Neurological Investigations

Neurophysiology

Electroencephalogram (EEG)

Used for recording the electrical activity of the brain via electrodes attached to the scalp.
Principal use of EEG is in the diagnosis and management of different seizure disorders with the EEG being performed during ictal and interictal phases.
Can be recorded with video monitoring.
Diagnostic yield can be increased with the use of forced hyperventilation (e.g. triggering spike and wave activity), photic stimulation, sleep deprivation and repeating the EEG.
While interpreting interictal EEG, background rhythm and paroxysmal changes need to be considered. Normal background rhythm is called ‘alpha’ at 8–13 Hz (mainly in the occipital lobes when one is awake and quiet). Slowing of the background rhythm can occur in metabolic encephalopathy (renal and hepatic failure), drug overdose and other degenerative conditions.
Paroxysmal changes (spikes <70 ms, sharp waves: 70–200 ms) can be focal (suggesting a structural lesion) or generalised. Generalised spike and wave activity at 3 Hz is seen in absence seizures (Table 4.1).

Remember: A normal interictal EEG does not exclude the diagnosis of epilepsy. A high proportion of patients with epilepsy may have a normal interictal EEG. Although specific, EEG has low sensitivity for diagnosing epilepsy.

Ictal EEG is ideal for diagnosing epilepsy. During a generalised tonic–clonic seizure, high-voltage synchronised discharges of 100 μV are seen. Continuous video EEG monitoring to localise a seizure focus is used as part of preoperative evaluation for surgical treatment of medically refractory epilepsy.
Other uses include non-convulsive status, confirming brain death (not essential) and burst suppression.
Memorise these characteristic EEG changes (Table 4.1) frequently featuring in examinations.

Table 4.1 Different clinical conditions of note with characteristic EEG changes.

Clinical conditions	EEG changes
Absence epilepsy	3 Hz spike and wave
West syndrome	Interictal hypsarrhythmia
Juvenile myoclonic epilepsy	Polyspike discharges
Creutzfeldt–Jakob disease	Bilateral sharp waves (1.5–2 Hz, later triphasic); can occur with simultaneous myoclonic jerks
Herpes simplex encephalitis	Bilateral periodic lateralizing epileptiform discharges (regular sharp waves with background slow activity)
Metabolic encephalopathy	Generalised slow activity
Subacute sclerosing panencephalitis	Body jerks and associated periodic high voltage

Evoked Potential Studies

These assess the sensory pathways from the sensory organ to the cerebral cortex, for example visual (VEP), brain stem auditory (BSAEP) and somatosensory (SSEP) evoked potentials.
Stimuli, for example clicking noise for BSAEP, are delivered while recordings are made from scalp electrodes.
A delay in the potential implies demyelination while a reduction in amplitude suggests axonal degeneration.
A delayed VEP may be due to a subclinical lesion of MS affecting the optic pathway. Similarly, delayed BSAEP may occur in the context of acoustic neuroma or MS.
SSEP involves assessing posterior column pathway of the sensory system. Painless electrical stimulation is applied at different points, for example above the clavicle or the lumbar and cervical spine with simultaneous recording from contralateral parietal sensory cortex.
An abnormal SSEP can therefore occur with MS or spinal cord lesions. Intraoperative SSEP is useful during spinal surgery. A drop in amplitude intraoperatively may signify neural damage.
Transcranial motor evoked potentials involves electrical stimulation of motor cortex with a recording of potentials from muscle. This again can be useful for surgery of the spinal cord.

Nerve Conduction Studies (NCS)

Assesses motor and sensory functions of myelinated fibres in terms of two principal measurements, conduction velocity and amplitude.
Motor element of NCS involves stimulation of motor nerve at two points (distal and proximal) and recording the amplitude of the compound muscle action potential (CMAP) at a fixed different point. The sensory element involves stimulation at one point with recording the potential at a different distant site.
Conduction velocity is measured using the distance and the latency.
Axonal disease (e.g. diabetic neuropathy) leads to normal velocity motor conduction with reduced amplitude while demyelinating disease, for example Guillain–Barré syndrome, increases the latency thereby decreasing the conduction velocity.

Remember: The point about the delay versus the decrease in amplitude seen in association with demyelinating versus axonal degeneration, respectively.

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