Age / Sex : 55 / M

Chief complaint: heel pain

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Two weeks later, you can see the final diagnosis with a brief discussion of this case.

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Courtesy: Young Cheol Yoon, Samsung medical center Seoul hospital

Baxter Neuropathy (Baxter’s nerve impingement, inferior calcaneal nerve entrapment)

Discussion

Baxter’s nerve is a mixed sensory and motor nerve, providing motor innervation to the abductor digiti minimi (ADM) muscle. Baxter’s nerve impingement can produce symptoms very similar to plantar fasciitis. While this diagnosis has been said to account for up to 20% of heel pain, it is often overlooked relative to other causes of heel pain.

Anatomy and pathology

Within the ankle tarsal tunnel, the posterior tibial nerve (PTN) bifurcates into medial (MPN) and lateral (LPN) plantar nerves. These nerves exit the tarsal tunnel and continue along the plantar aspect of the foot. The MPN travels anterior to the LPN, carrying sensory information from the medial two thirds of the plantar foot, and motor innervation to the flexor digitorum brevis, abductor hallucis, flexor hallucis brevis, and first lumbrical. The LPN carries sensory information from the lateral plantar aspect of the foot, 5th toe, and lateral half of the 4th toe. Motor innervation involves all the remaining foot muscles, not innervated by the MPN. The first branch of the LPN, Baxter’s nerve (also known as the inferior calcaneal nerve), originates from the LPN. The nerve courses vertically between the abductor hallucis and quadratus plantae muscles, then makes a sharp 90 degree horizontal turn, coursing laterally beneath the calcaneus to innervate the ADM muscle. Motor innervation supplies the ADM, occasionally to the flexor digitorum brevis and lateral half of the quadratus plantae.

Reported risk factors for Baxter’s nerve impingement include advancing age, the presence of a calcaneal spur, plantar fasciitis, underlying mass, vascular enlargement, muscular enlargement (such as in athletes), obesity, and foot hyper-pronation.

Imaging

MRI has been shown to be extremely valuable in demonstrating muscular changes associated with denervation. Acute and subacutely muscle denervation is best evaluated with fluid sensitive sequences exhibiting increased signal within the muscle belly compared to normal muscle, related to neurogenic muscular edema. In the setting of Baxter’s nerve impingement, muscular edema will occur selectively within the ADM, and potentially within the flexor digitorum brevis and quadratus plantae, depending on the innervation anatomy of the patient. Chronically denervated muscle will eventually undergo volumetric atrophy, and subsequent irreversible fatty infiltration. Typically, the atrophy and fatty infiltration occurs homogeneously within the muscle belly. In the case of Baxter’s nerve impingement, the ADM is typically involved homogeneously, unless dual innervation exists.

Reference; http://radsource.us/baxters-nerve/