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Diagnosis is generally determined by observation of evidence of facial weakness and a vesicular rash. Occasionally, a nerve conduction study will be done to determine the extent of damage to the facial nerve and potential for recovery.
The more severe the damage, the longer it will take to recover, and the lower the chance of complete return to normal function. Occasionally, blood tests are used to determine whether or not an infection with the varicella zoster virus has occurred.
There is a promising laboratory technique called PCR, which can detect very small amounts of viral DNA in the affected skin. However, it is mostly used for research nowadays.
The use of neuroimaging (pictures of the brain), particularly MRI (magnetic resonance imaging), may sometimes show inflammation of the facial nerves and determine whether the infection has spread to other nerves or the brain. A spinal tap is used in rare cases, especially when the diagnosis is not clear.
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