Ramsay Hunt, the disorder paralyzing Bieber’s face

Ramsay Hunt syndrome (RHS), which has forced singer Justin Bieber to cancel his world tour, is a rare and painful complication of the virus that causes shingles and chickenpox.

RHS was discovered in 1907 by the neurologist of the same name. It is a rare neurological disorder that can inflame and then paralyze the facial nerve and cause a painful rash around the ear or mouth.

Symptoms vary from person to person but can cause severe discomfort or pain.

Most sufferers become paralyzed on one side of the face and develop an ear rash, according to the U.S. National Organization for Rare Disorders (NORD).

The affected facial muscles may become weak or feel stiff, preventing the sufferer from smiling, frowning or shutting the eye on their paralyzed side. In certain cases, their speech may become slurred.

In many instances, a reddish, painful blistering rash appears on the outer ear and external ear canal.

"It's often diagnosed because of this ear rash," French infectious disease specialist, Benjamin Davido, told AFP.

Sometimes the blisters spread to the mouth, soft palate and upper throat and ear pain spreads to the neck.

Other possible symptoms include ringing in the ears (tinnitus), earache, hearing loss or hyperacusis where sounds appear much louder than normal nausea and vertigo.

RHS is caused by the varicella zoster virus, which also causes chickenpox in children and shingles in adults.

The virus can remain dormant for decades in a person who has had RHS is generally treated by antiviral drugs such as acyclovir and famciclovir, along with corticosteroids like prednisone.