Hemifacial spasm (HFS) is a rare neuromuscular disease characterized by irregular, involuntary muscle contractions (spasms) on one side (hemi-) of the face (-facial) The facial muscles are controlled by the facial nerve (seventh cranial nerve), which originates at the brainstem and exits the skull below the ear where it separates into five main branches. This disease takes two forms: typical and atypical. In typical form, the twitching usually starts in the lower eyelid in orbicularis oculi muscle.
Causes : Three theories exist to explain the facial nerve dysfunction found in hemifacial spasm. The first proposed theory is ephaptic transmission, which is electrical activity crossing from one demyelinated neuron to another resulting in a false synapse. The second theory involves abnormal activity of axons at the facial nerve root end zone secondary to compressive damage/demyelination. The third theory or "Kindling theory" involves increased excitability of the facial nerve nucleus.
Signs and symptoms : The first sign of hemifacial spasm is typically muscle movement in the patient's eyelid and around the eye. It can vary in intensity. The intermittent twitching of the eyelid, which can result in forced closure of the eye which gradually spreads to the muscles of the lower part of the face (Typical form- See Image). In atypical form the spasms start in the cheekbone area and spreads to the eyelid. Ultimately, all the muscles on that side are affected, nearly all the time.
Statistics The incidence of hemifacial spasm is approximately 0.8 per 100,000 persons.Hemifacial spasm is more prevalent among females over 40 years of age. The estimated prevalence for women is 14.5 per 100,000 and 7.4 per 100,000 in men. Prevalence for hemifacial spasm increase with age, reaching 39.7 per 100,000 for those aged 70 years and older. One study divided 214 hemifacial patients based on the cause of the disease.