27 Alteration of eyelid position and movement
INTRODUCTION
The palpebral fissure is the opening between the eyelids. It forms at 7–10 days in the newborn cat and dog. Opening and closing the palpebral fissure is called blinking. Blepharospasm is a sustained or intermittent, involuntary forced closure of the palpebral fissure. A ‘squint’, in medical parlance, describes strabismus, not the palpebral fissure narrowing of blepharospasm.
PRESENTING SIGNS
Innervation of the palpebral fissure
The size of the palpebral fissure is chiefly controlled by CN III and sympathetic neurons which elevate the upper eyelid (Fig. 27.1). Failure of one or both causes a drooping of the upper lid called ptosis (Table 27.1).
Eye closure is achieved by contraction of the orbicularis oculi muscle, innervated by the facial nerve (CN VII) (Table 27.2).
Table 27.2 Closing the palpebral fissure
| Muscle | Innervation | Action |
|---|---|---|
| Orbicularis oculi | Branch of CN VII | Sphincter-like closure of palpebral fissure. Depresses the eyebrow |
| Retractor anguli oculi lateralis | Branch of CN VII | Draws lateral palpebral angle caudally |
CN VII paralysis prevents blinking. The lack of orbicularis oculi muscle tone may cause the lower lid to droop, thus slightly widening the palpebral fissure in facial paralysis.
Stimuli for blinking
Reflex blinking occurs in response to:
Reflex eyelid contraction can occur in synchrony with the rate of spontaneous nystagmus of vestibular disease. Both features appear, and resolve, in concert with each other.
Excessive blinking or involuntary eye closure (blepharospasm) is most commonly caused by ocular pain. Additional clues such as epiphora and conjunctival hyperemia should be present. Photophobia, the dislike and avoidance of light, is due to ocular pain in the presence of bright light. It is a sign of meningeal irritation.
Dysfunction of the central controls of voluntary blinking may give rise to an increased blink rate, eyelid spasms or eyelid tic. This is known as ‘essential’ blepharospasm and is a form of dystonia reported in humans. Sometimes other facial muscles are involved and hemifacial spasm occurs.
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