Diagnostic Testing in Acute Neurological Disease
| Test | Comment |
|---|---|
| Lumbar puncture | Critical part of the investigation of CNS infection and subarachnoid haemorrhage. May provide supportive role in diagnosis of CNS and PNS inflammatory disorders. Contraindications, technique and interpretation are further discussed in Chapter 123. |
| CT brain | Invaluable in the workup of suspected haemorrhagic/vascular pathology (SAH, EDH/SDH, stroke, venous sinus thrombosis) and where raised ICP or a mass lesion are acute concerns. Benefits from availability, rapidity, and sensitivity in detecting acute haemorrhage. Spatial resolution of contrast CT-A exceeds that of MR-A. CT-V may also provide more diagnostic clarity than MR-V in suspected venous sinus thrombosis. |
| MRI brain | Provides improved visualization of posterior fossa/brainstem structures and spinal cord over CT. MR images may be virtually diagnostic of CNS demyelination and even HSV encephalitis. DWI/ADC sequences can be extremely useful for clarifying diagnostically difficult stroke/TIA presentations. |
| EEG | Most useful acutely in the evaluation of possible non-convulsive status or for differentiating (ongoing) epileptic from non-epileptic seizures. An inter-ictal EEG lacks the sensitivity or specificity to rule in or out epileptic seizures and it is prudent to carefully consider and/or discuss such requests in advance. |
| EMG/nerve conduction studies | EMG can help distinguish neurogenic from myogenic causes of weakness, but it is worth noting that spontaneous activity suggestive of denervation can take up to two weeks from the point of injury to develop. Likewise, nerve conduction studies can prove entirely normal, particularly in the early stages of an acute neuropathy such as GBS. Specialist protocols (repetitive stimulation and/or single fibre EMG) are required to evaluate the neuromuscular junction (e.g. in suspected myasthenia). Again, these tests do not make or break a diagnosis of in isolation, and will often require specialist input to interpret correctly. |
| Vital capacity | A spirometric assessment of vital capacity is an absolute requirement in the safe management of patients at risk of neuromuscular respiratory failure (typically GBS or myasthenia gravis in the acute setting). An adult vital capacity of <1.5L/<20 mL/kg is of immediate concern and warrants discussion with ICU as a minimum. At <1L/15 mL/kg, or with a fall of 50% from baseline on serial testing, prompt ICU involvement is required. Peak flow or arterial blood gas measurements are inadequate substitutes. |
EEG, electroencephalography; EMG, electromyography; GBS, Guillain-Barré syndrome.