Anatomy & Etiology

Abstract The anatomical position of the upper and lower eyelids is dominated by the balance between forces closing the eyelids (protractive) and forces opening the eyelids (retractive). The protractive forces of the lower eyelid are maintained by the orbicularis oculi muscle (OOM), innervated by the VII nerve, whereas the protractive forces of the upper eyelid are the gravitational traction as well as the action of OOM. In the case of VII nerve malfunction, an imbalance of protractive and retractive forces in favor of the latter. In the upper eyelid, this leads to elevation (retraction) of the eyelid margin with poor depression upon voluntary eyelid closure (lagophthalmos). In the lower eyelid, this results in defective support against gravitational traction, depression and progressive eversion of the lower eyelid margin (paralytic ectropion) as well as deficient tear drainage through the active mechanism of lacrimal pump, which is largely maintained by the action of OOM. The resulting epiphora leads to a persistent vicious circle of ocular irritation and digital rubbing by the patient, creating repeated mechanical stress on the anatomical supporting elements of the lower eyelid, i.e. the medial and lateral canthal tendons, which may become destabilized or completely detached. The combined action of the above mentioned components leads to damage to the ocular surface, such as corneal epithelial defects, which may threaten the integrity of the eye through infection or corneal perforation. The anatomy and pathophysiology of VII nerve malfunction as well as their clinical and imaging assessment are discussed.