Primary neurolymphomatosis presenting as paraparesis and diplopia in a young man.

A man in his mid-20s presented with a 5-month history of bilateral lower limb weakness and wasting along with bladder symptoms and erectile dysfunction. Additionally, he had binocular diplopia and progressive drooping of both eyelids. His clinical presentation was suggestive of bilateral third cranial nerve involvement with lumbosacral polyradiculopathy. Initial treatment with steroids for suspected idiopathic lumbosacral polyradiculopathy syndrome proved ineffective, as his symptoms progressively worsened without other systemic manifestations. We diagnosed a rare and treatable neurological condition, primary neurolymphomatosis, presenting as lumbosacral polyradiculopathy with bilateral third cranial nerve involvement. A definitive diagnosis was established only after nerve root biopsy, highlighting the crucial role of biopsy in confirming the diagnosis. Our case underscores the importance of early consideration for an invasive nerve root biopsy, enabling prompt treatment and an improved prognosis.