[A case of slowly progressive cauda equina syndrome caused by invasive lobular carcinoma of the left breast with leptomeningeal carcinomatosis].

Herein, we present the case of a 67-year-old woman who presented with a 14-month history of progressive lower limb paresthesia and muscle weakness. Bilateral sensorineural hearing loss also developed during the disease course. Six months prior to admission, cerebrospinal fluid cytology revealed findings suspect of malignancy. Although tumor screening was performed, the cause could not be identified. Due to symptom progression, the patient was re-admitted after which further examination revealed invasive lobular carcinoma of the left breast as the primary tumor, with further metastasis to the large intestine also confirmed. Immunostaining of the cerebrospinal fluid revealed cells with positive findings for estrogen receptor (ER) and AE1/3, leading to a definitive diagnosis of meningeal dissemination. This was a rare case of meningeal dissemination of breast cancer, initially presenting with slowly progressive cauda equina syndrome. This case highlights the importance of repeated tumor screening for neurological symptoms of unknown primary origin.