Encephalitis

 

• Background

  Inflammation of the brain.

  Causes

  Viral (50% of cases):

  • Commonest (in order): herpes simplex (HSV1 > HSV2), VZV, EBV.
  • Others: CMV, HIV seroconversion, measles, mumps, arboviruses (West-Nile, Japanese, tick-borne, St Louis), rabies.

  Autoimmune (20% of cases):

  • Often affects the medial temporal lobes, when it is known as 'limbic encephalitis'.
  • Often paraneoplastic.
  • Antibodies (and associations): anti-Hu (small cell lung cancer), anti-Ma2 (germ cell tumour of testes), anti-NMDAR (50% have ovarian tumour), anti-LGI1 (formerly 'anti voltage-gated potassium channel'; only 20% have cancer).

  Other causes:

  • Idiopathic (30% of cases).
  • Any bacterial meningitis can become a meningoencephalitis.
  • Spirochetes: Listeria, Lyme, syphilis.
  • TB
  • Protozoa: malaria, Toxoplasma.
  • Fungal: Aspergillus, Cryptococcus.

  Epidemiology

  • Annual incidence: 1/20,000.
  • Commonest under 1 years old or over 65.

• Signs and symptoms

  • Initially non-specific: fever, headache, nausea, vomiting, malaise.
  • Neuro symptoms: seizures, odd behaviour or confusion, ↓level of consciousness, focal signs.
  • In addition to cerebrum, may also affect brainstem, dorsal roots (sensory), and autonomic nervous system, with an associated wide range of symptoms.

• Investigations

  Take a careful history, including travel and bite exposure.

  Bloods:

  • Blood culture.
  • Serum viral PCR.
  • If suspected: toxoplasma IgM, malaria film.

  Image with MRI or contrast-enhanced CT:

  • Temporal lobe inflammation: usually HSV or autoimmune.
  • Meningeal irritation: meningoencephalitis.

  Special tests:

  • LP: ↑protein (most causes), ↑lymphocytes (viral, autoimmune), ↑PMNs (bacterial), ↓glucose (bacterial). Identify pathogen with viral PCR and Gram stain.
  • EEG: optional.

• Management

  • Aciclovir IV stat to cover HSV. Continued for 14-21 days if HSV confirmed.
  • Consider ganciclovir for CMV if immunosuppressed.
  • Anticonvulsants for seizures.
  • Autoimmune encephalitis: immunosuppression (steroids, IVIg, plasma exchange) and treat any underlying cancer.
  • If infectious, notify public health authorities.

• Complications and prognosis

  • Short term: seizures, ↑ICP, SIADH or diabetes insipidus.
  • Long-term: neurological complications are common, including motor and cognitive problems.
  • 10% mortality overall. For HSV, treatment reduces mortality from >50% to 20%.

• Brain abscess

  Pathophysiology

  • A focal infection of brain tissue, comprising encapsulated pus, in contrast to the generalized infection/inflammation of encephalitis.
  • Source of pathogen: direct inoculation (penetrating trauma, neurosurgery), contiguous spread (mastoiditis, sinusitis, dental infection) or haematogenous (endocarditis, IVDU, right-to-left shunt).
  • Often polymicrobial. Staophylococci and Streptococci are common. In the immunosuppressed, consider Aspergillus, Toxoplasma, and Nocardia.

  Signs and symptoms

  • Classic triad (25%): headache (70%), fever (60%), focal neurology (60%).
  • Others: vomiting, confusion, lethargy, seizures.

  Investigations

  • CT/MRI head with contrast shows ring-enhancement.
  • Avoid LP due to risk of brainstem herniation.
  • Image-guided needle aspiration for culture, but do not delay antibiotics while waiting.

  Management

  • Ceftriaxone plus metronidazole IV for 6-8 weeks.
  • Aspiration or surgical drainage needed for most, though can trial antibiotics alone if small (<2 cm).

• Acute disseminated encephalomyelitis (ADEM)

  Pathophysiology and epidemiology

  • Autoimmune CNS disease, defined by acute-onset encephalopathy, polyfocal neurology, and imaging evidence of multifocal demyelination.
  • Commoner in kids and young adults.

  Presentation

  • Encephalopathy: altered consciousness or behaviour.
  • Polyfocal neurology e.g. weakness, ataxia, gait abnormality, cranial nerve palsies, visual problems, seizures, spinal deficits.
  • Non-specific symptoms: fever, headache, nausea/vomiting.
  • ⅔ have a recent (<4 weeks) history of infection, which is thought to be a trigger.

  Investigations

  • MRI: multiple, large white matter lesions, usually brain, but spine affected in 1/3.
  • CSF: ↑WBC and ↑protein (both around 50% sensitive).
  • No definitive diagnostic test.

  Management

  • Steroids, IV then oral taper.
  • IVIg or plasma exchange 2nd line.

  Prognosis

  Most recover fully, but 25% develop a relapsing demyelinating disease (multiphasic ADEM, MS, or NMO).