Background: The frequency of sural sensory nerve action potential (SNAP) abnormalities in large fibre sensory neuropathy (LFSN) is uncertain. When sural SNAPs are normal, the sural/radial amplitude ratio (SRAR) was found to improve diagnostic yield in some studies. Motor parameters have been studied rarely, but may be helpful in this setting.
Methods: The electrophysiology of 105 consecutive patients with LFSN was reviewed and compared with that of 62 controls. We determined (i) the frequency of abnormal sural SNAPs, (ii) the proportion of patients with normal sural SNAPs but reduced SRAR <0.21 and (iii) the utility of motor studies (motor nerve conduction velocity, compound muscle action potential, F-wave).
Results: Reduced age-adjusted sural SNAPs were present in 54.3% of all patients. In those with normal age-adjusted sural SNAPs, SRAR <0.21 offered a sensitivity of 52.1% and specificity of 83.9%, and prolonged lower limb F-wave latency >105% of the upper limit of normal (ULN) offered a sensitivity of 29.2% and specificity of 90%. Independently and in combination, both parameters significantly improved diagnostic yield.
Discussion: The combined use of SRAR <0.21 and/or lower limb F-wave delay >105% of the ULN may be useful in patients with LFSN and normal sural SNAPs.