Phrenic Nerve Conduction Study: Not New but Improved
Abstract & Commentary
By Michael Rubin, MD Professor of Clinical Neurology, Weill Cornell Medical College, New York, NY Dr. Rubin reports that he is involved with grant/research support for Pfizer, and that he is on the speaker's bureau for Athena.
Synopsis: Phrenic nerve conduction studies are technically difficult, but are useful in the evaluation of patients with diaphragmatic paralysis.
Source: Resman-Gaspersc A, Podnar S. Phrenic nerve conduction studies: technical aspects and normative data. Muscle Nerve 2008;37:36-41.
All too often, diaphragmatic motor responses recorded following phrenic nerve stimulation are unobtainable, poorly reproducible, corrupted by concomitant brachial plexus stimulation, or demonstrate a compound muscle action potential of reversed polarity, with a positive rather than a negative peak. Can these difficulties be overcome or eliminated? Among 29 normal volunteers, 15 men and 14 women, ages 21 to 65 years, bilateral phrenic motor nerve conduction studies were performed using various stimulation sites, adjacent to the sternocleidomastoid (SCM) muscle and along the supraclavicular fossa, to determine which position resulted in a maximal, most reproducible result, at minimum stimulation strength. Motor responses were recorded using a fixed site, with the active, G1 electrode 5 cm above the xiphoid process, and the reference, G2 electrode 16 cm from G1, along the lower costal margin, ipsilateral to the side of stimulation. Measurements were obtained separately during inspiration and expiration. Supramaximal stimulation strength, motor response amplitude, onset latency, duration, and area of the negative phase were measured. Statistical analyzes included both parametric and non-parametric methods, using the paired t-test, Pearson's correlation coefficient, and mathematical transformations. Upper and lower limits were set at the mean +/- 2 standard deviations.
Bipolar surface stimulation, using firm pressure in the supraclavicular fossa between the 2 heads of the SCM muscle, yielded reproducible results using the lowest stimulation strength and without brachial plexus contamination. Momentary neck flexion facilitated observation of these two SCM heads, permitting better localization of the optimal stimulation site between the heads. Occasionally, stimulation at the posterior border of the SCM yielded a better response. Compound muscle action potential amplitude and duration, but not latency or area, varied with respiration, with 8 ms the upper normal limit for latency and 4.4 mVms the lower normal limit for area. The lower normal limits were 0.46 and 0.33 mV for amplitude on inspiration and expiration, respectively. Future phrenic nerve studies may benefit by recording motor response latency and area, rather than amplitude, as the former do not vary with respiration. Meanwhile, modification of stimulation and recording sites will improve routine amplitude measurements as presently performed.
Commentary
Phrenic neuropathy may improve with topiramate. Diaphragmatic paralysis developed in a 57-year-old type 2 diabetic, who presented with a year-long history of stable orthopnea. When peripheral neuropathy was subsequently diagnosed and treated with topiramate, orthopnea and neuropathic symptoms improved within 26 weeks. Regrowth of intraepidermal nerve fibers was demonstrable on skin biopsy. Topiramate may be effective for the treatment of phrenic neuropathy and diaphragmatic paralysis.1
Reference
1. Rice AL, et al. Reversal of phrenic nerve palsy with topiramate. J Diabetes Complications 2007;21:63-67.