Quantitative measurement of sympathetic neuropathy in patients with diabetes mellitus

Vasoconstriction occurs in the skin capillary blood flow of the healthy subject when posture changes from supine to standing. Using frequency analysis of the optical photoplethysmograph signal, a statistically significant difference (P less than 0.01) may be demonstrated between supine and standing positions in the lower frequency band (0.01-0.5 Hz) in the foot of normal subjects. This allowed us to develop a simple index: sympathetic power band change (SPBC). Patients with diabetes mellitus often suffer from degeneration in the sympathetic nervous system. This impairs the normal vasoconstrictor response to standing. We have applied the SPBC 'blind' to a group of diabetic patients. Such patients may be divided into three groups according to their SPBC indices: normals with SPBC greater than 2.6 (group A), intermediates with 2.6 greater than or equal to SPBC greater than or equal to 0.26 (group B) and poor with SPBC less than 0.26 (group C). All patients with retinopathy were in group C and five out of the six patients with electrophysiologically confirmed peripheral neuropathy were in group C. Frequency analysis of the photoplethysmograph signal has produced an index of sympathetic tone change when subjects move from supine to standing position. The application of this index to patients with diabetes mellitus shows some patients to have sympathetic vascular tone failure.