608 - Lateral Plantar Mononeuropathy with Fascicular Enlargement on Diagnostic Ultrasound

Lateral plantar mononeuropathy

Case Description:

A 46-year-old female with a prior history of bilateral radial tunnel syndrome (successfully treated with bilateral surgical decompressions) presented with two years of right medial foot and ankle pain. Pain was burning in character with associated paresthesia throughout the plantar medial hindfoot and midfoot. Symptoms had previously improved temporarily after a palpation-guided tarsal tunnel corticosteroid injection performed at an outside podiatry clinic. Physical examination showed reproduction of symptoms with percussion over the tarsal tunnel (positive Tinel’s sign). Diagnostic ultrasound revealed focal fascicular enlargement of the lateral plantar nerve just distal to the bifurcation of the tibial nerve. Given her history of multiple compressive mononeuropathies, neuromuscular medicine recommended electrodiagnostic studies, inflammatory markers to rule out mononeuritis multiplex, and genetic testing to rule out hereditary neuropathy with liability to pressure palsies (HNPP). The results of these studies were negative. Symptoms improved with conservative treatments and activity modification.

Discussions:

Lateral plantar mononeuropathy is a rare cause of foot and ankle pain. Mononeuropathies involving the tibial nerve, medial calcaneal nerve, inferior calcaneal (Baxter’s) nerve, and medial plantar nerve may also present with neuropathic pain of the medial foot and ankle. Diagnostic ultrasound of the medial foot and ankle enables direct visualization and localization of peripheral nerve pathology while also screening for underlying space-occupying lesions or anomalous anatomy causing focal nerve compression. Ultrasound-guided injections targeting the area of focal neuropathy may be considered in cases of persistent pain.

Conclusions:

Diagnostic ultrasound is an effective first-line imaging modality for cases of suspected mononeuropathy affecting the medial foot and ankle. Pathology of isolated fascicles of the lateral plantar nerve may cause symptoms without corresponding electrodiagnostic abnormalities. Additional workup should be considered in patients with a history of multiple mononeuropathies to rule out underlying genetic or inflammatory etiologies.