Neuralgic Amyotrophy
Neuralgic amyotrophy (NA) is an uncommon disorder of the peripheral nervous system characterized by the sudden onset of extreme pain in the upper extremity followed by rapid multifocal motor weakness and atrophy and a slow recovery in months to years. NA includes both an idiopathic (INA, also known as Parsonage-Turner syndrome) and hereditary (HNA) form.
Epidemiology
The minimum incidence of NA is estimated at 1/50,000-1/30,000 but under recognition and initial misdiagnosis is common. HNA is thought to be 10 times less common than INA.
Clinical description
NA can occur at any age but is most frequently seen in those between the 3rd-7th decades of life and is more frequent in men. Those with HNA usually present earlier than those with INA but clinically they are virtually indistinguishable. The classic presentation (71% of cases) manifests with sudden onset of aching, burning, or stabbing pains, most often in the shoulders, neck, and/or arm region, showing an upper brachial plexus distribution. Weakness in the periscapular and periglenohumeral muscles follows hours to weeks after the initial onset of pain. These pains are usually relentless, worse at night and last for about 3 weeks. Other manifestations may occur due to the involvement of nerves outside the brachial plexus such as the lumbosacral plexus or phrenic nerves. Certain patients with HNA (with a R88W point mutation) display characteristic physical features (ex. hypotelorism, slanted eyes, epicanthal folds, oval face, cleft palate). Some patients experience a relapsing/remitting course with symptom-free intervals while others have an incomplete recovery with persisting neurologic deficit. Recurrences can occur (75% in HNA and 25% in INA) and persistent musculoskeletal pain develops in 2/3 of patients.
Etiology
The exact etiology is unknown but genetic, autoimmune and external factors are thought to play a role. The brachial plexus has an underlying predisposition to mechanical injury and attacks caused by an immune-mediated response in this nerve. A viral infection or immunization can precede NA. Bacterial and parasitic infections, surgery, anesthesia, rheumatic disease, trauma, pregnancy and childbirth have all been implicated as possible contributing factors of NA attacks. HNA is associated with a point mutation or duplication of the susceptibility gene SEPT9 on chromosome 17q25.3 in 50% of cases.
Diagnostic methods
Diagnosis is based on the typical clinical features and the exclusion of other disorders (ex. neuroborreliosis, cervical radiculopathy or Pancoast syndrome) using laboratory tests, electromyography and imaging of the cervical spine and brachial plexus. In those where HNA is suspected, a molecular genetic test can be used to identify a SEPT9 mutation, but since HNA is genetically heterogeneous, a negative test does not exclude the diagnosis.
Antenatal diagnosis
Antenatal diagnosis is not routinely offered.
Genetic counseling
Genetic counseling is possible in families with the HNA susceptibility gene.
Management and treatment
Treatment in the acute stage involves pain management, relying mostly on a combination of long-acting opioids and nonsteroidal anti-inflammatory drugs. Those with chronic pain can be given co-analgesics. Oral prednisone decreases the duration of pain and accelerates recovery in some patients if given in the first weeks of an attack. A daily dose of 1mg/kg for 1 week which is tapered during the 2nd week is recommended. Follow-up is recommended every 6 months. Rehabilitation therapy is now considered important and excessive strain to the affected area should be avoided.
Prognosis
The prognosis is variable but thought to be good with patients recovering 70-90% of their previous health after 1-2 years. However, many are left with exercise intolerance and poor muscular coordination in the affected and compensating muscles. Quality of life can be affected in those with an incomplete recovery but early diagnosis and proper treatment increase the chances of a full functional recovery.