Unilateral leg weakness and pain secondary to metastatic anal squamous cell carcinoma

A 58-year-old man presented to colorectal clinic with intermittent rectal bleeding, weight loss, also pain and weakness affecting his lower back and right leg. On inspection, there were perianal warts (condyloma acuminata), with an additional palpable anal lesion on digital rectal examination, confirmed by colonoscopy. Subsequent imaging revealed a large right psoas abscess, and an associated paravertebral soft tissue component invading the right lumbosacral plexus and nerve roots at L4, L5 and S1. Biopsy of the paravertebral mass revealed metastatic squamous cell carcinoma. Given his symptomatology, and also as biopsy of the perianal warts revealed high-grade squamous intraepithelial lesion/grade III anal intraepithelial neoplasia on histology with infection by human papillomavirus type 6, the primary was presumed to be anal. This was a case of sciatic pain which proved to be diagnostically challenging on initial presentation to primary care. This mode of presentation and pattern of metastasis are uncommon in anal cancer.