Clinical Disorders Of The Sensory Systems
Disturbances in the
sensory pathways can produce one of two main symptoms:
· Negative ones, with a loss of
sensation such as numbness or analgesia;
· Positive ones, such as pins and
needles (paraesthesiae) or pain. These symptoms can arise from many different
sites along the sensory pathways, but it is often the distribution of sensory
change that points towards the likely site of pathology.
In order to determine
the nature and cause of the sensory disturbance a full history and examination
is needed along with appropriate tests. Most patients with isolated sensory
symptoms do not yield to a diagnosis but the most common causes are neuropathies
and multiple sclerosis.
A typical screen of
tests for patients with sensory symptoms involves blood tests, nerve
conduction studies (NCS) and magnetic resonance imaging (MRI) of brain and
spinal cord. In all cases it is important to remember that non-neurological
causes, e.g. hyperventilation.
Peripheral nerves
Diseases of the
peripheral nerves can cause sensory disturbance. This can either be caused by focal
nerve entrapment or a generalized neuropathy, in which
case the disease process can target either the large or small fibres or both.
Common focal nerve entrapments include:
• The
median nerve at the wrist (carpel tunnel syndrome). Patients
typically present with aching in the forearm especially at night, weakness of
some of the thumb muscles and loss of sensation over the thumb and adjacent two and a half
fingers. It can resolve spontaneously but in cases where it does not, simple
splinting, steroid injection or even surgical decompression is often curative.
· The ulnar nerve at the elbow.
Patients present with wasting of most of the intrinsic hand muscles with
weakness and loss of sensation in the hand involving the little and half of the
ring finger but without involvement of the forearm. It can be treated by surgical
transposition of the nerve in some cases.
· The common peroneal (or lateral
popliteal nerve) can be trapped around the knee. Patients typically present
with foot drop and numbness on the outer aspect of the foot.
Generalized
neuropathies may
be caused by many disorders and if large fibres are preferentially involved
then there is a loss of joint position sense, vibration perception and light
touch along with absent or reduced reflexes. These neuropathies are rarely
purely sensory and often associated with weakness and wasting. The typical
pattern of sensory loss in these neuropathies is ‘glove and stocking’ which, as
the name implies, reflects the symmetrical loss of sensation in all four limbs
to the wrist/forearm and to the ankle/shin.
In some cases patients
complain of much pain but paradoxically have reduced sensation for pain and
temperature. These patients are more likely to have small fibre
neuropathy. Rarely, the dorsal root ganglion cell (as opposed to the
peripheral nerve) is targeted by the disease process. In these instances there
is a devastating loss of proprioception which greatly compromises motor
function.
Peripheral pain
syndromes are
discussed in Chapters 32 and 33, but it is always important to remember that
pain is more often the result of non-neurological causes such as arthritis or
local tissue damage.
The nerves as they
emerge out of the spinal column can be trapped typically by bony spurs or
intervertebral discs and give sensory disturbance along that nerve root.
Patients normally complain of pain radiating down that nerve root with sensory
abnormalities confined to that dermatome (see Chapter 2). This commonly happens
in the cervical and lumbar region and may require surgical decompression
especially in cases where there is weakness, wasting and loss of the
appropriate reflexes.
Spinal cord
Syringomyelia
Syringomyelia is the
development, for a number of reasons, of a cyst or cavity around or near to the
central canal, usually in the cervical region, which tends to spread over time
up and down the spinal cord. The lesion typically disrupts the spinothalamic
tract (STT) fibres as they cross just ventral to the central canal, resulting
in a dissociated sensory loss, i.e. reduced temperature and pain sensation at
the level of the lesion but normal light touch, vibration perception and joint
position sense (see Chapter 31). In addition, there may be motor involvement
because of expansion of the cyst into the ventral horn or dorsolaterally into
the descending motor tracts and other ascending sensory pathways.
Subacute combined
degeneration of the spinal cord This is usually associated with pernicious anaemia and a lack of vitamin
B12. It is characterized by demyelination and eventually
degeneration of the dorsal columns (DCs), the spinocerebellar (SCT) and
corticospinal tracts (CoST) as well as damage to peripheral nerves (peripheral
neuropathy). Patients therefore develop a combination of paraesthesiae and
sensory loss (especially light touch, vibration perception and joint position
sense) with weakness and incoordination (see Chapter 55). The weakness may be
of both an upper and lower motor neurone type (see Chapter 55).
Brown–Séquard
syndrome
This describes a lesion
involving half of the cord such that there is an ipsilateral loss of position
and tactile senses (DC sensory information), a contralateral loss of
temperature sensation originating from several segments below the lesion (STT
sensory information), and ipsilateral spasticity and weakness because of
involvement of the CoST pathway (see Chapters 31, 32, 37).
Anterior spinal
artery syndrome
This syndrome describes
the situation when there is occlusion of the artery providing blood to the
anterior two-thirds of the cord. The patient has weakness and sensory loss to
temperature and pain with preservation of DC sensory modalities such as joint
position sense and vibration perception (see Chapter 6).
Transverse myelitis
Transverse myelitis (not
shown in figure) describes a complete lesion of the whole spinal cord at one
level that produces a complete sensory loss with weakness from that level down.
The weakness is characteristically caused by a disruption of both the
descending motor pathways and the spinal motor neurones. It is typically seen
as a part of multiple sclerosis or a secondary acute
demyelinating process in response to infection such as an atypical pneumonia.
Brain
Abnormalities in
supraspinal sites can result from a variety of causes and depending on the
disease process and site determines the type of sensory disturbance. Typically,
hemispheric lesions give a loss of sensation down the contralateral side of the
body. Brainstem lesions give rise to a range of sensory deficits depending on
the exact level of the lesion. For example, a pontine lesion can give
ipsilateral sensory loss of the face but contralateral sensory loss in the
limbs. Cortical lesions can give a loss of sensation if the primary somatosensory
cortex is involved, or can give more complex sensory deficits such as
astereognosis (an inability to recognize objects by touch) or even sensory
neglect or inattention. These latter abnormalities are typically seen with
lesions of the posterior parietal cortex (see Chapter 34).
In some cases,
irritative lesions of the primary sensory cortex give rise to simple partial
seizures (see Chapter 61) in which the patient experiences brief migrating
sensory symptoms up one side of the body. This can also be seen in some
patients with transient ischaemic attacks (TIAs).
Pain syndromes can also
develop with central lesions and this is best seen in small thalamic vascular
events, where dysaesthesia is found in the contralateral limb in a typically
diffuse distribution (see Chapter 33).
