Thickening fraction as a measure of ultrasonographic diaphragm dysfunction in amyotrophic lateral sclerosis

Respiratory failure is the most common cause of death in ALS patients secondary to diaphragmatic dysfunction. Herein, we report three ALS patients, and we sought to determine the diaphragm dysfunction by the measurement of ultrasonographic diaphragmatic thickness fraction (DTf).

High-resolution linear US probe of 10 MHz (Philips Healthcare EPIQ 7 Ultrasound System Inc.) was used to measure the diaphragm thickness (DT) using B mode at the Zone of Apposition. Phrenic nerve compound muscle action potential measured stimulating the nerve, posterior to the sternocleidomastoid muscle, approximately 3 cm above the clavicle and recording the diaphragm with electrode G1 placed fingerbreadth above the xiphoid process and electrode G2 placed over the anterior costal margin 16 cm from G1.

The diaphragmatic thickening fraction (DTf) measured in these three patients recorded was less than 15%. Diaphragm dysfunction was also suggested by low amplitude of the diaphragmatic compound muscle action potential in each patient.

Diaphragm dysfunction, secondary to lower motor neuron loss, was mirrored by the low amplitude of the diaphragm CMAP in the 3 patient case reports. These cases suggest that a thickening fraction ≤15% is associated with severe diaphragm weakness and risk of respiratory failure.

Before appropriate data obtained in a population of ALS patients are available, we propose DTf (%) <20% as a possible indicator of diaphragm dysfunction in ALS patients.