Teaching Case

A 66-year-old woman with history of chronic recurrent painful heel. On physical examination, there was no evidence of sensory loss.

Magnetic resonance imaging of the right foot performed with a high-resolution surface coil. Patient scanned in magnet of 1.5 T. The technical parameters for all magnet strengths were as follows: sagittal, coronal and axial short-tau inversion recovery (STIR) turbo spin echo, coronal turbo spin-echo proton density and T2-weighted and sagittal and coronal turbo spin echo T1-weighted.
MR images show marked thickening of the proximal plantar fascia with increased intrasubstance signal intensity. Also perifascial and mild calcaneal marrow oedema.
In association with these findings, there is selective fat tissue replacement of the abductor digiti minimi muscle.

Baxter's neuropathy is an entrapment syndrome of the inferior calcaneal nerve (ICN), which is the first branch of the lateral plantar nerve. It accounts for 20% of causes of medial heel pain, occasionally irradiating into the lateral aspect of the foot [1, 2, 3].

Atrophy of the abductor digiti minimi muscle reflects chronic compression of the ICN, which consist of loss of muscular mass of the affected muscle with fat tissue replacement (Fig. 1) [4]. MRI plays a key role since nerve entrapment at the foot and ankle involves thin and complex anatomic structures and is underdiagnosed because clinical symptoms and electrophysiologic findings may not contribute to the diagnosis [1]. In addition, opposite to MRI electromyography may not allow differentiation of lateral plantar nerve entrapment at the level of the tarsal tunnel from ICN entrapment [1].

Entrapment of the ICN occurs at three possible places: (a) adjacent to the fascial edge of a hypertrophied abductor halluces muscle, (b) as the nerve passes between the deep fascia of the abductor hallucis muscle and the medial caudal margin of the medial head of the quadratus plantae muscle or (c) most commonly where the nerve passes just anterior to the medial calcaneal tuberosity, where it is sometimes related with calcaneal spur and marked thickening of the proximal plantar fascia, as it is shown in our case (Fig. 2) [1, 4].

Compression of the ICN may result from altered biomechanics, reflected by posterior tibial tendon dysfunction or Achilles tendinosis, or may result from direct mechanical compression of the nerve due to plantar fasciitis and/or plantar calcaneal spurs (Fig. 3) [2]. Recht et al reported plantar fasciitis in 37% of cases with abductor digiti quinti atrophy [4].
Plantar fasciitis is referred in the literature as the most common cause of plantar heel pain [5, 6]. This condition generally occurs in obese middle-aged or elderly patients as a result of repetitive trauma from sport activities, excessive standing and walking [5, 6].

In view of the present case, and in order to identify the proper imaging approach to the patient with heel pain, we recommend to assess findings of fatty infiltration of the abductor digiti minimi muscle, since it reflects chronic ICN entrapment.