(See also Overview of Foot and Ankle Disorders.)
Interdigital neuralgia is a common cause of metatarsalgia. The interdigital nerves of the foot travel beneath and between the metatarsals, extending distally to innervate the toes. Neuralgia of the interdigital nerve along its distal innervation near the ball of the foot develops primarily as a result of improper or constrictive footwear or, less commonly, nerve traction resulting from abnormal foot structure (eg, splayfoot deformity). As a result of chronic repetitive trauma, a benign thickening of the nerve develops (Morton neuroma).
Interdigital neuralgia is characterized by pain around the metatarsal heads or the toes. Early interdigital neuralgia often causes an occasional mild ache or discomfort in the ball of the foot, usually when wearing a specific shoe, such as those that are too narrow at the front. Neuralgia is usually unilateral. As the condition progresses, the nerve thickens. The pain becomes worse, often with a burning or lancinating quality or paresthesias. In time, patients are unable to wear most closed-toe shoes. While walking, patients often falsely sense a pebble in their shoes, which they take off for relief. Neuroma most frequently affects the 3rd interspace. Only slightly less common is involvement of the 2nd interspace. Sometimes both interspaces or feet are involved simultaneously.
The symptoms of interdigital neuralgia are often specific, and the diagnosis is confirmed by tenderness on plantar palpation of the interdigital space and by reproduction of the radiating burning pain, often accompanied by a notable click, by squeezing the space (Mulder sign). Although MRI does not usually confirm neuroma, it may be useful to rule out other interspace lesions or arthritis causing similar symptoms.
Neuralgia of recent onset usually resolves quickly with properly fitting shoes and insoles or with local anesthetic injection. Using a metatarsal pad placed proximally to the metatarsal heads of the affected interspace may also help reduce symptoms.
Neuromas may require one or more perineural infiltrations of long-acting corticosteroids with a local anesthetic. Injection is at a 45° angle to the foot, into the interspace at the level of the dorsal aspect of the metatarsophalangeal joints. Orthotics with neuroma pads, rest, cold packs, and properly fitting shoes often relieve symptoms. Nerve ablation techniques, such as injecting 20% alcohol with a local anesthetic directly into the nerve with ultrasonographic guidance, or cryogenic freezing of the nerve may help relieve symptoms. If other treatments are ineffective, excision often brings complete relief. Another neuroma occasionally develops at the site of nerve excision (amputation or stump neuroma), which may require additional surgery.
Metatarsal pain can result from irritation or benign thickening of the interdigital nerves.
Initially, mild pain caused by wearing narrow shoes can worsen and become lancinating, sometimes with paresthesias and/or a foreign body sensation.
Diagnose the disorder by clinical findings, including tenderness and reproduction of symptoms with palpation of the interdigital space.
Treat by modifying footwear, giving local anesthetic injections and sometimes corticosteroid injections, doing nerve ablation techniques, or doing surgery.