Diagnostic Testing and Neuroimaging
Choosing the Right Test
Understand what each modality shows. CT is fast and best for acute hemorrhage, fractures, and mass effect; it is the first-line test in acute stroke to exclude bleeding. MRI provides superior soft-tissue detail for demyelination, tumors, and spinal cord lesions. Angiography visualizes vessels and is used for aneurysm and stenosis evaluation. Lumbar puncture analyzes CSF for infection, inflammation, and subarachnoid hemorrhage; it is contraindicated when signs of increased intracranial pressure or herniation are present.
CSF findings: bacterial meningitis shows elevated white cells (neutrophils), high protein, and low glucose; viral meningitis shows lymphocytes with normal glucose. Subarachnoid hemorrhage shows xanthochromia. EEG evaluates seizure activity and brain function; EMG and nerve conduction studies assess peripheral nerve and muscle disorders.
Traps
A frequent question asks the first imaging test in acute stroke - answer CT, not MRI, because speed and hemorrhage exclusion matter. Another trap: lumbar puncture before CT in a patient with focal deficits or papilledema risks herniation. Remember contrast precautions in renal impairment and allergy, and that a normal CT does not exclude early ischemia.
Worked Example
A patient presents with sudden severe headache. CT is negative for hemorrhage but suspicion remains high. The next step may be lumbar puncture to look for xanthochromia, but only after imaging excludes mass effect and increased intracranial pressure.
Rule of Thumb
"CT first for bleeding and speed; MRI for detail; LP only after you have ruled out pressure."