Mechanism of Diaphragm Weakness in Parsonage-Turner Syndrome
Parsonage-Turner syndrome causes diaphragm weakness through direct autoimmune-inflammatory injury to the phrenic nerve, resulting in phrenic nerve palsy and subsequent diaphragmatic paralysis that can be unilateral or bilateral. 1, 2
Pathophysiological Mechanism
Direct Phrenic Nerve Involvement
The phrenic nerve is directly affected in at least 7% of Parsonage-Turner syndrome cases through the same autoimmune-inflammatory process that targets the brachial plexus. 3 This represents a multifocal neuropathy that predominantly affects proximal nerve segments.
The autoimmune response produces focal scarring along the affected phrenic nerve, creating what is termed an "hourglass constriction" that causes focal compression and prevents spontaneous nerve recovery. 4
This nerve injury occurs anywhere along the phrenic nerve course from the cervical spine (C3-C5 nerve roots) to the diaphragm, resulting in impaired neural transmission to the diaphragm muscle. 4
Clinical Presentation Pattern
Patients typically present with dyspnea that follows (rather than accompanies) the characteristic severe shoulder and upper limb pain of Parsonage-Turner syndrome. 1, 2
Neurological examination of the upper limb may be completely normal at the time of respiratory symptom presentation, making the diagnosis easily missed. 1 This occurs because the phrenic nerve involvement can be isolated or the upper extremity symptoms may have already resolved.
The diaphragmatic paralysis can manifest as unilateral, bilateral, or even recurrent on alternating sides. 2
Diagnostic Confirmation
Pulmonary Function Testing Findings
Forced vital capacity demonstrates characteristic reduction between supine and sitting positions (mean change 23%, range 22-46%), which is pathognomonic for diaphragmatic dysfunction. 1
Maximal inspiratory pressures are reduced (mean 61% predicted, range 43-86%), while maximal expiratory pressure remains preserved (mean 107% predicted, range 83-130%). 1
Sniff nasal inspiratory pressure is reduced (mean 88 cm H2O, range 66-109 cm H2O). 1
Objective Testing
Phrenic nerve conduction studies confirm phrenic nerve palsy and distinguish this from primary diaphragm muscle disease. 1, 2
Diaphragmatic fluoroscopy or ultrasound demonstrates unilateral or bilateral diaphragmatic paralysis with paradoxical movement. 1
Needle electromyography of the diaphragm documents denervation changes consistent with phrenic nerve injury. 2
Critical Clinical Pitfall
The diagnosis of Parsonage-Turner syndrome as the etiology of diaphragmatic weakness is frequently overlooked because clinicians fail to elicit the history of preceding severe ipsilateral shoulder and upper limb pain, particularly when neurological signs in the upper extremity have resolved by the time respiratory symptoms develop. 1, 3 This results in up to 20% of diaphragmatic pareses being incorrectly classified as "idiopathic" when they are actually due to unrecognized neuralgic amyotrophy. 3
Management Implications
Conservative management is strongly advocated over surgical intervention (such as diaphragmatic plication) because most patients demonstrate gradual spontaneous resolution over 6 months to 3 years. 1
Early-stage corticosteroid therapy should be considered when the diagnosis is made acutely, though this opportunity is often missed due to delayed recognition. 3
Surgical repair with internal neurolysis of hourglass constrictions may be considered only in patients who fail to recover spontaneously after an adequate observation period. 4
Follow-up phrenic nerve conduction studies may remain abnormal for up to 4 years even with clinical improvement. 2