Leptomeningeal metastasis of lung adenocarcinoma presenting as cauda equina syndrome: a case report and literature review.

Objective: This study aims to improve the understanding of the imaging characteristics of leptomeningeal metastasis, particularly in patients presenting with cauda equina syndrome as the initial manifestation, in order to improve diagnostic accuracy and reduce the misdiagnosis rate. Methods: A 69-year-old male presented with a 4-month history of lower back pain accompanied by progressive lower limb numbness and gait instability. Non-contrast lumbar MRI performed at another hospital revealed punctate abnormal signal intensity in the cauda equina region, which was overlooked. The patient was treated as having lumbar disc herniation for 3 months without improvement. Chest CT subsequently revealed a left lower lobe pulmonary mass, which was surgically resected. Postoperative pathology confirmed poorly differentiated adenocarcinoma with neuroendocrine differentiation. Following surgery, the patient's lower limb symptoms worsened, with an inability to walk and onset of urinary and fecal incontinence, prompting referral to our hospital. Physical examination revealed bilateral lower limb muscle strength of grade 2. Laboratory tests showed markedly elevated ferritin and mildly elevated NSE. Whole-spine and brain contrast-enhanced MRI were subsequently performed. Results: Spinal contrast-enhanced MRI demonstrated linear and nodular thickening and enhancement of the leptomeninges from the cervical to lumbar segments, dural thickening and enhancement, and soft tissue within the cauda equina region exhibiting cast-like filling with marked post-contrast enhancement. Brain contrast-enhanced MRI revealed multiple small nodular enhancing lesions. Based on the pathological confirmation of lung adenocarcinoma and the characteristic imaging findings, a probable (type II) diagnosis of extensive intracranial and leptomeningeal metastases was made on imaging grounds alone. The patient received systemic chemotherapy in combination with bevacizumab but had a poor response with rapid disease progression, and died one month after discharge. Conclusion: Cauda equina syndrome can be caused by leptomeningeal metastasis from malignant tumors, and the latter should be considered an important differential diagnosis for cauda equina syndrome in elderly patients. Contrast-enhanced MRI is essential for diagnosis, commonly revealing linear or nodular leptomeningeal enhancement, with cast-like thickening and enhancement representing a characteristic manifestation of advanced disease.