Phrenic nerve examination in healthy individuals and patients with acute acquired polyneuropathy

Diaphragm is the most important muscle of respiration which consists of a muscle layer in the periphery and a central tendon in the middle and separates the abdominal and thoracic cavities. The posterior part of the diaphragm, called crural part, also plays an important role in the mechanism of esophageal reflux. Its motor innervation is provided by the phrenic nerve originating from the C3-C4-C5 nerve roots. Electrophysiological examination of the phrenic nerve is used for the investigation of post-surgical phrenic nerve injury, neuromuscular diseases, and lung diseases. Although there are a limited number of research to determine the normal values of phrenic nerve conduction, the points of stimulation and recording also differ in the publications. Recording from the posterior parts of the diaphragm, which has been claimed to have an important role in gastroesophageal functions, is not a well-established method to study with superficial electrodes, and measurements with a gastroesophageal catheter are difficult to tolerate for the patients. One of the most commonly known neuromuscular diseases affecting the diaphragm is GBS. GBS is an acute inflammatory polyradiculopathy of dysimmune origin, which is the most common cause of neuromuscular paralysis, with symmetrical arm and leg weakness, decreased reflexes and sensory findings. Electrophysiological examinations play an important role in diagnosing the disease, determining its prognosis, distinguishing between axonal and demyelinating features and determining the presence of respiratory involvement. In the present study, it was aimed to determine the followings: the most ideal stimulation and recording sites for the diaphragm studies, normal limits for the electrophysiological parameters obtained from Turkish people, and the recordability of the posterior part of the diaphragm with superficial electrodes. With these data, the correlation of diaphragmatic conduction with clinical findings and respiratory involvement in patients with GBS and the importance of phrenic nerve conduction in GBS subtype determination would be investigated. Sixty-three healthy participants and 23 patients with GBS were recruited into our study. Compound muscle action potentials (CMAP) were recorded by 2 separate channels to record from the anterior part of the diaphragm and by 2 other channels to record from its posterior part, in healthy participants. A pair of recording electrodes was placed over biceps brachii muscle to monitor the contribution of brachial plexus simultaneously during stimulation behind sternocleidomastoideus muscle. The traces with indiscernable motor responses from the latter was admitted for evaluation. In the patient group, examinations were performed bilaterally in the initial electrode position. Phrenic nerve is stimulated using a bipolar stimulator at the posterior edge of sternocleidomastoideus muscle. At the initial electrode position which provided the highest amplitude responses, the upper limit of phrenic nerve latency was 7.7 milliseconds (ms), the lower limit of base to peak amplitude was 0.30 millivolts (mV), the lower limit of peak to peak amplitude was 0.45 mV, the upper limit of negative peak duration was 24.15 ms and lower limit of area was 3.05 mV*ms. The posterior part of diaphragm provided lower amplitude and longer latency responses as compared to the anterior part. Negative peak durations and areas of the phrenic nerve motor responses were significantly correlated with clinical and respiratory parameters in the patients. If the upper limit of phrenic nerve response latency was taken as 110%, acute inflammatory demyelinating polyneuropathy (AIDP) subtype of GBS group could be differentiated with 84.6% sensitivity and 87.5% specificity with nerve conduction studies. According to the present study, we recommend to use the points 5 cm above the xiphoid process (active) and 16 cm laterally (reference) on the costal margin, as the recording sites in the phrenic nerve conduction studies. It was also shown that posterior part of the diaphragm can be recorded with superficial recordings. It is recommended that phrenic nerve conduction studies be added to routine electrophysiologic examinations in patients with GBS, because of the good correlation of electrophysiological data with clinical and respiratory parameters and their high specificity and sensitivity in distinguishing GBS subgroup. This study was funded by Scientific Research Projects Coordination Unit of Istanbul University. Project number: TTU-2020-36952