Plantar Dermal Deposition of Wild-Type Transthyretin Amyloid (ATTR): A Case Report of a Unique and Challenging Histopathological Manifestation of Cutaneous ATTR Amyloidosis.

A 74-year-old man presented with a cutaneous lesion on the plantar surface of his right foot. His medical history included spinal canal stenosis and bilateral carpal tunnel syndrome. The lesion was clinically suspected to be a fibroma and was subsequently resected. Histopathological examination revealed mild acanthosis and hyperkeratosis, along with eosinophilic changes and thickening of collagen bundles at the base of the specimen. Based on the patient's medical history and the pathological findings, transthyretin amyloid (ATTR) deposition was suspected. Congo red staining confirmed the presence of amyloid deposits, which were further identified as ATTR through immunohistochemistry and proteomic analysis. Genetic testing of the transthyretin gene revealed no mutations, leading to a diagnosis of wild-type ATTR (ATTRwt) amyloidosis. Cardiac function tests showed no significant abnormalities. Given that ATTRwt deposition is frequently associated with musculoskeletal disorders, amyloid may preferentially accumulate in connective tissues subjected to repetitive mechanical stress. As observed in other musculoskeletal tissues, amyloid deposition in the plantar dermis may aid in the early detection of ATTRwt. Therefore, detailed histopathological evaluation of plantar cutaneous lesions is important, especially in older adults and/or individuals with symptoms suggestive of ATTRwt amyloidosis.