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.
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.
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).
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 (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.
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).