In the clinical setting testing of somatosensory function will be dictated by the findings from the history. The sensory examination should follow the motor examination. The sensory examination will be somewhat subjective as it relies on a patients perception of the sensory stimuli. The object of the sensory examination is localisation.
Somatosensory function can be divided in to three broad categories:
- Superficial sensation
- Warm sense
- Cold sense
- Touch, tactile sense
- Deep/Proprioceptive sensation
- Vibratory sense
- Joint position sense
- Movement sense
- Deep pressure sense
- Cortical sensation
- Two-point discrimination
- Touch localisation
In terms of anatomy of somatosensory function the important things to note are:
- Group I and II- Largest fibres. Responsible for proprioceptive and vibratory function, light touch and touch.
- Group III and IV - Smaller and responsible for temperature and nociceptive (perception of pain) function.
- All sensory nerves enter the cord through the dorsal root ganglion
- Pain, temperature and touch sensation are carried on the spinothalamic tracts- Nerves carrying pain, temperature and touch enter the cord and synapses on the secondary neuron and then crosses over at or just above the level it enters. It then ascends as the spinothalamic tract and synapses on the ventral posterolateral nucleus of the thalamus on the tertiary neuron. Then it ascends to the post central gyrus.
- Vibratory and proprioception carried on the dorsal columns- Nerves carrying vibration and proprioception enter the cord and ascend as the dorsal columns to the medulla where it synapses on the secondary neuron. It then crosses over and ascends as the medial lemniscus to the ventral posterolateral nucleus of the thalamus where it synapses on the tertiary neuron and then ascends to the postcentral gyrus.
Types of sensory abnormality:
Hypoaesthesia is the diminished ability to perceive pain, temperature, touch or vibration.
Anaesthesia is the complete inability to perceive pain, temperature, touch or vibration.
Hypalgesia is the decreased sensitivity to painful stimuli.
Analgesia is the complete insensitivity to painful stimuli.
Hyperaesthesia is the increased sensitivity to painful stimuli.
Allodynia is the increased sensitivity to non-painful stimuli.
What a patient will complain of:
Numbness and pins and needles are used interchangeably by patients so it’s important to identify which it is- more for management.
Balance problems- identify if worse in the dark.
Burning pain, increased sensitivity to any touch.
Radicular pain- sciatica, neck pain radiating into the arms.
Examination of the Somatosensory System:
- Pain sense- sterile pins
- Temperature sense- tuning fork
- Tactile sense- light touch and stronger touch
- Vibration sense- tuning fork
- Joint position sense- Ask patient to close the eyes and then move the joint up and down. Hold the joint on either side rather than top or bottom to avoid the effects of pressure sense
Examination of Cortical Sensation:
- Two-point discrimination- ability to identify two point touched simultaneously.
- Graphaesthesia- ability to identify a letter or number drawn on the palm.
- Stereognosis- ability to identify an object by touch alone.
- Sensory extinction- ability to identify both sides of the body receiving a sensory stimulus at the same time.
When performing the examination of the sensory modalities you are trying to get a sense of the pattern. Things to note are symmetry/asymmetry, one or more limbs, one or more nerve distribution, one side of the body. Also when testing the sensation in the legs it is important to establish a gradient- so when checking pain and temperature for example it is important to ask the patient if there is a change in the intensity of the cold or pain sensation as you move up the limb.
- Dermatomal patterns- these are important when you are considering a radiculopathy. So if you have a diminished knee reflex then when checking dermatomes pay particular attention to L4 and L5. But there is really no point doing a dermatome check without a working theory as to the underlying cause of sensory loss.
- Length dependent neuropathy- this is possibly the commonest pattern of sensory loss that you will encounter. This should be predominantly distal and the sensory abnormalities should reach the knees before the hands. It usually starts by affecting the smaller fibres (pain and temperature) before the larger fibres (vibration and proprioception). If the large fibres are predominantly affected then consider causes that have a predilection for the dorsal columns such as vitamin B12 deficiency.
- Non-length dependent neuropathy- This pattern is seen in conditions such as chemotherapy induced neuropathy especially the platinum compounds. Also this pattern is seen in acute inflammatory demyelinating polyneuropathy such as Guillain-Barre syndrome. Here though based on the history you will have a condition such as Guillain Barre syndrome high on your list of differential diagnoses.
- Mononeuropathy- When the question is of a mononeuropathy the history will have helped you formulate a working diagnosis and the sensory examination is there to help you confirm this theory. Mononeuropthies you are likely to encounter are:
- Median neuropathy at the wrist
- Ulnar neuropathy at the elbow
- Peroneal neuropathy at the fibular neck
- Lateral femoral cutaneous neuropathy
- Mononeuritis multiplex- this causes multiple mononeuropathies. It is uncommonly encountered but if the cause is a vasculitis pain will often be a predominant symptom.
- Sensory neuronopathy- This is an uncommonly encountered pattern of neuropathy. Even though motor function is usually intact it is extremely debilitating. It results from degeneration at the level of the dorsal root ganglion. The common finding on examination is a sensory ataxia whereby the patient has no concept of where their hands and feet are when the eyes are closed. They are often idiopathic but a paraneoplastic and autoimmune aetiology need to be ruled out.
- Spinal cord lesions-
- Cape distribution sensory loss- this is uncommon but well described and is due to cervical cord lesions. There may be loss of temperature and pain sensation along the shoulders and upper arms (cape distribution). In this case a cervical MRI needs to be done to rule out a syrinx, tumour or demyelinating lesion.
- Brown-Sequard syndrome- Lesion involving one side of the spinal cord so you have ipsilateral dorsal column and contralateral spinothalamic loss. Spinal cord imaging needs to be done looking for demyelination or tumours.
- Brainstem lesions- There are several brainstem patterns of sensory loss but the one to know about is lateral medullary syndrome. Here the abnormality is in the lateral medulla and classically causes ipsilateral pain and temperature loss on the face and contralateral pain and temperature loss on the limbs and trunk. There may also be ipsilateral loss of proprioception and vibration sensation as well as dysphagia, ipsilateral Horner syndrome, vertigo and nystagmus.
- Thalamic lesions- These may cause a contralateral sensory loss involving all modalities. It often occurs as a result of a lacunar infarct. The thalamic pain syndrome is a type of central post stroke pain that can occur in the recovery period from this type of stroke.
- Sensory cortex- Discrete lesions of the sensory cortex are uncommon. Cortical signs and symptoms such as neglect and cortical sensory loss help localise to the sensory cortex. However, the examination of cortical sensory signs is only possible when there is adequate sensation.
the Neurologic Examination. Scientific basis for Clinical Diagnosis. Shibasaki H and Hallett M.
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