Working Diagnosis:
Right peroneal neuropathy

Treatment:
The patient was prescribed an Ankle-Foot Orthosis. She was advised to avoid sitting cross-legged and recommended to refrain from gym class, dance, and running. At four week follow-up, the patient reported a recent diagnosis of anorexia nervosa. On chart review her initial weight was 103 lbs (BMI 19.5 kg/m2), and over the course of several months, she had dropped to 88 lbs (BMI 16 kg/m2). The onset of the acute foot drop coincided with this significant weight loss. She was enrolled in a comprehensive pediatric eating disorder clinic for further treatment.
She had orthopaedic surgical consult at six weeks from her initial sports medicine consultation. Her weight had increased and her symptoms were improving. Conservative management was recommended as it was unlikely that the cystic lesion visualized on MRI was contributing to her symptoms.

Outcome:
At four week follow-up, the patient reported improvement in her gait with use of the Ankle Foot Orthosis, and the thigh pain had resolved. However, she still had significant weakness with ankle dorsiflexion and persistent numbness. At three month follow-up, she reported significant return of strength and only required the Ankle Foot Orthosis during gym. She continued treatment in the comprehensive pediatric eating disorder clinic with continued weight gain, although she was not yet weighing herself. She was not yet allowed to return to running or dance. At four month follow-up, she only had mild weakness of ankle dorsiflexion and a small area of numbness on the distal lateral leg. At six month follow-up, she had only subtle weakness, and had returned to her prior weight of 103 lbs. She was cleared from wearing the Ankle Foot Orthosis. She continued active treatment of her anorexia. With the okay from her psychologist, we progressed her back to running and dance.

Author's Comments:
This case illustrates an acute peroneal neuropathy as a rare manifestation of anorexia nervosa [1,2]. Not only is this an atypical, but it also highlights the importance of treating the patient as a whole including diet and lifestyle factors.

Editor's Comments:
While this case highlighted anorexia nervosa as a contributing cause, issues with less severe disordered eating leading to poor energy availability should always be considered in female, and male athletes. Oftentimes, training levels are so high that even the best-intentioned athlete may have difficulty maintaining appropriate caloric intake to meet their energy needs. RED-S (Relative Energy Deficiency in Sport) describes this issue in male and female athletes. [3]

References:
1. Lutte I, Rhys C, Hubert C, Brion F, Boland B, Peeters A, Van Den Bergh P, Lambert M. Peroneal neuropathy palsy in anorexia nervosa. Acta Neurol Belg. 1997 Dec;97(4):251-4.
2. MacKenzie JR, LaBan MM, Sackeyfio AH. The prevalence of peripheral neuropathy in patients with anorexia nervosa. Arch Phys Med Rehabil. 1989 Nov;70(12):827-30.
3. Mountjoy M, Sundgot-Borgen JK, Burke LM, et al IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update Br J Sports Med 2018;52:687-697.

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