Bell palsy

What it is

Bell palsy is defined as idiopathic, peripheral facial paralysis or paresis of acute onset. It is an isolated lower motor neurone paresis of the VIIth cranial nerve leading to facial weakness, and a lower motor neuron lesion affects the ipsilateral upper and lower part of the face.

Causes and risk

Bell palsy is the idiopathic form of facial nerve palsy. The secondary causes given are trauma, such as temporal bone fracture; infection, including herpes zoster (Ramsay Hunt syndrome, a vesicular rash around the ear with bells palsy), HSV reactivation and malignant otitis externa; tumors, such as parotid gland tumors and acoustic neuroma; diabetes mellitus; and sarcoidosis. Risk of Bell palsy is more in diabetics and pregnant women. A differential table sets unilateral against bilateral and upper motor neuron against lower, and it fills four cells. Unilateral upper motor neuron: stroke, demyelination and space-occupying lesions. Unilateral lower motor neuron: Bell's palsy (idiopathic), cerebellopontine angle tumor (i.e. acoustic neuroma), Ramsey Hunt syndrome (herpes zoster oticus) and parotid pathology (iatrogenic injury, tumors). Bilateral upper motor neuron: pseudobulbar palsy and motor neuron disease. Bilateral lower motor neuron: sarcoidosis and Lyme disease, DM, GBS, and myasthenia gravis with myopathies. Another page gives the lower motor neuron causes as Bell's palsy, Ramsay-Hunt syndrome (due to herpes zoster), acoustic neuroma, parotid tumours, HIV, multiple sclerosis and diabetes mellitus, with stroke as the upper motor neuron cause. The causes of bilateral facial nerve palsy are sarcoidosis, Guillain-Barre syndrome, Lyme disease and bilateral acoustic neuromas as in neurofibromatosis type 2. As Bell's palsy is relatively common it accounts for up to 25% of cases of bilateral palsy, but this represents only 1% of total Bell's palsy cases. Ramsay Hunt syndrome, that is herpes zoster oticus, is defined as reactivation of VZV in the geniculate ganglion, and because that ganglion sits where it does the reactivation involves the seventh (facial) and the eighth (vestibulocochlear) cranial nerves together. That is what tells it apart from Bell palsy: beside the ipsilateral facial paralysis there is fever and skin symptoms as in shingles in the auditory canal and pinna, and the vestibulocochlear involvement adds vertigo and sensorineural hearing loss. A facial palsy with an ear rash and hearing symptoms is therefore not idiopathic and should not be called Bell palsy.