Sensitivity and specificity of diagnostic ultrasound in the diagnosis of phrenic neuropathy

Abstract

Objectives: To determine the sensitivity and specificity of B-mode ultrasound in the diagnosis of neuromuscular diaphragmatic dysfunction, including phrenic neuropathy.

Methods: A prospective study of patients with dyspnea referred to the EMG laboratory over a 2-year time frame for evaluation of neuromuscular respiratory failure who were recruited consecutively and examined with ultrasound for possible diaphragm dysfunction. Sonographic outcome measures were absolute thickness of the diaphragm and degree of increased thickness with maximal inspiration. The comparison standard for diagnosis of diaphragm dysfunction was the final clinical diagnosis of clinicians blinded to the diaphragm ultrasound results, but taking into account other diagnostic workup, including chest radiographs, fluoroscopy, phrenic nerve conduction studies, diaphragm EMG, and/or pulmonary function tests.

Results: Of 82 patients recruited over a 2-year period, 66 were enrolled in the study. Sixteen patients were excluded because of inconclusive or insufficient reference testing. One hemidiaphragm could not be adequately visualized; therefore, hemidiaphragm assessment was conducted in a total of 131 hemidiaphragms in 66 patients. Of the 82 abnormal hemidiaphragms, 76 had abnormal sonographic findings (atrophy or decreased contractility). Of the 49 normal hemidiaphragms, none had a false-positive ultrasound. Diaphragmatic ultrasound was 93% sensitive and 100% specific for the diagnosis of neuromuscular diaphragmatic dysfunction.

Conclusion: B-mode ultrasound imaging of the diaphragm is a highly sensitive and specific tool for diagnosis of neuromuscular diaphragm dysfunction.

Classification of evidence: This study provides Class II evidence that diaphragmatic ultrasound performed by well-trained individuals accurately identifies patients with neuromuscular diaphragmatic respiratory failure (sensitivity 93%; specificity 100%).

© 2014 American Academy of Neurology.

Figures

Figure 1. Transducer position

The ultrasound transducer is positioned so that a linear probe can be placed over one of the most caudal intercostal spaces in approximately the anterior axillary line, spanning 2 ribs.

Figure 2. Recruitment flow diagram

Flow diagram shows patient recruitment and hemidiaphragm ultrasound results.

Figure 3. Ultrasound of the diaphragm

The diaphragm (D) is the 3-layered structure situated deep to the intercostal (IC) muscles that span the 2 ribs. The diaphragm muscle tissue is hypoechoic (dark) on ultrasound, and the 2 layers of connective tissue encasing the muscle (peritoneum and parietal pleura) are hyperechoic (bright) on ultrasound. (A) A normal diaphragm at end-expiration (0.29 cm thick). (B) A normal diaphragm, in the same patient, at maximal inspiration (0.6 cm thick, giving a diaphragm thickening ratio of 2.1). (C) An atrophic diaphragm (0.05 cm thick) in a patient with phrenic neuropathy. R = rib.