Encephalitis and other CNS diseases

Infectious diseases rarely cause acquired brain injury (ABI) in developed countries thanks to major advances in vaccines and antibiotic treatments. Nevertheless, the severity of the possible aftereffects and the existence of effective treatments in most cases mean that it is crucial to be aware of the symptoms, clinical course and treatment of infectious diseases.

Classifying infectious diseases is difficult because of the large number of germs involved and the range of symptoms they produce. The central nervous system (CNS) may be attacked by bacteria, viruses, fungi and other microorganisms.


Bacterial Meningitis

  • The most common bacterial infection, acute bacterial meningitis, consists of an inflammatory response to the presence of bacteria in one of the layers surrounding the brain, namely the leptomeningeal space.
  • In some cases, these processes become chronic over time and may last several weeks, leading to what is known as subacute or chronic meningitis. Tuberculous meningitis, brucellosis and neurosyphilis are examples of subacute or chronic meningitis.
  • The most frequent cause of meningitis varies with the patient’s age, whether the disease was contracted inside or outside the hospital and the patient’s history of associated diseases (diabetes, alcoholism, etc.). Most acute cases cause what is known as meningeal syndrome (headaches, fever, stiff neck, nausea and vomiting). In the case of the subacute or chronic symptoms, meningeal syndrome may not be detected, which hinders early diagnosis and may cause delays in treatment.
  • The mortality rate for acute bacterial meningitis is still as high as 20%, especially in very young or very old patients. Up to 30% of surviving patients suffer aftereffects, with the most common aftereffects being hearing loss, paralysis of cranial nerves, hydrocephalus, epilepsy and focal neurologic signs caused by necrosis or cerebral infarction. Paediatric patients require close monitoring because there have been cases where patients have subsequently suffered problems in their physical or intellectual development.
  • The prognosis for tuberculous meningitis and neurosyphilis depends predominantly on the patient’s age and the time elapsed before the patient starts treatment. Serious aftereffects may occur in as many as 20% of patients. The aftereffects are similar to those of other forms of meningitis. In addition, patients may suffer aftereffects from cerebral infarction caused by the inflammation of arteries in the brain (arteritis) owing to the infection and inflammation process. In the case of neurosyphilis, inflammation can extend to the brain parenchyma, causing chronic encephalitis, which may lead to cognitive deterioration, behavioural changes, hallucinations, delirium or psychotic episodes.

Abscesses and empyemata

  • Brain abscesses and empyemata are caused by a collection or gathering of pus in the inflammatory tissue due to an infectious process in the surrounding area (mastoiditis, otitis, sinusitis, etc.) or another part of the body (heart disease, etc.).
  • The clinical syndrome depends heavily on the location of the abscess or empyema. Some cases go completely undetected and are diagnosed by chance. In other cases, symptoms are similar to those of a stroke (frailty, loss of vision, trouble articulating words, etc.), generally accompanied by headaches or epileptic seizures.
  • The rate of mortality due to abscesses is close to 20%. The prognosis in patients who survive depends on, among other factors, the speed with which the patient starts treatment, whether surgery is necessary and the patient’s overall health.


  • Fungal infections of the CNS are rare in developed countries. They generally spread to the brain tissue from other organs in patients with serious alterations in their immune system. These germs can cause subacute or chronic meningitis, abscesses or even strokes due to the inflammation of blood vessel walls as a result of the infection and inflammation process (vasculitis).


Viral meningitis

  • Viral meningitis causes symptoms similar to those of bacterial meningitis, albeit to a lesser degree. Most cases can be successfully cured with no aftereffects.

Viral encephalitis

  • Viral encephalitis is caused by the effect of a viral infection on blood vessels (vasculitis) or the brain tissue (demyelinating disease, necrosis, etc.).
  • Initial clinical symptoms are usually preceded by flu-like symptoms (fever, sickness, headaches, etc.) and include an altered state of consciousness, epileptic seizures and focal neurologic deficits similar to those caused by strokes.
  • The viruses that most frequently cause ABI are the herpes simplex virus-1, which causes herpetic encephalitis, the herpes-zoster virus and HIV, which causes AIDS.
  • Up to 70% of patients who have suffered herpetic encephalitis suffer aftereffects. Up to 10% of patients cannot live independently because of the severity of the aftereffects. The most frequent problems are cognitive difficulties (learning and memory problems) and behavioural disorders, frequently accompanied by epileptic seizures. Other less common problems include trouble recognising objects, words or faces (prosopagnosia).
  • HIV can cause isolated cognitive problems of varying intensity that may be as severe as dementia. Both its prognosis and its treatment depend principally on the control of the disease using antiretroviral therapy.


This type of infection includes toxoplasmosis caused by the parasite toxoplasma gondii. Toxoplasmosis acquired as an adult or in healthy children is usually asymptomatic, but it may lead to symptoms in cases of patients with AIDS or in congenital cases such as pregnant women who suffer an initial infection during the first few weeks of pregnancy.

  • The manifestations of toxoplasmosis in patients with AIDS vary greatly and include epileptic seizures, confusional state or focal symptoms similar to those of a stroke.
  • The manifestations of congenital toxoplasmosis commonly entail cognitive problems and intellectual disability.