use sniff test to diagnose diaphragmatic paralysis


This patient's presentation suggests unilateral diaphragmatic paralysis likely due to phrenic nerve injury during cardiac surgery.  The phrenic nerves originate from C3-C5 to innervate the diaphragm on each side.  Unilateral diaphragmatic paralysis is more common than bilateral paralysis and is usually due to the following:

  • Phrenic nerve injury during cardiac surgery
  • Viral infections (eg, herpes zoster, poliomyelitis)
  • Other (eg, trauma, tumor, cervical spondylosis, neck surgery)
  • Idiopathic

Patients with unilateral diaphragmatic paralysis and no underlying lung disease are usually asymptomatic at rest but can develop dyspnea with exertion.  Patients with underlying lung disease may report dyspnea at rest, orthopnea, or sleep-disordered breathing.

Chest x-ray usually reveals elevated hemidiaphragm.  However, this can be missed in some postoperative patients due to other changes (eg, pleural effusions, infiltrates, atelectasis).  Diagnosis is confirmed by asking patients to sniff forcefully under fluoroscopy (sniff test).  The normal diaphragm moves downward during inspiration and the paralyzed side has paradoxical upward movement.

Spirometry shows a restrictive defect (low forced vital capacity), but the normal decrease of <10% in the supine position is exacerbated and may fall > 50%.  This results from the cephalad displacement of the abdominal viscera.  Patients may also have supine hypoxemia, but hypercarbia is rare in the absence of lung disease.

Asymptomatic patients with unilateral diaphragmatic paralysis do not require treatment.  Many patients with mild-to-moderate dyspnea may improve spontaneously within 1-2 years.  However, those with significant impairment may benefit from ipsilateral surgical diaphragmatic plication.

  • measurement of supine to sitting vital capacity change can detect diaphragmatic dysfunction
  • measurement of muscle strength can screen for diaphragmatic weakness1

The patient has right hemidiaphragm dysfunction or paralysis as is suggested by the chest radiograph showing an elevated right hemidiaphragm and confirmed by dynamic imaging of the diaphragm. In the past, this was done by fluoroscopic visualization during a brisk inspiratory effort ("sniff test") demonstrating paradoxical movement of the right diaphragm cephalad as opposed to the normal downward descent of the diaphragm on inspiration. Currently, this is performed by point-of-care ultrasound imaging of diaphragm thickening (choice B is correct). Injury to her right phrenic nerve is likely a sequela of the surgical decortication, although the right-sided pneumonia and pleural space infection could also be contributors. Diaphragm weakness or paralysis is an underappreciated cause of dyspnea and tracks with the onset of exercise limitation in this patient following her thoracic surgery. The shortness of breath is often worse in the supine position. In addition to surgery and trauma, other causes of diaphragm dysfunction include neuromuscular diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, myasthenia gravis and myopathies, medications (glucocorticoids) and toxins, hyperinflation from obstructive lung diseases, and the effects of critical illness such as mechanical ventilation and critical illness polyneuropathy.

While the chest radiograph is very sensitive for detecting unilateral diaphragm paralysis, it is not specific and may not be helpful if bilateral diaphragm dysfunction is present because the chest radiograph might simply suggest "poor inspiratory effort" or "small lungs." Additional testing that would further elucidate this diagnosis includes measurement of respiratory muscle strength, which is reduced in diaphragm weakness and paralysis (her maximum inspiratory pressure was −30 cm H2O and maximum expiratory pressure was +37 cm H2O), but while this confirms that respiratory muscle weakness is present, it does not delineate the etiology (choice C is incorrect). Upright and supine spirometry with a significant fall in the FVC in the supine position >20% (her FVC fell 21% when she went from sitting to supine) is especially useful in assessing the presence of bilateral diaphragm paralysis. In this setting, fluoroscopy is not helpful because it depends on the paradoxical movement of the dysfunctional diaphragm, which will not be seen in bilateral diaphragm disorders. Both for suspected bilateral diaphragm dysfunction and easy availability, direct ultrasound visualization of both diaphragms looking for normal diaphragm thickening with inspiration, as opposed to lack of thickening if weakness is present, is now the most commonly used diagnostic test. Phrenic nerve electromyography showed a right phrenic nerve neuropathy in this patient, but its role in diagnosis is limited by availability and technical issues.

Bubble echocardiogram is not indicated; while she has incompletely treated hypertension, there is no indication of additional cardiac disease or unexplained hypoxia (choice A is incorrect). The right lung nodular opacity required follow-up on serial imaging but was consistent with residual scar from her chest infection, so bronchoscopy is not indicated (choice D is incorrect).

Because unilateral diaphragm paralysis is an unlikely solitary cause of significant dyspnea and exercise limitation, it is likely her obesity and hypertensive heart disease contributed to her respiratory symptoms. In addition to obesity, concomitant cardiopulmonary diseases are thought to be associated with dyspnea in this setting and can be investigated by cardiopulmonary exercise testing. She has improved over time with control of her BP and with weight loss and has benefitted from the institution of nocturnal CPAP for OSA, which is seen in patients with diaphragm dysfunction. Improvement in her pulmonary function and walk tests have occurred with regeneration of phrenic nerve function as occurs in some patients over time. Diaphragm plication (only effective in unilateral diagram dysfunction) or pacing can be considered in severe cases, but she was uninterested in additional surgery.2

Footnotes

  1. Murray and Nadel Ch 136 Noninvasive Support of Ventilation

  2. SEEK Questionnaires