Neurological Investigation — LP, EMG, NCS, EEG and Neuroimaging

The answer first

Neurological investigations are an extension of the clinical examination: they refine a localisation hypothesis, exclude mimics, and change what you do next. The cardinal error — at the bedside and in the exam — is ordering the wrong test for the wrong question. The physician must be able to choose the modality that discriminates, defend the expected result, and interpret the report against the clinical picture. ^[1] The five modalities tested relentlessly are lumbar puncture (LP) and cerebrospinal fluid (CSF) analysis, nerve conduction studies (NCS) and needle electromyography (EMG), single-fibre EMG, electroencephalography (EEG), evoked potentials, and structural neuroimaging (CT versus MRI). ^[4]

A physician-level answer rests on four pillars: ^[1]

1. Localise first, then investigate. "Numbness" is not an indication — a dorsal column lesion, a mononeuropathy, a polyneuropathy, a radiculopathy and a cortical lesion each demand a different pathway. Spend the time on the examination so the investigation answers the question.
2. Match the test to its time window. NCS are normal in the first 1 to 2 weeks of Guillain-Barré; F-waves are the earliest abnormality, then conduction block, reduced conduction velocity, and prolonged distal latencies. Diffusion-weighted MRI shows acute ischaemic stroke within minutes, while CT can be normal for the first 6 hours. ^[11]
3. Respect thresholds and patterns, not isolated numbers. A CSF white-cell count of 5 cells/µL in viral meningitis, a protein of 1.0 g/L in Guillain-Barré, an opening pressure of 250 mm CSF in suspected idiopathic intracranial hypertension — these are diagnostic only in context. ^[3]
4. Treat the patient, not the report. An EEG can be normal in established epilepsy; an EMG can be normal in early motor neuron disease. The diagnosis is clinical, and the investigation either supports or refutes a hypothesis — it does not replace one. ^[9]

DWE high-yield: Three exam-tested facts. First, CSF opening pressure is measured in the lateral decubitus position with the legs relaxed — a normal adult value is up to 250 mm CSF (280 mm in obesity); a pressure above 250 mm with the appropriate phenotype defines idiopathic intracranial hypertension per the modified Friedman criteria. ^[3] Second, demyelinating neuropathy on NCS shows reduced conduction velocity, prolonged distal latency, conduction block and temporal dispersion; axonal neuropathy shows reduced amplitudes with preserved velocity — the distinction drives the differential (Guillain-Barré and CIDP versus diabetic or alcoholic neuropathy). ^[4] Third, a DWI-FLAIR mismatch identifies patients within the 4.5-hour thrombolysis window when the onset time is unknown (wake-up stroke). ^[12]

References12ShowHide

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