1504 - Lumbosacral plexopathy as a paraneoplastic syndrome secondary to gastrointestinal stromal tumor (GIST).

Lumbosacral plexopathy as a paraneoplastic syndrome secondary to gastrointestinal stromal tumor 

Case Description:

An 82-year-old man with hypertension, hyperlipidemia, and diabetes presented with acute right leg pain, numbness, and weakness. The Initial work-up, completed over three ER visits, included  X-ray, CT, and MRI imaging of the lumbar spine, which were all unrevealing. He was diagnosed with presumed lumbar radiculopathy. Symptoms progressed, and he required a walker. MRI of the lumbosacral plexus demonstrated right femoral nerve enlargement and T2 hyperintensity. Electrodiagnostic testing showed patchy right upper and lower plexopathy. CT abdomen incidentally showed a proximal gastric mass, confirmed as GIST on biopsy. Despite a negative antibody panel, paraneoplastic plexopathy was suspected. The patient received IVIG with limited benefit and participated in inpatient rehabilitation. Surgical resection was initially deferred but later performed for worsening symptoms. Postoperatively, he demonstrated significant improvement in pain and mobility during rehabilitation. 

Discussions:

Paraneoplastic lumbosacral plexopathy due to GIST is extremely rare, with only isolated reports in the literature. Diagnosis is challenging and requires exclusion of direct tumor infiltration, perineural spread, or radiation-induced plexopathy. Up to one-third of paraneoplastic neurologic syndromes are antibody-negative, as in this case. MRI is the most sensitive imaging modality for plexus involvement, while EMG confirms axonal loss and denervation. Pathogenesis is believed to be immune-mediated, involving cross-reactivity between tumor antigens and neural tissue. Immunomodulatory therapy (steroids, IVIG, plasma exchange) may be attempted, though efficacy is uncertain. Definitive treatment of the underlying malignancy offers the greatest chance for neurological recovery.

Conclusions:

This case highlights the diagnostic complexity of a lumbosacral plexopathy, the importance of a broad differential and thorough exam and work-up when seeing patients with lower extremity neuropathic pain. It also highlights an extremely rare etiology of lumbosacral plexopathy: paraneoplastic secondary to GIST. Clinicians should maintain suspicion for paraneoplastic plexopathy in patients with subacute weakness and malignancy.