Reduced Phrenic CMAP Amplitudes in Obesity Despite Preserved Diaphragm Ultrasound Measures

Phrenic nerve conduction studies (NCS) are regularly used in the evaluation of neuromuscular respiratory dysfunction; however, the impact of body mass index (BMI) on phrenic compound muscle action potential (CMAP) amplitudes remains poorly characterized. We evaluated the relationship between BMI and phrenic CMAP amplitudes in patients without evidence of neuromuscular diaphragmatic dysfunction.

We performed a retrospective secondary analysis of patients derived from two previously published cohorts from Mayo Clinic (Rochester, Minnesota) and the University of Alberta (Edmonton, Alberta) evaluating diaphragm ultrasound in suspected neuromuscular respiratory insufficiency. Patients without evidence of neuromuscular disease affecting the diaphragm who underwent bilateral phrenic NCS and diaphragm ultrasound were included.

Seventy-nine patients met inclusion criteria, including 36 with BMI ≥ 30 kg/m². Median phrenic CMAP amplitudes were significantly lower in patients with BMI ≥ 30 kg/m² compared to BMI < 30 kg/m² (0.25 mV [IQR 0.10-0.38] vs. 0.50 mV [0.30-0.65], p < 0.001). Low phrenic CMAP amplitudes (< 0.3 mV) were more common in patients with obesity (58% vs. 21%, p < 0.001). BMI demonstrated a moderate negative correlation with phrenic CMAP amplitude (ρ = -0.50, p < 0.001). Diaphragm thickness at functional residual capacity did not differ significantly between BMI groups, while diaphragm thickening ratio was modestly lower in obesity.

Elevated BMI is associated with lower phrenic CMAP amplitudes in patients without evidence of neuromuscular diaphragmatic dysfunction. Reduced phrenic CMAP amplitudes in obesity may reflect technical recording limitations rather than clinically meaningful diaphragmatic dysfunction. Low-amplitude phrenic responses in patients with obesity should be interpreted cautiously and alongside adjunctive assessments such as diaphragm ultrasound.