Several causes for upper eyelid droop exist and have been characterized as:
1) Myogenic (dysmyogenic) ptosis: most common congenital type, due to the faulty development of the eyelid muscle that pulls the eyelid up (levator muscle).
2) Neurogenic ptosis: results from faulty innervation of upper eyelid lid retractors; ptosis is profound with complete interruption of innervation to the levator muscle.
3) Mechanical or traumatic ptosis: results from any traumatic injury that interferes with levator muscle or its innervation;