MRI of the brachial plexus: what it shows and how it is used

The brachial plexus is a network of nerves that begins as five large nerve roots, C5, C6, C7, C8 and T1, leaving the spinal cord between the bones of the neck. The Scottish National Brachial Plexus Injury Service explains that these roots mix together in the neck and divide into branches that pass under the collarbone, into the armpit and along the arm. The upper roots, C5 and C6, are those most commonly affected in an injury. When a nerve is injured, the muscles it controls become weak even though the muscle itself is not damaged.

Doctors work out which roots have been hurt mainly by examining the pattern of weakness and sensory loss. Imaging adds information about the roots, the spinal cord and the surrounding structures, and helps plan whether and when surgery should be considered.

The Scottish service's adult guidelines, written for referring clinicians, list risk factors for brachial plexus injury: high-velocity road traffic collisions, especially motorbike; fracture or dislocation of the shoulder, scapula or elbow; open or penetrating injury to the neck, upper trunk or arm; arterial injury in the upper arm; and traction injury to the upper limb. Signs include swelling above or below the collarbone, Horner's sign, severe pain in the upper limb, paralysis and sensory loss.

The guideline lists chest and cervical spine radiographs as mandatory initial investigations, with MRI of the cervical spine or CT myelography as optional tests. It says both are useful for root avulsions but that neither is 100 percent accurate, and that MRI is easier to perform early after injury. Neurophysiology, which tests nerve function, is not usually helpful in the acute situation.

It also explains that where the injury follows shoulder dislocation or other shoulder trauma, the nerve damage is usually below the clavicle, so MRI of the cervical spine is unnecessary. MRI of the shoulder is more useful in that setting, particularly for diagnosing associated rotator cuff tears which may warrant early repair.

The service's protocol says high-resolution imaging of the cervical spine from C3 to T2 is the priority, because MRI is most useful for defining root avulsions or spinal cord damage, and imaging further out along the plexus is harder to interpret. Its current protocol combines MRI of the cervical spine with MRI of the brachial plexus, using several image types.

The limits matter. The same document states neither MRI nor CT myelography is fully accurate. An NHS research summary from Leeds Teaching Hospitals describes a study of a new MRI technique that aims to diagnose severe nerve injury so that exploratory surgery could be avoided and reconstructive surgery performed sooner. Until such methods are established, a normal-looking scan does not entirely exclude nerve injury, and clinical examination and nerve testing remain essential.

The NHS says an MRI scan is done in hospital by a radiographer, usually takes between 15 minutes and 1 hour but can take longer, and involves lying flat on a bed that moves through a tunnel. You are given ear plugs or headphones because the scanner is loud, and you need to lie still. The radiographer can hear and speak to you from another room. You may be asked to remove metal items, change into a gown if your clothes have metal, and complete a safety form.

Because a brachial plexus scan includes the neck and shoulder, lying still can be uncomfortable if you are in pain or have a weak arm. Tell the department about this beforehand so that support and positioning can be arranged. Some scans use a contrast injection; the NHS says allergic reactions are rare and you may be asked to wait about 30 minutes afterwards.

• Bring a list of your medicines and details of any implants or previous surgery.

• Say if you cannot lie flat for long because of pain or breathlessness.

• Tell the team about claustrophobia, as support or medication may be possible.

• Remember that you should not drive for 24 hours after a relaxing medicine.

The NHS says images are examined by a radiologist, who writes to the doctor who referred you, and results usually take 1 to 2 weeks. For people with a suspected major nerve injury the report is often prioritised, but the timing depends on the local service. A specialist then combines the scan with clinical examination and nerve testing to decide on treatment.

The Scottish service describes a multidisciplinary approach that includes assessment, surgical reconstruction, physiotherapy, occupational therapy, nurse specialists and counselling. Outcome depends on the type and severity of the injury, because, as the service explains, nerves outside the spinal cord may be able to repair themselves after injury depending on how bad the damage is.

Seek urgent medical help through NHS 111 or A&E for any sudden loss of strength or feeling in the arm, particularly after an injury.

The Scottish National Brachial Plexus Injury Service describes itself as a specialist multidisciplinary service for Scotland and the UK. It accepts referrals from medical professionals, preferably after the patient has been seen by an orthopaedic surgeon, plastic surgeon or paediatrician. Initial scans are usually arranged by the local hospital team. Private MRI is also available in the UK, but decisions about nerve surgery are made by a specialist team.