No single demographic is affected, as there are numerous causes of a phrenic nerve palsy. The epidemiology will therefore match that of the underlying aetiology.
Clearly presentation will depend on the underlying cause. In many cases of unilateral (majority of cases) phrenic nerve paralysis, patients are asymptomatic 7. When presentation is directly attributable to phrenic nerve palsy, then patients typically report dyspnoea and orthopnoea. Respiratory function tests tend to have a restrictive pattern 1.
In cases of bilateral phrenic nerve paralysis, symptoms are not surprisingly more pronounced, but are particularly exacerbated when recumbent 1.
Many cases are idiopathic or thought to be post viral 8,9. Common causes include:
- trauma and iatrogenic
- penetrating injury
- chiropractic manipulation 1
- post operative: especially cardiac: up to 10% of cases 7,9,11
- central venous catheters 2,6
- direct trauma
- compression by haematoma
- local anaesthetic infiltration
- forceps delivery (newborn) 5
- neuromuscular diseases
- direct compression
- aortic aneurysm
- cervical osteophytes
In some cases the diagnosis is obvious. However, as diaphragmatic position is not symmetric, an understanding of the normal level of the diaphragms is important (see normal position of diaphragms on chest radiography). If the left hemidiaphragm is higher than the right or the right is higher than the left by more than ~ 2 centimetres, one of the many causes of diaphragmatic elevation should be sought. This, of course, includes phrenic nerve palsy 10.
CT has little role in the diagnosis of an elevated hemidiaphragm, as supine position and variable inspiratory effort can significantly alter diaphragmatic height. Despite this, it excels at identifying many of the causes of phrenic nerve palsy and is able to acquire volumetric data from base of skull to upper abdomen.
MRI is particularly suited to evaluation of superior sulcus tumours, and better able to determine tumour extension in such cases.
Fluoroscopic examination of the diaphragm ("sniff test") is useful in assessing diaphragmatic function, and involves rapid inspiratory effort and observation of the hemidiaphragms.
In normal subjects both hemidiaphragms descend with inspiration. In cases of phrenic nerve paralysis the affected side demonstrates paradoxical upward movement 10.
Conduction and electromyographic studies
Phrenic nerve conduction studies can be carried out with placement of an oesophageal electrode to record diaphragmatic contractions and stimulation of the the nerve in the neck either with surface stimulation or with a monopolar needle electrode at the level of the cricoid cartilage 7.
Alternatively diaphragmatic electromyography may be carried out. The two tests are complementary 7.
Fluoroscopy may also be employed at the time of conduction studies to confirm diaphragmatic movement.
Treatment and prognosis
In symptomatic unilateral cases, or more often bilateral involvement, diaphragmatic pacing may be used. This can either take the form of distal phrenic nerve stimulation or direct muscular stimulation with implanted electrodes 8.
Prognosis is influenced by the underlying cause. Nerve function does not usually recover in cases of neoplastic involvement. Like other compressive neuropathies, paralysis is due to compressive lesions, gradual improvement may take place, although this often takes many months or even years 1.
The differential of a phrenic nerve palsy is essentially that of an elevated hemidiaphragm with common entities to be considered being:
- masses of collections pushing the diaphragm up from below
- hepatic mass
- subphrenic collection
- intra-abdominal fat
- reduced volume of the lung pulling the diaphragm up
- radiation induced fibrosis
- diaphragmatic eventration
In cases of bilateral diaphragmatic paralysis, in addition to bilateral phrenic nerve palsy a number of conditions should be considered 8:
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