Neuroimaging, EEG, EMG and other diagnostic methods
This domain tests whether you can choose the right investigation, interpret its pattern, and recognise its limitations. Questions pair a clinical problem with an imaging or electrophysiological finding.
Imaging
Non-contrast CT is first-line in acute stroke to exclude haemorrhage; look for the hyperdense vessel sign and calculate ASPECTS. CT angiography identifies large-vessel occlusion. MRI: DWI becomes positive within minutes and shows restricted diffusion with low ADC; FLAIR detects established lesions; SWI shows microbleeds and venous thrombosis; gadolinium marks blood–brain barrier breakdown; perfusion identifies mismatch. In MS, typical lesions are ovoid, periventricular, juxtacortical, infratentorial or spinal. In tumour imaging, assess enhancement, oedema, mass effect and multiplicity. Catheter angiography remains the reference for vascular malformations.
EEG
Electrodes are placed in the 10–20 system; longitudinal bipolar and referential montages are standard. The normal posterior alpha rhythm is 8–13 Hz and reacts to eye opening. Activation procedures are hyperventilation, photic stimulation and sleep deprivation. Abnormal findings: focal slowing (structural lesion), epileptiform discharges (spike, sharp wave, spike-and-wave), periodic discharges, burst suppression, triphasic waves (metabolic encephalopathy). Non-convulsive status requires EEG. Electrocerebral silence supports brain death but must be interpreted with the clinical criteria.
EMG and nerve conduction studies
Motor studies measure distal latency, amplitude and conduction velocity; sensory studies are usually antidromic. F-waves assess proximal segments; the H-reflex assesses the S1 root. Axonal loss reduces amplitude with relatively preserved velocity; demyelination slows velocity, prolongs latency, and may produce conduction block or temporal dispersion. Needle EMG assesses spontaneous activity (fibrillations, positive sharp waves, fasciculations, myotonic discharges), motor unit potential morphology and recruitment. Repetitive stimulation at 3 Hz shows a decrement in myasthenia gravis; high-frequency stimulation produces an increment in Lambert–Eaton syndrome. Single-fibre EMG measures jitter.
Evoked potentials and CSF
VEP P100 delay occurs in optic neuritis; SEP and MEP assess central conduction. Lumbar puncture: opening pressure 10–20 cm H₂O in the lateral decubitus position; send cell count, protein, glucose, cytology, PCR, oligoclonal bands and IgG index. Avoid lumbar puncture with mass lesions, coagulopathy or local infection.
Worked example
A patient with acute hemiparesis has a normal CT at 2 hours. Normal CT does not exclude ischaemia - proceed to MRI with DWI, which shows restricted diffusion in the MCA territory, and CT angiography to look for a large-vessel occlusion.
Traps
Over-reading non-specific EEG slowing; excluding epilepsy because the EEG is normal; diagnosing neuropathy without nerve conduction studies; forgetting that a normal early CT does not exclude stroke.