Specific Imaging Findings. Owing to their isodensity with brain, hypothalamic hamartomas may be difficult to identify on CT scan when their size is small. On MRI, they appear as round to oval solid masses located within or attached to the tuber cinereum, interposed between the pituitary infundibulum anteriorly and the mammillary bodies posteriorly. They are attached with a sessile or pedunculated base to the floor of the third ventricle and typically project caudally into the interpeduncular cistern or, rarely, bulge upward into the third ventricle. The lesions vary in size from a few millimeters up to 4 centimeters. Smaller hamartomas are both T1 and T2 isointense with gray matter, while larger ones tend to be of lower T1 and higher T2 signal, probably because of a larger glial component. The lesions do not enhance with contrast material. Diffusion imaging shows normal to increased ADC values, MR spectroscopy primarily reveals increased myo-inositol levels, which correlate with glial component. In rare cases, a large cystic component is associated, extending uni- or bilaterally into the middle cranial fossa. Pertinent Clinical Information. Patients with hypothalamic hamartomas may be asymptomatic with the lesion found incidentally. However, most patients typically present with either isosexual precocious puberty, caused by overproduction of luteinizing hormone-releasing hormone, or with a peculiar form of partial complex epilepsy called gelastic seizures, manifesting with clonic movements of the chest and diaphragm that simulate laughing. There is no clear-cut correlation between lesion size and clinical presentation, although there is a tendency for larger lesions to present with gelastic epilepsy.
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