occur in roughly predictable patterns with bacterial or viral meningitis, and pathogens may be visible using Gram and other stains (see Chapter 94 Infectious Disease Emergencies ) Rapid testing with agglutination studies or polymerase chain reaction tests might also be used to identify pathogens CSF pleocytosis in encephalitis is variable and, if present, is usually mild (less than 500 cells/mm3), with normal levels of glucose and protein being common In the absence of signs of infection, bloody or xanthochromic CSF under increased pressure indicates subarachnoid hemorrhage Metabolic Abnormalities The presence of a metabolic disorder leading to coma is usually apparent once the results of routine laboratory tests are available These values for glucose, sodium, potassium, bicarbonate, calcium, magnesium, and phosphorus make any deficiency or excess of these serum components readily apparent and treatable Blood gas analysis for evaluation of acidosis or alkalosis from metabolic or respiratory causes may also be indicated Decreased consciousness caused by diabetic ketoacidosis may initially worsen because of a paradoxical temporary decrease in CSF pH and/or cerebral edema complicating the disease Renal and hepatic functions should be quantified with analysis of blood urea nitrogen, creatinine, and ammonia Markedly elevated serum blood urea nitrogen and creatinine, oliguria, hypertension, anemia, acidosis, and hypocalcemia indicate the presence of uremic coma as a result of renal failure Hyperammonemia with decreased mental status may be caused by hepatic failure, acetaminophen ingestion with resultant hepatotoxicity, valproic acid toxicity, Reye syndrome, or inborn metabolic errors The hyperammonemia of Reye syndrome is accompanied by a history of antecedent viral illness and likely treated with aspirin (see Chapter 97 Neurologic Emergencies ) Unremitting vomiting is soon accompanied by encephalopathy, in the absence of jaundice, scleral icterus, focal neurologic signs, or meningeal irritation Hyperammonemia without accompanying liver failure in the young infant may indicate the presence of a congenital urea cycle defect Coma of Unknown Origin Patients with coma of unknown origin not falling into any of the diagnostic categories discussed previously usually benefit from a noncontrast brain CT scan, CSF analysis, and neurologic consultation, in that order The emergence of new infectious diseases such as West Nile virus and eastern equine encephalitis virus means that diagnosis may require consultation with infectious disease experts and the Centers for Disease Control If meningeal irritation is present without fever or