14th June 2023

Investigations

Initial investigations consist of blood tests, imaging and CSF analysis.

Bloods

Routine bloods tests, including full blood count, urea and electrolytes, liver function tests, CRP, glucose and coagulation, can be useful in the first instance to investigate for a cause of the patient’s symptoms. A lymphocytosis may be seen in viral encephalitis. Serum microbiological cultures, serology and PCR may be useful in determining a causative organism.

In some cases, it may be necessary to rule out toxic causes, in which case bloods for paracetamol, salicylates, tricyclics and heavy metals may be required.

Specialist neurological blood tests may be required in some cases to rule out autoimmune or inflammatory causes. These may include complement levels, lupus anticoagulant, cardiolipin antibodies, thyroglobulin, thyroperoxidase antibodies, ferritin, fibrinogen, triglycerides, voltage-gated potassium channel complex and NMDA receptor antibodies.

Imaging

CT brain is normally the first neurological imaging performed as it is quick to arrange and can help to exclude other causes of symptoms such as brain tumour or abscess. CT brain may appear normal, although in some cases of herpes simplex encephalitis, a low attenuating area in the temporal lobes with surrounding oedema may be seen.

MRI brain is the imaging of choice in suspected viral encephalitis. Typical features of herpes simplex encephalitis include focal oedema of the temporal lobes, insular cortex, and inferolateral frontal lobes.

CSF analysis

Once brain imaging has ruled out evidence of raised intracranial pressure, a lumbar puncture to obtain CSF for analysis is important in supporting the diagnosis and ruling out other differentials, such as bacterial meningitis.

In viral causes of encephalitis, there is typically lymphocytic pleocytosis with a normal glucose and normal or mildly elevated protein. CSF culture and viral PCR can help identify a causative organism.

Other investigations

Electroencephalogram (EEG) can be useful in the diagnosis as well as monitoring of encephalitis and can be useful if distinguishing between encephalitis and metabolic encephalopathy.

In rare cases, brain biopsy may be performed.