Sensation
General
BACKGROUND
There are five basic modalities of sensation (Table 21.1).
The posterior column remains ipsilateral up to the medulla, where it crosses over. The spinothalamic tract mostly crosses within one to two segments of entry (Fig. 21.1).
Figure 21.1 Spinal cord section showing sensory input (blue) from and motor output (black) to the right (R) side
Vibration, joint position and temperature senses are often lost without prominent symptoms.
Light touch and pinprick loss is usually symptomatic.
Sensory examination should be used:
Sensory examination requires considerable concentration on the part of both patient and examiner. Vibration sense and joint position sense are usually quick and easy and require little concentration, so test these first. This also allows you to assess the reliability of the patient as a sensory witness.
In all parts of sensory testing it is essential first to teach the patient about the test. Then perform the test. In most patients you will be confident they have understood and that their responses are reliable. Sometimes you will need to check that the patient has understood and carried out the test appropriately. With all testing, move from areas of sensory loss to areas of normal sensation.
Remember that sensory signs are ‘softer’ than reflex or motor changes; therefore less weight is generally given to them in synthesising these findings with associated motor and reflex changes.
Arms
There are four individual nerves which are commonly affected in the arm. The relevant sensory loss is illustrated in the fingers for the median nerve, ulnar nerve, radial nerve and axillary nerve (Fig. 21.2A–C). There may be loss beyond the core sensory distributions illustrated.
Figure 21.2 A. Sensory loss in the hand: median (blue) and ulnar (black) nerves
B. Sensory loss in the hand: radial nerve
C. Sensory loss in the arm: axillary nerve
The dermatomal representation in the arms can be remembered easily if you recall that the middle finger of the hand is supplied by C7. This is illustrated in Figure 21.3.
Figure 21.3 Dermatomes in the arm
Legs
Individual sensory deficit is most commonly seen in the following individual nerves:
• lateral cutaneous nerve of the thigh (Fig. 21.4A)
• common peroneal nerve (also referred to as the lateral popliteal nerve) (Fig. 21.4B)
The dermatomes most frequently affected are L4, L5 and S1.
A ‘dance’ to help you remember the leg dermatomes is given in Figure 21.5.
Dermatomes
An overview of the root innervation is given in Figure 21.6. The key dermatomes to be remembered are shown in blue.
