of inflammatory and autoimmune markers in the serum and CSF and meningeal and parenchymal abnormalities on magnetic resonance imaging (MRI) of the brain.

Autoimmune encephalopathy may take many forms, including paraneoplastic or idiopathic limbic encephalitis that is defined by characteristic serologic and neuroimaging abnormalities. Idiopathic autoimmune encephalopathy is also often defined on the basis of a clinical response to steroids. Steroid-responsive encephalopathy associated with autoimmune thyroiditis (SREAT), often referred to as Hashimoto encephalopathy, was initially described by Brain et al. Although high serum levels of thyroperoxidase (TPO) antibody, formerly known as antimicrosomal antibody, are often found in patients with SREAT, this organ-specific autoantibody is a serologic marker for the common condition of autoimmune (Hashimoto) thyroiditis and a common marker of autoimmunity in most autoimmune neurologic disorders.

This retrospective study was reviewed and approved by the Mayo Clinic institutional review board. The authors identified consecutive patients in whom a diagnosis of SREAT was made at Mayo Clinic Rochester or Mayo Clinic Scottsdale between November 1995 and July 2003. For the purpose of this study, the diagnosis of SREAT required fulfillment of the following criteria: During this same approximately 8-year period, we treated additional patients who fulfilled the same criteria described in this article, except that no significant improvement occurred following high-dose corticosteroid treatment (ie, steroid-unresponsive encephalopathy associated with autoimmune encephalopathy). We present data on these patients and argue that they likely have a nonautoimmune or noninflammatory origin for their encephalopathy. Twenty patients met criteria for the diagnosis of SREAT as noted herein. The symptoms and findings for the 20 patients all but 1 patient required hospitalization during the acute phase of illness because of the severity of their deficits. The median age at onset was 56 years (range, 27-84 years). There was a preponderance of female patients (70%). Nine patients had a history of hypothyroidism that antedated the onset of neurologic symptoms, and 2 had euthyroid goiter. Five additional patients developed hypothroidism after the resolution of encephalopathy. Other autoimmune disorders were present in 6 patients and included diabetes mellitus type 1 in 2 (10%), systemic lupus erythematosus in 1 (5%), and Crohn disease in 1 (5%). Two patients had symptoms of sicca syndrome (10%).

The clinical, laboratory, and radiologic findings associated with SREAT are more varied than previously reported. Misdiagnosis at presentation is common. This treatable syndrome should be considered even if the serum sensitive thyroid-stimulating hormone level and erythrocyte sedimentation rate are normal, the cerebrospinal fluid profile does not suggest an inflammatory process, and neuroimaging results are normal. Until the pathophysiologic mechanism of this and other autoimmune encephalopathies is better characterized, we believe that descriptive terms that reflect an association rather than causation are most appropriate for this syndrome.

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