Revision 6 for 'Rabies encephalitis'

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Rabies encephalitis

Rabies encephalitis

Epidemiology

Infection results most frequently from transmission of the virus from an infected animal to a human via a bite. Occasionally infection can also occur via an open wound, mucous membranes or even as the result of organ transplantation 1

In developed countries, most cases are the result of bites from rabid bats, whereas in the developing world, rabid dogs are the main source of human infection 1

Clinical presentation

Involvement of the CNS by rabies can take two forms 1

  1. classic rabies encephalitis (80%)
  2. paralytic rabies (20%)
Classic rabies encephalitis

Encephalitis is by far the most common presentation of CNS involvement by rabies, accounting for 80% of cases. Symptoms are initially non-specific with general systemic symptoms, inspiratory spasms, autonomic dysfunction and altered mental status. With time classic symptoms and rabies encephalitis develop including: hydrophobia, aerophobia and hypersalivation 1

Paralytic rabies

Paralytic rabies is relatively uncommon accounting only for 20% of CNS infections in humans. It is characterized by bilateral global motor weakness resulting in bilateral facial weakness and quadriparesis, with relative sparing of the sensory system  1

Radiographic features

As is the case with other encephalitides, MRI is the only modality of any use in the diagnosis of CNS rabies, as CT is usually normal. 

CT

Usually normal. Occasionally, non-specific hypoattenuation in the brainstem, mesial temporal lobes, basal ganglia and periventricular white matter may be evident 1. If imaging is only obtained late in the course of the disease, hemorrhage and cerebral swelling may be evident 1

MRI

The main imaging feature is increase in T2 signal (best seen on FLAIR) in the affected parts of the brain and spinal cord, with a predilection for grey matter structures. As the disease progresses, swelling becomes more marked and petechial hemorrhages occur, as well as contrast enhancement 1. The distribution of imaging changes, naturally, depends on the type of involvement. 

In classic rabies encephalitis, increased T2 signal has a predilection for the basal ganglia, thalami, hypothalami, brainstem, and spinal cord as well as the frontal
and parietal lobes 1.  In paralytic rabies, involvement of the spinal cord and medulla are more pronounced, although no specific imaging features exist to allow differentiation form classic encephalitic form 1

Treatment and prognosis

Differential diagnosis

The differential diagnosis depends on the form of CNS involvement. 

For classic rabies encephalitis, especially during the early phases, before the development of classic symptoms, imaging differential includes:

For paralytic rabies, the main differential, and the cause of frequent delay in diagnosis, is Guillain Barre syndrome  1

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