What tests should be done for a patient with metabolic encephalopathy in the Intensive Care Unit (ICU)?

Diagnostic Testing for Metabolic Encephalopathy in the ICU

For ICU patients with metabolic encephalopathy, perform urgent EEG (within 60 minutes if possible) to exclude nonconvulsive seizures, obtain arterial blood gas with ammonia level, comprehensive metabolic panel, and brain imaging (MRI preferred, CT if unavailable) to exclude structural lesions. 1, 2

Essential Initial Laboratory Tests

Blood gas analysis with specific metabolites:

• Arterial blood gas (ABG) to assess pH, PaCO2, and PaO2—hypoxemia (PaO2 < 70 mmHg) should be avoided in the first 24-48 hours 1
• Plasma ammonia level—though a normal value should prompt investigation for other etiologies, it helps confirm hepatic encephalopathy when elevated 2, 3
• Blood glucose measurement—continuous monitoring recommended in critically ill patients, with samples from arterial or central venous catheters analyzed via blood gas analyzer or central laboratory 1

Comprehensive metabolic assessment:

• Electrolytes (sodium, potassium, magnesium, phosphate)—these disturbances are common precipitating factors 2, 3
• Liver function tests and renal function (BUN, creatinine) to identify hepatic or uremic causes 2, 4
• Blood lactate and ketones to assess metabolic derangements 5

Additional screening:

• Toxicology screen including alcohol level and common drug intoxicants—essential to identify drug-induced or alcohol-related encephalopathy 2
• Infection markers if sepsis suspected 2, 4

Neurophysiological Monitoring

EEG is the cornerstone neurophysiological test:

• Urgent EEG (within 60 minutes) is strongly recommended for all comatose ICU patients with unexplained altered consciousness to exclude nonconvulsive seizures 1
• Continuous EEG monitoring is preferred over routine EEG when feasible, as routine EEG will miss nonconvulsive seizures in approximately half of patients 1
• EEG is particularly important in patients with severe sepsis, renal failure, or hepatic failure 1
• EEG findings correlate with encephalopathy severity: progressive slowing, triphasic waves (though not specific), and decreased amplitude 1, 6

Evoked potentials as adjunctive tests:

• Somatosensory evoked potentials (SSEPs) can assess both brainstem and cortical function 1
• These are relatively resistant to environmental electrical noise and anesthesia levels compared to EEG 1
• Consider when EEG is technically difficult or to supplement EEG findings 1

Pupillometry:

• Automated pupil assessment should be considered if available for objective neurological monitoring 1

Neuroimaging

Brain imaging is mandatory:

• MRI is preferred to exclude structural lesions, intracranial hemorrhage, or other non-metabolic causes 2, 3
• CT scan if MRI unavailable or patient too unstable for transport 2, 3
• Early neuroimaging recommended for patients at risk based on physical examination and neuromonitoring findings 1

Additional Diagnostic Considerations

Lumbar puncture:

• Perform if infection (meningitis/encephalitis) cannot be excluded clinically 2, 3
• Critical caveat: Only perform after ruling out increased intracranial pressure via imaging and correcting coagulopathy 2

Cerebral oximetry:

• Continuous cerebral oximetry recommended for ongoing trend monitoring, especially in VA ECMO patients at risk for differential hypoxia 1

Monitoring Parameters During ICU Stay

Serial assessments:

• Perform serial ABG sampling in the first 24 hours 1
• Continuously monitor core temperature and actively prevent fever 1
• Follow glucose, potassium, magnesium, and phosphate levels closely 3
• Monitor blood pressure—maintain MAP > 70 mmHg, individualized based on cerebral autoregulation 1

Common Pitfalls to Avoid

• Do not rely solely on ammonia levels—routine ammonia testing is NOT recommended for diagnosis or monitoring, though it can support the diagnosis when elevated 2
• Do not delay EEG—nonconvulsive seizures are common and will be missed without EEG monitoring 1
• Do not use capillary glucose samples in critically ill patients—arterial or central venous samples analyzed by blood gas analyzer are required 1
• Do not use BIS monitoring—data do not support bispectral index score for brain-injured ICU patients due to high variability and interference 1
• Do not perform lumbar puncture before imaging—always exclude increased intracranial pressure first 2

Grading and Risk Stratification

Use West Haven criteria for hepatic encephalopathy grading:

• Grades III-IV require ICU management with capability for intracranial pressure monitoring 2
• Glasgow Coma Scale should be added for grades III-IV 2
• Patients with declining consciousness require urgent ICU assessment for airway protection 3