LESIONS

A Guide to Localisation in Neurology

Case 7. A weak hand on waking

What is the lesion?

The differential for mononeuropathy includes extrinsic compression (neurapraxia), damage from fractures and crush injuries, entrapment (e.g. against ligaments and muscle borders), tumours in the nerve (e.g. schwannoma) or around them, infections, and vascular damage (box )

Most of the time individual peripheral nerve lesions (mononeuropathies) are compressive. Nerves require soft padding, and prolonged pressure on them can cause damage. Our patient slept in a chair then woke with radial nerve weakness, and this is very suggestive of compression.

The classic cause is Saturday night palsy, after someone falls asleep in a chair after a night of drinking alcohol, with their arm hanging over the back of the chair, compressing the radial nerve – whether in the axilla, against the posterior humerus, or at the elbow. In this case there was no alcohol involved but her arm had been on the hard armrest of the chair all night, which will have caused compression around the elbow. Similar is seen in people who spend much of their time in wheelchairs with hard armrests – and can complicate pre-existing neurological conditions such as stroke (causing fear of a further stroke). It can even been seen in healthy people who fall asleep during air travel, even without alcohol on board!

Weight loss, which this patient has had, is a risk factor for compressive radial palsy, exposing the nerves to compression between external hard objects and bony eminences. This is also the case for other compressive nerve palsies, especially common peroneal nerve – the so-called slimmer’s palsy ). Weight gain, on the other hand, is a risk factor for entrapment-related palsies, for example of the sciatic nerve or lateral cutaneous nerve of the thigh under the inguinal ligament (meralgia paraesthetica).

Concerning the other causes listed above, they don't sound likely.

Fracture seems unlikely - there is no history of trauma, even low-impact - elderly people often have osteoporosis so minor injuries can cause fracture. There is also no pain, and fractures are usually painful - bars in people with pre-existing neurological damage, particularly neuropathy, who can sustain fractures with no awareness.

Entrapment is a major aetiology in some nerve lesions - the commonest mononeuropathy, carpal tunnel syndrome, is due to entrapment - but not usually in a radial lesion such as this, as the likely location is not within a specific vulnerable site. This is unlike the PIN, or the superficial radial nerve near the wrist - compression of which causes a painful condition called cheiralgia paraesthetica).

A tumour would not tend to present so abruptly, and even though she has a degree of weight loss which might raise the question of malignancy, it would more commonly point to a systemic cancer than a regional one in the limb.

Nerve infection is not likely - this case happened in Britain and the patient had minimal prior travel history of note.

Vasculitis is unlikely but not impossible - it is usually very painful, due to infarction within the nerve. However, it would be sensible to check basic bloods to rule out an inflammatory response.

In summary - weight loss has likely predisposed to compression palsy of the radial nerve, with a night sleeping in the chair as the trigger.