Brain inflammation still leaves the culprit unknown in many cases
Encephalitis means inflammation of the brain, spanning mild confusion to seizures, aphasia, and memory loss. Viruses such as herpes simplex and rabies are leading infectious causes, yet bacteria, fungi, parasites, autoimmunity, and drugs also trigger it—and thirty to forty percent of cases never get a clear etiology. In 2015 it affected an estimated 4.3 million people and killed about 150,000.
Adults often present with sudden fever, headache, confusion, and sometimes seizures; infants may show irritability, poor feeding, and fever. A stiff neck suggests meningeal irritation. Diagnosis combines clinical findings with bloodwork, imaging, and cerebrospinal fluid analysis. Vaccines prevent some forms; hospital care may include acyclovir, anticonvulsants, steroids, and occasionally mechanical ventilation, followed by rehabilitation.
Limbic encephalitis targets emotion and memory circuits, yielding disorientation, disinhibition, seizures, and medial temporal T2 bright signals on MRI; some cases are autoimmune. Encephalitis lethargica swept the world from 1917 to 1928 with a still-unknown cause. Herpes simplex is the most common identifiable viral culprit; rabies, polio, and measles viruses also appear. Weak immunity raises risk.
Because so many cases stay idiopathic, clinicians treat empirically while labs race. Prevent what vaccines can, and treat fever with altered mental status as a neurologic emergency rather than mere flu.
A formal diagnosis needs a lowered or altered level of consciousness, lethargy or personality change lasting at least a day with no better explanation. Doctors then use MRI to spot inflammation, EEG to catch abnormal brain signals, a spinal tap to sample cerebrospinal fluid, and PCR on that fluid to find viral DNA. The list of culprits is long: arboviruses such as West Nile and St. Louis encephalitis, bacteria behind syphilis and Lyme disease, and, in people with weakened immunity, toxoplasmosis or malaria. Free-living amoebae cause rare, usually fatal forms. The commonest autoimmune type, anti-NMDA receptor encephalitis, comes with an ovarian teratoma in 58 percent of affected women aged 18 to 45.
Source: Encephalitis
