A 52-year-old woman with a 6-week history of subacute cognitive decline, myoclonus, and fluctuating consciousness is evaluated. She has known Hashimoto thyroiditis on levothyroxine with TSH in target range. Her EEG is shown. The abnormality marked **B** — generalized rhythmic delta activity with diffuse slowing (FIRDA) — is most consistent with which diagnosis?
A. Hepatic encephalopathy, characterized by triphasic waves and diffuse background slowing
B. Hashimoto encephalopathy (SREAT), supported by elevated anti-TPO antibodies and expected dramatic response to high-dose corticosteroids
C. Metabolic encephalopathy from hypothyroidism, since TSH elevation is the primary driver of neurological symptoms
D. Creutzfeldt-Jakob disease, which presents with identical myoclonus and periodic sharp waves on EEG
Explanation
Why option 1 is correct
The clinical presentation of subacute cognitive decline, myoclonus, and fluctuating consciousness in a patient with Hashimoto thyroiditis, combined with the EEG finding marked B (FIRDA with diffuse background slowing and intermittent triphasic waves), is pathognomonic for Hashimoto encephalopathy (SREAT). The key diagnostic feature is that the patient is EUTHYROID (TSH in target range), ruling out hypothyroid encephalopathy. The markedly elevated anti-TPO and anti-thyroglobulin antibodies, normal or nonspecific MRI, mildly elevated CSF protein with normal cell count, and expected dramatic clinical and EEG improvement with high-dose corticosteroids (methylprednisolone 1 g IV daily × 3–5 days followed by oral prednisone taper) confirm the diagnosis. The pathogenesis is autoimmune/inflammatory rather than thyroid hormone–mediated. Per Harrison's 21e and Castillo 2006, FIRDA with diffuse slowing is the characteristic EEG pattern in SREAT, and steroid responsiveness is the hallmark of the condition.
Why each distractor is wrong
Option 2 (Creutzfeldt-Jakob disease): While CJD also presents with rapidly progressive dementia and myoclonus, it typically shows periodic sharp wave complexes (PSWCs) or periodic sharp wave complexes on EEG, NOT diffuse FIRDA. CJD is rapidly progressive (weeks to months) with a much worse prognosis and does not respond to corticosteroids. The normal MRI and absence of focal findings argue against CJD.
Option 3 (Hepatic encephalopathy): Although hepatic encephalopathy does produce triphasic waves and diffuse slowing, there is no mention of liver disease, elevated ammonia, or hepatic dysfunction in this patient. Hepatic encephalopathy would not be associated with elevated anti-TPO antibodies or a history of Hashimoto thyroiditis.
Option 4 (Hypothyroid encephalopathy): The patient's TSH is explicitly stated to be in target range on levothyroxine, meaning she is euthyroid. Hypothyroid encephalopathy occurs in the setting of overt hypothyroidism with elevated TSH. The diagnosis of SREAT is specifically defined by the presence of encephalopathy in a euthyroid patient with Hashimoto thyroiditis and autoimmune antibodies.
High-YieldNEET PG
FIRDA with diffuse slowing in a euthyroid patient with Hashimoto thyroiditis = SREAT (Hashimoto encephalopathy); treat with high-dose IV corticosteroids, not thyroid hormone adjustment.
