F-Wave Abnormalities in a Patient with Possible Chronic Exertional Compartment Syndrome

Abstract

The F-waves demonstrated here were obtained from a 20-yr-old woman referred for several months of bilateral exertional calf pain (Fig. 1). In addition, she reported that 5 days before testing, she developed severe bilateral calf pain after walking 1 mile, and by the next day, she had developed left lateral foot numbness and weakness at the left ankle. On physical examination, sensation was impaired to light touch and pinprick over the left lateral malleolus and the lateral aspect of the foot. Reflexes were intact and symmetric at the bilateral patella and achilles. Strength was impaired with all movements of her left ankle and toes only.FIGURE 1: F-wave responses in the left lower limb. Runs 1 and 2 were measured at the extensor digitorum brevis, run 3 was measured at the tibialis anterior, and the final run was measured at the abductor hallucis.Sural sensory, fibular motor, and tibial motor nerve conduction studies were unremarkable. Interestingly, no F-waves could be obtained when measured from the fibular nerve at the extensor digitorum brevis bilaterally. However, when measured from the fibular nerve at the tibialis anterior and the tibial nerve at abductor hallucis, F-wave responses were obtained with normal latency and penetrance. Needle examination was not performed because weakness had been present only for several days. Chronic exertional compartment syndromes (CECSs) can affect any lower limb compartment, and it seems to affect the lower leg anterior compartment most frequently.1 Although early research on acute compartment syndromes documented resultant neuropathy,2,3 electrodiagnostic findings in CECS have not been definitively characterized, and previous studies of electrodiagnostic evaluations in patients with a presumptive diagnosis of CECS have been unremarkable.4 Therefore, it is likely that elevated compartment pressures may cause neuropathy if pressures of sufficient severity and duration are produced, although this threshold remains uncertain. In our patient, we observed the absence of F-wave responses from the bilateral extensor digitorum brevis only. These findings are suggestive of a subtle neuropathy of the deep fibular nerve in a clinical scenario strongly suggestive of CECS of the anterior compartment of the lower leg, and we propose that these findings may represent the mildest findings of CECS that may be appreciated on electrodiagnostic studies. To summarize, CECS of the lower leg is a well-recognized diagnosis, and although not the diagnostic test of choice for this diagnosis, these patients are referred not infrequently for electrodiagnostic testing, as symptoms may include pain, weakness, and numbness. As such, the electromyographer should be aware of this entity and potential electrodiagnostic abnormalities.