NEURODIAGNOSTIC TESTING Lumbar Puncture (LP) Most often performed in the L3 or L4 interspaces (at the level of the superior iliac crests). The needle is advanced to the subarachnoid space. Opening pressure should be measured but is valid only when obtained with the patient in the lateral decubitus position (i.e., with the spinal needle located at the same level as the heart). Patients with papilledema, focal neurologic signs, or immunosuppression should have imaging prior to LP to evaluate for mass effect and herniation risk. Imaging of the spine should precede LP in patients with spinal cord signs or symptoms. Electroencephalography (EEG) Conditions with notable EEG findings include the following: Metabolic encephalopathy: Hepatic encephalopathy is the classic metabolic coma. EEG typically shows generalized periodic triphasic waves. Viral encephalitis: HSV encephalitis: The classic EEG finding consists of periodic lateralizing epileptiform discharges (PLEDs) originating over one or both temporal lobes. Subacute sclerosing panencephalitis (SSPE): EEG typically shows a flat background punctuated by periodic generalized large-amplitude slow-wave discharges. Prion disease: EEGs in patients with Creutzfeldt-Jakob disease show periodic generalized sharp waves Computed Tomography (CT) CT imaging of the brain is inferior to MRI for most studies but is the imaging study of choice for investigating acute hemorrhage (e.g., SAH, epidural hematoma) and bone pathology (e.g., skull or vertebral fractures). Magnetic Resonance Imaging (MRI) The best imaging modality for most diseases of the brain and spinal cord, including neoplastic, vascular, demyelinating, infectious, and structural diseases (e.g., spondylosis of the spine). Cerebral Angiography The gold standard for investigating vascular abnormalities of the CNS, including stenosis, aneurysms, AVMs, and cerebral vasculitis. Also useful for preoperative evaluation of vascular supply to intracranial tumors (e.g., meningiomas). Cerebral venography is the gold standard for diagnosing venous sinus thrombosis. Evoked Potentials (EPs) Obtained by measuring the time course of a specific CNS response to a given stimulus. Visual EPs are generated by recording cortical response (using EEG electrodes) elicited by a visual stimulus. A delay in response suggests that the conduction velocity along the visual pathway is slow, often a sign of demyelination (e.g., in MS). Brain stem and sensory EPs are useful for evaluating potential demyelinating lesions of the brain stem and dorsal columns of the spinal cord. Often used to obtain supportive evidence of CNS demyelination. Sensory and motor EPs are used for intraoperative monitoring during neurosurgical procedures involving the spinal cord or brain stem and can be helpful prognostically in hypoxic-ischemic encephalopathy.