Sural-to-medial femoral cutaneous amplitude ratio in early diagnosis of uremic neuropathy.

Introduction: Medial femoral cutaneous (MFC) sensory nerve action potentials (SNAPs) can be easily recorded using distal stimulation. This study aimed to identify a new parameter using MFC SNAPs for the early electrophysiological diagnosis of length-dependent axonal polyneuropathy (LDAP) associated with uremic neuropathy.

Methods: Patients with chronic renal failure who were referred to the electrodiagnostic laboratory due to symptoms suggesting polyneuropathy were included. Assessments encompassed neurological examination, Michigan Neuropathy Screening Instrument (MNSI), and Semmes-Weinstein monofilament test. Antidromic radial, median, ulnar, MFC, sural, and superficial peroneal sensory; median, ulnar, tibial, and peroneal motor nerve conduction studies were performed. Sural-to-radial amplitude ratio (SRAR) and sural-to-medial femoral cutaneous amplitude ratio (SMFCAR) were calculated, and their diagnostic sensitivities were compared with the age and sex matched healthy controls.

Results: Thirty-two chronic renal failure patients (mean age 60.0 ± 9.6 years) and 37 controls (60.6 ± 9 years) were included. MNSI indicated clinical polyneuropathy in 59.4% of patients, while sural SNAP amplitude was diagnostic in 78%. Median SRAR and SMFCAR values were significantly lower in patients than controls (p < .001 for both). The cut-off values for SMFCAR and SRAR were <1.82 and <0.30, respectively, both with a sensitivity of 59% and a specificity of 94%.

Discussion: Sural SNAP is the most sensitive parameter in the diagnosis of LDAP. SMFCAR is not superior to SRAR. If the sural SNAP amplitude is normal, SMFCAR can serve as an alternative to SRAR in dialysis patients with bilateral arteriovenous fistulae or in those unable to undergo radial NCS.