A case of splaying toes caused by Morton’s neuroma indicates early rheumatoid arthritis

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Abstract
Morton’s neuroma is an entrapment neuropathy of the plantar digital nerve in the intermetatarsal space by a soft tissue tumor, bursitis, or synovitis. The most common location is the third intermetatarsal space, but splaying toes are rare in typical Morton’s neuroma. However, the location, character, and histological findings are different in Morton’s neuroma with rheumatoid arthritis (RA). RA is seen in 40% of patients with Morton’s neuroma, most of which develop during treatment for RA. However, a Morton’s neuroma that precedes the occurrence of RA is rare. The case of a 68-year-old woman with a burning sensation in her third toe and splaying toes at the second intermetatarsal space is reported. A dumbbell-shaped tumour was seen on magnetic resonance imaging and ultrasound. Needle biopsy was performed under ultrasound guidance, and histopathology was negative for malignancy. Morton’s neuroma was diagnosed and treated with an insole and exercise therapy. After 1 month, she developed swelling and tenderness in the metacarpophalangeal joint of her right thumb. She had a power Doppler positive sign, high rheumatoid factor, and other increased inflammatory markers. RA was diagnosed, and treatment with methotrexate 6 mg/week, bucillamine 100 mg/day, and prednisolone 5 mg/day was begun. After 1 year, the tumour in the second intermetatarsal space of the right foot was smaller, and the widening of the second interdigital space had disappeared. When a patient presents with splaying toes and swelling in the intermetatarsal space, especially the second, clinicians should look for rheumatoid nodules, synovitis, and bursitis and consider RA.

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