Ultrasonographic diaphragm thickness correlates with compound muscle action potential amplitude and forced vital capacity

Abstract

Noninvasive evaluation of respiratory function in patients with various neuromuscular disorders is important for predicting life expectancy. We performed B-mode ultrasonography (USG) and nerve conduction studies in 37 patients (16 had amyotrophic lateral sclerosis; 11, myopathy; and 10, neuropathy) and 10 controls. USG of the diaphragm was performed in the supine position using a linear probe over the intercostal space at the anterior axillary line. Diaphragm muscle thickness was measured at end-expiration. The amplitude of diaphragm compound muscle action potentials (CMAP) was obtained by phrenic nerve stimulation with a surface electrode. Respiratory function was measured with standard pulmonary function tests including forced vital capacity (FVC). Diaphragm thickness was significantly correlated with FVC (r = 0.74) and CMAP amplitude (r = 0.53). Diaphragm USG is useful for objective evaluation of pulmonary function in neuromuscular disorders without requiring undue patient effort or cooperation.