Neurogenic thoracic outlet syndrome

Neurogenic thoracic outlet syndrome is the most common cause of thoracic outlet syndrome accounting for approximately 90% of cases. It is caused by compression of the brachial plexus as it passes between the scalenus muscles, over the first rub and posteroinferior to the clavicle on its way to the axilla. 

Epidemiology

Prevalence of neurogenic thoracic outlet syndrome has been estimated as high as 8% of the population and most frequently seen in women (female to male ratio 4:1) ². Most often it presents in adulthood (30 - 40 years of age is typical) and it is rare in childhood ². 

Clinical presentation

Patients present with pain around the shoulder girdle, extending into the arm and even up into the neck and/or face ^1,2. The distribution of pain will depend on which part of the brachial plexus is affected; upper thoracic outlet syndrome (C5-C7) is more proximal, whereas lower thoracic outlet syndrome (C8 - T1) involves the hand ².

In the vast majority of cases, no objective findings can be identified and these are referred to as the painful neurological form of thoracic outlet syndrome. In a very small proportion of cases, muscular atrophy may be present, typically of the thenar eminence, and later, other intrinsic muscles of the hand ¹. Some authors have termed this "true" neurogenic thoracic outlet syndrome ¹. 

Radiographic features

As the vast majority of cases of neurogenic thoracic outlet syndrome have no anatomical abnormalities, imaging is primarily used to exclude unexpected and rare mechanical causes of compression as well as to exclude other potential causes of pain (e.g. foraminal stenosis). 

Plain radiograph

Chest radiography is typically performed to exclude an underlying bony abnormality.

CT and MRI

Both CT and MRI are able to assess regional anatomy. Specific anatomical features to be sought include:

• cervical rib
• hypertrophic C7 transverse process
• first rib callus, exostosis or osseous lesion
• fibrous band
• muscle insertional variation
• supernumerary muscles

It is essential to remember, however, that identifying an anatomical abnormality (e.g. a cervical rib) in a patient with painful neurogenic thoracic outlet syndrome does not mean that it is the cause ¹. 

Treatment and prognosis

Preventative changes and rehabilitation remain the mainstay treatment for most cases of neurogenic thoracic outlet syndrome, focusing on reducing risk factors (e.g. occupational health and safety), orthoses, physiotherapy etc... 

Injection of anaesthetic agents or botulinum toxin into the anterior scalene muscle can also be of benefit ². 

Surgical decompression is controversial ¹. Options depend on the site of postulated compression. Resection of the first rib, cervical rib and scalenectomy are all performed using a variety of approaches. Success rate varies and improvement, if attained, is not always permanent, with 15-30% recurrence of symptoms ^1,2. In addition to the usual surgical risks, post-operative reflex sympathetic dystrophy is also encountered ¹.