When Endurance Takes Its Toll: Ultrasound Diagnosis of Femoral Nerve Neuropathy in a Sport Horse

Femoral Nerve Neuropathy of an Endurance Horse as a Sequela of Iliopsoas Muscle Myopathy Diagnosed by Transrectal Ultrasonography

M. Puccetti, F. Beccati, N. Pilati

Background:
Femoral nerve neuropathy in horses is an uncommon cause of pelvic limb lameness, characterized clinically by reduced stifle extension and impaired quadriceps function. Peripheral nerve imaging in equine practice remains limited, and ultrasonographic descriptions of femoral nerve injury are scarce. This case report describes femoral neuropathy secondary to iliopsoas muscle myopathy in an endurance horse and highlights the diagnostic and monitoring value of transrectal ultrasonography.

Methods:
A 13-year-old Arab gelding developed acute, non–weight-bearing left pelvic limb lameness during a 120-km endurance competition. Clinical and neurologic examinations were performed, followed by limited serum biochemical analysis focusing on muscle enzymes. Transcutaneous and transrectal ultrasonography of the pelvis were conducted using a portable ultrasound unit with linear and convex probes. Serial ultrasonographic examinations were performed during recovery to monitor nerve and muscle changes.

Results:
Clinical examination revealed inability to extend the left stifle, reduced medial thigh sensitivity, and quadriceps atrophy, consistent with femoral nerve dysfunction. Serum creatine kinase and lactate dehydrogenase concentrations were markedly elevated. Ultrasonography identified right gluteus medius myopathy and increased echogenicity of the left iliopsoas muscle. The left femoral nerve showed increased cross-sectional area, heterogeneous echogenicity, and perineural edema compared to the contralateral side. Follow-up imaging at 12 days demonstrated further nerve enlargement with loss of fascicular structure, and at 4 months showed persistent but improved enlargement without edema. The horse underwent conservative management with rest and anti-inflammatory therapy, gradually returning to full training at 8 months and competition at 12 months post-injury. The horse subsequently completed two 160-km endurance races.

Limitations:
Advanced diagnostics such as magnetic resonance imaging, electromyography, nerve conduction studies, or muscle biopsy were not performed to definitively characterize the type and severity of nerve injury. Power Doppler imaging was not utilized to assess hypervascularity. Complete visualization of the entire femoral nerve course was not possible with ultrasonography.

Conclusions:
This case describes femoral nerve neuropathy likely secondary to compressive iliopsoas myopathy sustained during endurance competition. Transrectal ultrasonography proved valuable for identifying nerve enlargement, surrounding edema, and structural alterations, as well as for longitudinal monitoring of recovery. The findings support the use of transrectal ultrasonography as a practical diagnostic modality for suspected femoral nerve dysfunction in horses under field conditions.

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