The phrenic nerve is among the most important nerves in the body due to its role in respiration. The phrenic nerve provides the primary motor supply to the diaphragm, the major respiratory muscle. Phrenic nerve injury, such as may occur from cardiothoracic surgery, can lead to diaphragmatic paralysis or dysfunction. The presentation of phrenic nerve injury is non-specific, and the diagnosis may easily be missed. Phrenic nerve injury can be identified by a number of imaging modalities including ultrasound, electromyography, and fluoroscopy.
Ultrasound is ideal and emerging as a diagnostic test for this condition as the modality is non-invasive, poses no risk of ionizing radiation, is easily reproducible, low cost and may be performed at the bedside. This is especially ideal for critical patients. Ultrasound is more commonly being used to evaluate phrenic nerve injury due to its ability to evaluate the function and structure of the diaphragm. Ultrasound of the diaphragm can help clinicians identify diseases caused by phrenic nerve injury including diaphragmatic paralysis and diaphragmatic dysfunction. Ultrasound can sometimes identify the etiology behind the disease and provide data for determining prognosis. Diaphragmatic ultrasound is also useful in the assessment of prolonged mechanical ventilation and assessment of the failure of weaning from mechanical ventilation. This modality can additionally help select patients for early surgical plication (which can decrease intensive care unit length of stay and ventilator-associated pneumonia).
Injury of the phrenic nerve can occur by multiple mechanisms. One common etiology of phrenic nerve injury is from surgery, primarily thoracic and cardiac surgery. The left phrenic nerve descends anteriorly between the pericardium and mediastinal pleura. Therefore, it can be injured while dissecting near the area of an internal thoracic artery. Canbaz et al. identified that a major factor that causes injury during cardiac surgery is the icy slush that is used for myocardial protection. The phrenic nerve can also be damaged from blunt or penetrating trauma, metabolic diseases like diabetes, infectious causes such as Lyme disease and herpes zoster, direct invasion by tumor, neurological diseases such as cervical spondylosis and multiple sclerosis, myopathy (i.e., muscular dystrophy) and immunological disease (e.g., Guillain-Barre syndrome).
Phrenic nerve injury may present as diaphragmatic dysfunction, unilateral diaphragmatic paralysis, or bilateral diaphragmatic paralysis. One of the most frequent causes of unilateral diaphragmatic paralysis is iatrogenic. In bilateral diaphragmatic paralysis, one of the most common causes is a motor neuron disease, including amyotrophic lateral sclerosis and post-polio syndrome.
Few studies have shown that diaphragmatic paralysis occurs more often in male individuals. Scharf et al. found that 1% to 7% of patients with significant blunt trauma present with a diaphragmatic injury. Up to 3% of abdominal injuries also involve the diaphragm.
The diagnosis of phrenic nerve injury requires high suspicion due to nonspecific signs and symptoms including unexplained shortness of breath, recurrent pneumonia, anxiety, insomnia, morning headache, excessive daytime somnolence, orthopnea, fatigue, and difficulty weaning from mechanical ventilation. In unilateral diaphragmatic paralysis, the patient is often asymptomatic at rest and has dyspnea only during exertion. This unilateral diaphragmatic paralysis is typically found incidentally on chest radiograph. In comparison, patients with bilateral diaphragmatic paralysis always present with shortness of breath.
On physical examinations, findings may include decreased breath sounds on the affected side, dullness to percussion of the affected side of the chest and inward movement of the epigastrium during inspiration.
Phrenic nerve injury can be evaluated with multiple modalities, for example, esophageal and gastric manometry, diaphragmatic electromyography or ultrasound. Diaphragmatic ultrasound is primarily performed with a low-frequency curvilinear transducer. A high-frequency linear transducer is the best choice for an intercostal view since it has a small footprint that can fit between the ribs.
Patients are typically examined in the supine position during spontaneous inspiration and expiration. The pleural and peritoneal membranes should be visualized during imaging of the diaphragm for thickness measurements. The supine position limits any compensatory active expiration by the anterior abdominal wall, which may mask paralysis.
On ultrasound, the diaphragm appears as a thick echogenic line. The left hemidiaphragm can be evaluated through the splenic window. The right hemidiaphragm is visualized through the liver window. Interestingly, pathologic conditions such as a left upper quadrant mass, splenomegaly or hepatomegaly may make an evaluation of the left hemidiaphragm easier. Diagnostic criteria for diaphragmatic paralysis include paradoxical movement or significantly decreased diaphragmatic excursion.
Diaphragm thickness and thickening during respiration can be assessed with ultrasound, typically with a high-frequency linear transducer. The ultrasound transducer is placed at the anterior axillary line between the seventh and eighth or eighth and ninth ribs, which is the Zone of Apposition (ZOA). Diaphragm thickness can be used to assess for atrophy, especially as might occur while on mechanical ventilation. Diaphragmatic thickness values less than 0.2 cm at the end of expiration are considered to define diaphragmatic atrophy. In healthy volunteers, the average thickness of the diaphragm is typically between 0.22 to 0.28 cm, however in a paralyzed diaphragm, the diaphragmatic thickness is reported to be less, between 0.13 to 0.19 cm. The Thickening Fraction can also be calculated and used as a predictor of success of weaning from mechanical ventilation.
Most patients with asymptomatic unilateral diaphragmatic paralysis do not require treatment. When identified, the underlying cause should be treated. Surgical options are considered if the underlying cause is treated and the patient still has symptoms, or if the patient has bilateral diaphragmatic paralysis. There are various treatment options including plication and phrenic nerve stimulation. Plication of the affected site is a very useful treatment method that allows weaning from mechanical ventilation. Plication is preferably performed in unilateral diaphragmatic paralysis in non-morbidly obese patients. Phrenic nerve stimulation is performed in intact phrenic nerve without evidence of myopathy. This procedure can be performed in patients with bilateral diaphragmatic paralysis with cervical spine injuries.
Since the symptoms of phrenic nerve injury are not specific, we have to consider all causes of shortness of breath including pulmonary, cardiac, metabolic, and hematologic causes. In bilateral diaphragmatic paralysis, the following additional diagnoses should be considered: anterior horn cells and neuromuscular junction diseases. Hypoventilation secondary to cervical spine disease is also difficult to differentiate from phrenic nerve dysfunction.
The mortality and morbidity of phrenic nerve injury or diaphragmatic paralysis depend on the underlying causes and status of pulmonary function. Generally speaking, unilateral diaphragmatic paralysis has a good prognosis unless the patient is experiencing severe shortness of breath. If recovery occurs in bilateral diaphragmatic paralysis, it usually takes more than one year for partial or full recovery.
There is potentially significant mortality and morbidity associated with phrenic nerve injury. Recovery is heavily dependent on an interprofessional effort of nurses, respiratory therapists, and clinicians educating and motivating the patients while providing quality care.
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