Tumor-Induced Oblique Abdominal Muscle Paralysis Syndrome: A Case of Position-Dependent Abdominal Distension and Umbilical Displacement.

Unilateral paralysis of the oblique abdominal muscles is commonly linked to non-malignant conditions, such as herniated discs, shingles, and phrenic nerve injury, each leading to localized muscle weakness through nerve impairment. However, oblique abdominal muscle paralysis caused by malignant tumors is exceedingly rare. This report presents a unique case of left oblique abdominal muscle paralysis induced by malignant pleural mesothelioma (MPM), demonstrating a distinct pattern of position-dependent abdominal bulging and umbilical displacement. The patient, a male in his 70s, was initially treated with chemotherapy for left-sided MPM and later received radiation therapy (RT) for pleural lesions causing pain. As the disease progressed, he developed a visible bulging of the left lateral abdomen, which was more pronounced in upright positions (standing or sitting) and resolved in the supine position. CT imaging showed atrophy of the left oblique abdominal muscle and a displacement of the umbilicus to the right, findings were consistent with intercostal nerve impairment due to tumor invasion. This case underscores the importance of conducting abdominal examinations in various positions when nerve impairment by a tumor is suspected. The presence of position-dependent lateral abdominal bulging, lateral displacement of the umbilicus, and lack of intraperitoneal abnormalities should raise suspicion for tumor-induced oblique abdominal muscle paralysis. We propose defining such cases as "tumor-induced oblique abdominal muscle paralysis syndrome" to improve diagnostic accuracy and aid in the effective management of malignancy-associated abdominal neuropathies.