Which combined nerve conduction study scores are best suited for polyneuropathy in diabetic patients?

Abstract

Introduction/Aims: Nerve conduction studies (NCS) are widely used in diagnosing diabetic polyneuropathy. Combining the Z scores of several measures (Z-compounds) may improve diagnostics by grading abnormality. We aimed to determine which combination of nerves and measures is best suited for studies of diabetic polyneuropathy.Methods: Sixty-eight patients with type 1 diabetes and 35 controls were included in this study. NCS measurements were taken from commonly investigated nerves in one arm and both legs. Different Z-compounds were calculated and compared with reference material to assess abnormality. A sensitivity proxy, the accuracy index (AI), and Cohen's d were calculated.Results: Z-compounds with the highest AI consisted of the tibial and peroneal motor, and the sural, superficial peroneal, and tibial medial plantar sensory nerves in one or two legs. All Z-compounds were able to discriminate between diabetic subjects and nondiabetic controls (mean Cohen's d = 1.42 [range, 1.03-1.63]). The association between AI and number of measures was best explained logarithmically (R 2 = 0.401), with diminishing returns above approximately 14 or 15 measures. F-wave inclusion may increase the AI of the Z compounds. Although often clinically useful among the non-elderly, the additional inclusion of medial plantar NCS into Z-compounds in general did not improve AI.Discussion: Performing unilateral NCS in several motor and sensory lower extremity nerves is suited for the evaluation of polyneuropathy in diabetic patients. The use of Z-compounds may improve diagnostic accuracy in diabetic polyneuropathy and may Abbreviations: AI, accuracy index; NCS, nerve conduction study; PN-NCS, polyneuropathy as indicated by nerve conduction study; Z-compound, aggregate of the Z scores of certain NCS measures.

Combining multiple measures into a summary index: A step toward more reliable measurement

“…The article by Dunker and colleagues 1 in this issue of the Journal, in which they combine multiple nerve conduction study (NCS) measures into a single summary variable (Z-compounds), is a continued step forward in developing a more reliable approach toward detecting and quantifying polyneuropathy. This approach, which has been used by others as well, [2][3][4] is attractive for a number of reasons.…”

“…Another limitation is that determining the Z score, particularly when using the appropriately sophisticated transformations described by Dunker et al, 1 requires a robust reference data set and relevant software and statistical capacity, which may not be available in all electrodiagnostic laboratories. For instance, the use of Z scores requires a normal, Gaussian, or bell‐shaped distribution, which is not typically present in NCS data 7,9 .…”