The sensory action potentials of the tibial nerve at the medial malleolus were studied by averaging in 51 patients with chronic renal failure treated by hemodialysis. Vibratory sense was also tested quantitatively on the dorsum of the foot with a pallesthesiometer. 2. Good correlation was found between sensory tibial nerve potentials and vibration sense in subclinical as well as in clinical uremic polyneuropathy. A biphasic potential correlated with unaffected vibration sense in 18 out of 23 patients, and impaired vibratory sense with a polyphasic response in 20 of 28 patients. Maximal nerve conduction of sensory fibres was faster (mean 37.4 m/sec) in cases with normal vibratory sense, but slower (mean 31.3 m/sec), when vibratory sense was impaired. Furthermore there was a correlation between the threshold of vibratory perception and sensory nerve conduction. 3. Sensory function, tested with conventional methods, was impaired only 5 times in 28 patients with altered vibratory perception. 4. The earlier impairment, especially of the vibratory sense, may be explained by the following neurophysiological mechanisms: a) Because of the polyphasic prolonged response of the sensory potentials, no rhythmical groups of impulses reach the central nervous system, but only a continual stream of small peaks arrives, so that vibration perception does not develop. b) A multiplication of the frequency of discharges caused by alternating firing of different sensory fibres is impossible due to the reduction of the number of axons. c) The prolongation of the relatively refractory period due to demyelinization of the surviving fibres prevents the transmission of frequent impulses. 5. Alterations of the sensory action potentials of the tibial nerve, as well as of vibratory perception tested quantitatively, are earlier signs of uremic polyneuropathy than the prolonged motor nerve conduction velocity. Since not all patients give accurate information when tests of vibratory sense are performed both methods should be applied. Physiological polyphasia of sensory action potentials and diminishing vibration perception in advanced age must be taken into account.
