Phrenic nerve stimulation in the evaluation of ventilator-dependent individuals with C4- and C5-level spinal cord injury.

Three individuals with C4 or C5 spinal cord injuries (SCI) were seen in follow-up for management of their late complications, which included impaired ventilation. Electrodiagnostic studies were performed on all three as part of the assessment of the function of their phrenic nerves and diaphragm muscles in relation to their need for mechanical ventilator support. Each patient had evidence of lower-motor neuron injury to the phrenic nerves. Two of the patients who initially displayed small-amplitude (<0.1 mV) compound muscle action potentials (CMAP) bilaterally were later reevaluated during the course of their observation in the outpatient rehabilitation clinic. The CMAP amplitude of the diaphragm increased in these two cases during the 3-11 mos after SCI. Evidence of nerve recovery occurred in parallel with improvements in pulmonary function testing and was followed by successful weaning from the ventilator. These individuals both gained ventilator independence after the CMAP amplitude of least one hemidiaphragm was >0.4 mV. In the third case, early failure of ventilator weaning was reported to the patient as a poor prognostic sign. At the time of our first evaluation 11 mos after injury, a CMAP of 1.0 mV was seen on the right, with an absent response on the left. In case 3, the needle electromyogram demonstrated voluntary active motor unit action potentials that provided additional electrophysiologic support for phrenic nerve function. Phrenic nerve-conduction studies can provide useful measures in assessing the recovery of lower-motor neuron diaphragm function in relation to impaired ventilation in individuals with C4- or C5-level SCI.