**6.7 Benign essential blepharospasm**

It is a progressive neurological disorder differentiated by involuntary muscle contractions and spasms of the eyelid muscles.

## **6.8 Canavan disease**

It is one of the most general cerebral degenerative diseases of infancy, is a genelinked, neurological birth disorder where the white matter of the brain degenerates into spongy tissue puzzled with microscopic fluid that filled the spaces. This disease is known as leukodystrophy a group of genetic disorders, which cause faulty development or growth of myelin sheath, the fatty covering that work as an insulator around nerve fibres in the brain. Canavan disease is caused by a mutation in the gene used for an enzyme called aspartoacylase.
