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Clinical Neurophysiology in Diagnostics
Principles
- Clinical neurophysiology is a branch of medicine concerned with the measurement of the electrical activity of the central nervous system, peripheral nerves and muscles.
- The measurements of electrical activity are usually sensitive but, as far as the aetiology of the lesion is concerned, non-specific.
- The investigations are labour-intensive, and they are consultations by nature.
- Some of the investigations are uncomfortable for the patient. Neurophysiological investigations are therefore not suitable for screening purposes.
- There are no contraindications or age limitations.
- The significance of any positive findings should always be assessed against the patient's clinical details.
- Neurophysiological investigations are used in neurological diagnostics, together with neuroradiological and other tests, after a provisional anatomic diagnosis has been made with the aid of history taking and physical examination.
Electroencephalography (EEG)
- Indications
- Episodic symptoms, particularly suspicion of epilepsy and classification of epilepsy
- Encephalitis
- Delirium, acute encephalopathy, more rarely chronic encephalopathies (e.g. dementia-inducing Creutzfeldt-Jakob disease)
- Antiepileptic medication need not be discontinued for the EEG recording. The dose of benzodiazepines, however, should be decreased.
- In a patient with suspected epilepsy, there is a strong correlation between an epileptiform EEG pattern and the diagnosis of epilepsy and its type (primary generalised epilepsy or focal epilepsy) especially in adults. The correlation of other abnormalities with epilepsy is more unreliable.
- In temporal lobe epilepsy, a sleep EEG (awake from early hours) will increase the frequency of epileptiform abnormalities as will a sleep-deprived EEG (awake all night) in generalised epilepsy. A recording of a sleep EEG is usually attempted in children and neonates.
- Special EEGs (to be arranged with the department of clinical neurophysiology): ambulatory EEG, videotelemetry EEG, quantitative EEG analysis, investigations in the aim of epilepsy surgery
- Continuous EEG monitoring with a few channels may be used, for example, during status epilepticus in order to assess treatment response. In the treatment of prolonged status epilepticus, the possibility of EEG monitoring is always indicated.
- EEG is also used in the monitoring of the neonatal phase, as well as in assessing hypoxic-ischaemic damage and the depth of anaesthesia.
Electroneuromyography (ENMG)
- An ENMG is an examination of the functioning of anterior horn cells, nerve roots, nerve plexuses, peripheral nerves, neuromuscular junctions and muscle cells.
- The investigation will not yield information regarding the malfunction of the central nervous system (brain, spinal cord).
- If the patient presents with symptoms suggestive of an upper motor neurone lesion (brisk reflexes, abnormal Babinski sign) and/or, for example, progressing weakness of the lower extremities, the initial investigations should consist of neuroradiological tests, examination of the cerebrospinal fluid etc., as considered appropriate by a neurologist.
- The most common indications for an ENMG are various combinations of pain, loss of sensation and muscle weakness.
- It is usually possible to suspect the cause (working hypothesis) and the level of the lesion based on the presenting neurological findings (paraesthesia and weakness).
- Clinical examination of a nerve entrapment (entrapment of the median nerve in the wrist in particular) may reveal nothing abnormal, but the patient gives a history of repeated episodes of numbness e.g. at the site of the median nerve.
- Suspected axon damage is the most common reason for referral. Needle electromyography (EMG) findings indicative of axon damage (fibrillation, denervation) take 2-3 weeks to develop. If ENMG investigations are required earlier than this, a clinical neurophysiology physician should be consulted. Needle EMG may increase the concentration of muscle enzymes (CK) in the blood.
- In order to avoid unnecessarily extensive and unpleasant investigations, the referral should clearly state the diagnostic hypothesis and the reason for the investigation.
- Most common indications
- Nerve entrapment, local damage to the nerve: entrapment of the median nerve around the carpal tunnel, ulnar neuropathy around the elbow region, radial neuropathy in the upper arm, peroneal neuropathy around the knee, Morton's metatarsalgia in the foot
- Nerve root injury: lumbosacral radiculopathy, cervical radiculopathy
- Polyneuropathies, neuromyopathies, motor neurone disease etc.
- Nerve plexus injury, for example after a shoulder injury, and other trauma
- Polyradiculitis (also during an acute phase), plexus neuritis, radiculitis, mononeuritis
- Diseases affecting myocytes and nerve junctions (rare, may require specialist investigations)
- Neurography, i.e. nerve conduction velocity measurement alone, carried out by a neurophysiology technician: e.g. for establishing or excluding the presence of median nerve entrapment. The investigation does not test the presence of cervical radiculopathy.
- Latency measurement screening: a totally normal test result will exclude significant (i.e. suitable for surgery) carpal tunnel syndrome. Other findings are of uncertain significance, and ENMG testing is usually required to clarify the results.
- Botulinum toxin injection therapy in EMG control: indications include e.g. dystonia, spasticity in some cases
Quantitative sensory testing (QST)
- Small fibre neuropathy, verification of a neuropathic component (with e.g. sensitisation as a symptom) in prolonged pain states; this type of abnormality is not detected by ENMG examination
- Usually suggested as a follow-up examination after ENMG.
Evoked potential studies
- In neurological diagnostics, evoked potential (EP) studies are used to investigate demyelination (in suspicion of MS) relating to the visual pathway (visual evoked potentials, VEP) or sensory pathway (sensory evoked potentials, SEP).
- Motor tracts may be studied with the aid of magnetic stimulation of the cerebral cortex. It is also used for therapeutic purposes in the treatment of depression, pain, etc.
Diagnosis of organic sleep disorders
- The patient's history will usually suffice when treating sleep disorders, and additional information regarding night time behaviour should be obtained from family members.
- A sleep diary is often useful when exploring daytime tiredness and it will also be beneficial when the reasons for insomnia are considered.
- If an organic sleep disorder leads to, for example, diminished work capacity and the possibility of active treatment is considered, the patient needs diagnostic recordings taken during nocturnal sleep.
- Polysomnography (PSG): several simultaneous measuring probes, including measurements of breathing, oxygen saturation etc.
- Extended polysomnography: including sleep analysis, EEG recording and several measurement probes.
- An overnight, labour-intensive study at a sleep laboratory
- Can also be performed at home with fairly good quality
- Prior to MSLT e.g. in suspicion of narcolepsy
- Limited overnight polygraphy: no sleep analysis and therefore easier to execute
- Recordings taken at the patient's home to establish the degree of various organic sleep disturbances (sleep apnoea, snoring, restless legs). Useful also in differential diagnosis.
- The extent and range of various recordings vary greatly between different units. The test should be sufficiently extensive.
- Multiple Sleep Latency Test (MSLT) and Maintenance of Wakefulness Test (MWT): the tests take a full day to complete and aim to establish, with the aid of 4 or 5 recordings taken at a sleep laboratory, the degree of daytime tiredness.
- MSLT in suspicion of narcolepsy
- MWT in the evaluation of the ability to maintain alertness; e.g. a professional driver
- In actigraphy, the patient's activity and movements are monitored over several days with a wrist-watch-like sensor in order to document e.g. sleep-wake cycle, delayed sleep phase, insomnia and daytime tiredness.
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