Figure 4.10 summarises principal patterns of sensory loss.
Peripheral Nerve Lesions
A lesion of an individual nerve produces symptoms and signs within its distribution. Demarcation is clear‐cut. Areas of sensory loss are discussed in Chapter 10. The quality of sensory disturbance varies between numbness, tingling and painful pins and needles. Painful tingling in the distribution of a damaged nerve when it is percussed, is known as a positive Tinel’s sign, for example in some carpal tunnel cases.
Neuralgia (Chapter 23) describes severe pain in the distribution of a nerve or root. In trigeminal neuralgia (tic douloureux; Chapter 13), the paroxysmal nature of pain, and its distribution are diagnostic.
Causalgia (Complex Regional Pain Syndrome, Chapter 23) describes chronic pain after nerve section or crush injury, sometimes following amputation. Anaesthesia dolorosa is pain in an anaesthetic area.
Polyneuropathy
Symmetrical, four limb, distal tingling, numbness or deadness are typical of polyneuropathy (Chapter 10).
Figure 4.10 Principal patterns of sensory loss.
Sensory Root and Root Entry Zone
Spinal and Vth nerve dermatomes are shown in Figure 4.11. There is sometimes overlap between adjacent dermatomes. Root pain is typically perceived both within the dermatome and within the myotome but tends to be less demarcated than pain with a single nerve lesion. For example, with an S1 root lesion from a lumbosacral disc, the sensory disturbance is down the back of the leg, without clear dermatome demarcation. Stretching the root by straight leg raising typically makes matters worse.
When a root entry zone is affected, within the cord, such as in tabes dorsalis, intense stabbing pains involve one or more spots, typically on the ankle, calf, thigh or abdomen – the lightning pains of tabes, seldom seen today.
Neuralgia, persistent burning root pain can follow shingles (post‐herpetic neuralgia, Chapter 23).
Figure 4.11 Spinal and V nerve dermatomes.
Cord Lesions: Sensory Changes
Posterior Columns
Patients describe:
Band‐like sensations, around trunk or limbs
Limb clumsiness, deadness
Numbness and burning
Electric shock‐like sensations.
Joint position sense, vibration, light touch and two‐point discrimination become diminished below the lesion. Stamping gait and pseudochorea of the outstretched hands are products of failing position sense.
Lhermitte’s sign is a sudden electrical sensation down the back, into the limbs produced by bending the head forward. Lhermitte’s suggests posterior column damage or occasionally caudal medulla. Lhermitte’s is seen in:
MS, typically in exacerbations
Cervical myelopathy, radiation myelopathy, trauma
Subacute combined degeneration of the cord
Occasionally: Behçet’s, Chiari malformations.
Spinothalamic Tracts
A lesion within these tracts produces changes in pain and temperature sensation below its level. With progressive compression from outside the cord, such as by an enlarging thoracic meningioma (extramedullary cord compression), the sensory level will tend to commence in the feet and rise to the level of the tumour – because of lamination of spinothalamic fibres in the cord. The patient may notice they cannot gauge water temperature with a foot. Extramedullary cord compression tends to affect both principal cord sensory pathways – both posterior column and spinothalamic.
When a lesion is within the cord (intramedullary) such as a syrinx (Chapter 16) sensory loss can initially be confined entirely to the spinothalamic pathways. The sensory loss is described as dissociated. Suspended sensory loss describes another aspect also seen with a syrinx: the dissociated sensory loss does not extend to the lower limbs – it is thus hanging, on the thorax or abdomen.
Sacral sparing is the phrase used to capture preserved sacral and perineal sensation when a central cord lesion expands centrifugally, damaging first centrally placed fibres and reaching last the spinothalamic sacral fibres on the periphery of the cord.
As a cavity develops within one side of the cord, dissociated sensory loss on one side occurs with pyramidal signs such as a spastic lower limb on the other. This carries the eponym Brown‐Séquard, from a treatise in 1849 on traumatic hemisection of cord. Brown‐Séquard findings mean spinothalamic signs on one side with pyramidal and dorsal column signs on the other. They point to a cord lesion, on the same side as the pyramidal and dorsal column loss. The patient may report: ‘I cannot feel the bathwater with my left foot, but it is my right that drags’.
Brainstem Lesions and Sensation
Various patterns are seen: trigeminal sensory loss (Chapters 2 and 13), dissociated (spinothalamic) sensory loss in the limbs, and/or lower limb numbness. The site of a lesion is usually determined more from signs from cranial nerve nuclear damage than by the sensory loss.
Thalamic Lesions
Destructive lesions of the complex thalamic nuclei are relatively unusual causes of sensory symptoms. When the ventral posterior lateral (VPL) and ventral posterior medial (VPM) thalamic nuclei (Chapters 2 and 5) are damaged, such as following a thrombo‐embolic stroke, contralateral hemi‐anaesthesia follows immediately. Sometimes, however, during the weeks or months following the stroke, highly unpleasant disabling persistent pain (post‐stroke central pain, a.k.a. thalamic pain, Chapter 23) develops in partially anaesthetic limbs. Pain is usually permanent.
Mononeuropathy, Polyneuropathy