Criteria for demyelination based on the maximum slowing due to axonal degeneration, determined after warming in water at 37 degrees C: diagnostic yield in chronic inflammatory demyelinating polyneuropathy

Abstract

The diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP) is based on clinical and laboratory results and on features of demyelination found in nerve conduction studies. The criteria that are currently used to reveal demyelinative slowing in CIDP have several limitations. These criteria were only determined in lower arm and lower leg nerve segments, were not defined with respect to nerve temperature, and the relationship with distal compound muscle action potential (CMAP) amplitudes is unclear. The aim of our study was to determine criteria for demyelinative slowing for lower arm and leg segments as well as for upper arm and shoulder segments at a temperature of 37 degrees C, and to assess whether criteria have to be modified when the distal CMAP is decreased. Included were 73 patients with lower motor neuron disease (LMND), 45 patients with CIDP and 36 healthy controls. The arms and legs were warmed in water at 37 degrees C for at least 30 min prior to an investigation and thereafter kept warm with infrared heaters. The proposed criteria for demyelinative slowing were based on the maximum conduction slowing that may occur as a consequence of axonal degeneration and consisted of the upper boundary (99%) or the lower boundary (1%) of conduction values in LMND. In LMND, the maximum conduction slowing was different for arm and leg nerves and for segments within the arm nerves. Moreover, distal motor latency and motor conduction velocity were slower in nerves with distal CMAP amplitudes below 1 mV than in nerves with distal CMAP amplitudes above 1 mV. For these reasons, separate criteria were proposed for arm nerves, for leg nerves and for different segments within arm nerves, and more stringent criteria were proposed for distal motor latency and motor conduction velocity when the distal CMAP amplitude was below 1 mV. The diagnostic yield in CIDP was assessed using the nerve, and not the patient, as the unit of measurement. Thus, whether demyelinative slowing was present was determined for each nerve. Compared with other criteria, our criteria increased the specificity without affecting sensitivity. We conclude that the present criteria, based on the maximum slowing that may occur as a result of axonal degeneration, allow more accurate detection of demyelinative slowing in CIDP compared with other criteria. It should be emphasized that the proposed criteria can only be applied if the method of warming in water at 37 degrees C for at least 30 min is adopted.