The laterality index in the evaluation of distal sensorimotor neuropathy

Electrodiagnostic evaluation of distal sensorimotor neuropathy can be technically challenging. Conventional nerve conduction studies (NCS) often include unilateral upper and lower limb evaluation for patients with suspected sensorimotor peripheral neuropathy. For patients with predominantly lower extremity complaints, NCS of both lower limbs are performed occasionally. Side-to-side NCS parameters have not been adequately addressed.

We performed NCS prospectively on 132 patients presenting with complaints of bilateral numbness or weakness in the extremities and 45 normal controls. The laterality index (LI) was defined as ratio of the smaller to the larger amplitude in a sensory or motor NCS, as a novel side-to-side comparison parameter.

Ten patients had at least 1 sensory or motor NCS with LIs exceeding that of normal controls. Patients 1-7 (group 1) had diagnoses of autoimmune or neoplasia-related conditions and all had 3 or more NCS with abnormal LIs. In contrast, patients 8-10 (group 2) had diabetes mellitus and all had 2 or less NCS with abnormal LIs (unpaired t test; P = 0.002). In group 1, patients 1-4, 6, and 7 all had both upper and lower limb LI abnormalities. In contrast, only 1 patient (patient 9) had upper limb LI abnormality in group 2.

Patients with abnormal LIs were uncommon in our cohort. Patients with diabetes mellitus had significantly less abnormal LIs than those with autoimmune or neoplasia-related etiologies. There is evidence of vasculitic inflammation documented in diabetic neuropathy. However, this process may be significantly less prominent compared with the former 2 conditions. Our findings of predominantly lower limb LI abnormalities suggest the presence of a length-dependent distal neuropathic process in diabetes mellitus. The LI is a simple and useful adjunct to detect asymmetrical NCS abnormalities. It may also provide helpful information regarding the underlying etiology.