LESIONS

A Guide to Localisation in Neurology

Case 7. A weak hand on waking

Where is the lesion?

This patient awoke with paralysis of the wrist and of finger movements, though not all of them. She has a wrist drop and finger drop - drop simply refers to a body part hanging down instead of staying in its usual position, due to weakness in extensor muscles. She also has some sensory loss in the dorsal hand.

Weakness in a hand can be due to lesions in various sites, in both the central and peripheral nervous systems. The problem can be localised according to which muscles are affected and the distribution of sensory loss (if present - it isn't always).

The hand is an important part of human anatomy. It is capable of making many different movements, and when it becomes weak, the consequences are very disabling.

Given its importance, and the complicated hand functions we can make with the dozens of muscles involved, there is a distinct area of the motor cortex (precentral gyrus) dedicated to it, visible on axial imaging as a ‘knob’.

Lesions in the this area of the cortex (such as a stroke) can paralyse the hand – but the pattern is global hand muscle weakness. A lesion here would not selectively spare some of the finger movements this patient is still able to make such flexion and abduction, which are totally intact in this patient - while making the others so dramatically weak.

With the exception of the subcortical fibres immediately below the hand area of the motor cortex - which, again, would not tend to only affect the extensors - deeper brain structures are unlikely to produce isolated hand and wrsit weakness because the corticospinal tract travels together with fibres innervating the rest of the arm as well as the leg and face on the same side. For example, an internal capsule posterior limb lesion would not tend to spare the rest of the arm and would often also affect the leg and face.

This is true likewise within the brainstem and the spinal cord – a lesion would not usually cause isolated hand and wrist weakness.

Root pathology is unlikely to selectively paralyse the hand and wrist and spare the other muscles or areas of cutaneous sensation beyond the hand:

• C7 innervates more proximal muscles (e.g. triceps) in addition to finger extensors, and also innervates skin over the dorsal forearm
• C8 innervates finger flexors and skin over the medial hand - so isn't the correct pattern
• T1 innervates finger and thumb abductors and also the skin over the ventral forearm and the medial aspect of the arm above the elbow - not only the wrong weakness pattern but also a sensory zone not affected here

So while unilateral root pathology affecting one or several of the roots at C7-T1 might paralyse parts of the hand, it would also affect other muscles, which aren’t involved here, and a broader area of skin. The retained triceps reflex (C7) is also evidence against a C7 lesion – reflex changes are helpful clues to lesions in certain roots, though only some roots have testable reflexes. Root lesions are also usually painful in the acute phase. Finally, while two consecutive root lesions are sometimes be seen (e.g. a paraspinal collection or dual disc prolapses), three would be unlikely – and again, we’d see other weak muscles and numb regions.

The plexus is another possible site, but similar issues apply as with the roots. There is no obvious site in the plexus that would account for this combination of features, and other, more proximal, weakness would be expected as well as sensory abnormalities. The posterior cord supplies the fibres that become the radial nerve - including those that extend the wrist and fingers - but also the more proximal muscles (e.g. triceps) which are spared here, and in addition the axillary nerve, which is spared, so a lesion here does not fit.

This leaves the peripheral nerves. (Note - a neuromuscular junction or muscle disorder would not acutely present with such focal weakness - and sensation would not be affected.)

Ulnar (finger abduction) and median (finger flexion, thumb abduction) groups are spared, whereas all the muscles affected are innervated by the radial nerve, so a radial nerve lesion seems likely here.

This is a long nerve – the pathway spans the humerus, elbow and forearm until the hand. It gives off many branches on its path, and we can try to localise the affected site based on the weak muscles and numb skin areas.

The muscle groups and sensory area involved in this case are distal, so this is not likely to be a high radial lesion – e.g. at the axilla or around the humeral shaft. High/proximal lesions would affect triceps - with weak elbow extension and reduced/absent triceps reflex - as well as sensation over the posterior arm and forearm via the posterior cutaneous nerves of arm and of forearm respectively. These emerge quite high up, even though they innervate the forearm skin, so only a proximal radial lesion will affect them.

If the lesion were distal to where the triceps supply branches off, but still above the elbow, we would also expect brachioradialis involvement - with some elbow flexion weakness and loss of the ‘supinator’ reflex. This reflex is poorly-named and really tests the brachioradialis, by stimulating near its insertion site in the distal radius. However, there are no signs of a lesion at this level here so the problem must be more distal.

The last muscle innervated by a branch of the radial nerve emerging just above the elbow (above the lateral epicondyle) is the extensor carpi radialis longus (ECRL). Then the nerve crosses the elbow. Just below the elbow the radial nerve innervates the extensor carpi radialis brevis (ECRB).

Then it divides into superficial and deep branches, separating sensory and motor functions:

• The superficial branch, or ‘sensory branch’ - innervating the skin over the dorsal hand and fingers
• The posterior interossueous nerve (PIN), or ‘motor branch’ - a continuation of the deep branch, which travels beneath the supinator and between the ulna and radius (hence interosseous), innervating many extensor muscles

The PIN can be come trapped by the arcade of Frohse, a membranous edge of the supinator, as it travels below this structure. The PIN is a motor-only nerve, so sensation is not affected in lesions. Our patient has numbness, so the lesion must be at a point before this division - but not much higher up.

In addition, the PIN innervates extensor carpi ulnaris (ECU) - but as we’ve just seen, not ECRB or ECRL. In a PIN lesion there is partial wrist extension weakness but not complete – if the patient’s hand is placed on a flat surface and extension is attempted, the wrist deviates radially due to the extensor carpi radialii muscles, which are spared. This patient however has 0/5 power in wrist extension.

In summary - this patient’s weakness and sensory loss sensory is due to a radial nerve lesion around or just above the elbow, but no more proximally - given the sparing of other muscles, and the intact reflexes - and is it clearly proximal to the division into superficial and deep branches, as sensation is affected and wrist extension is totally absent.