Eyelid closure

At the Eyelid closure The upper and lower eyelids meet until the fissure of the eyelid is completely closed and the eye is no longer visible. The seventh cranial nerve of the mimic muscles is primarily involved in eyelid closure, which protects the eye from drying out and from dangerous stimuli with the help of the eyelid closure reflex. If the nerve is paralyzed, the eyelid closure is incomplete.

What is the eyelid closure?

In addition to an upper eyelid, the human eye is equipped with a lower eyelid. The so-called eyelid gap, through which the eye is visible, lies between the lids. When the upper and lower eyelids meet, the fissure of the eyelid is completely closed and the eye is fully covered. The active merging of the upper and lower eyelids is also known as eyelid closure.

The human eye is protected and moistened by closing the eyelid. As part of the so-called eyelid closure reflex, the eyelid closure takes place automatically in response to certain stimuli, in the form of an external reflex.

The eyelid closure is carried out via the facial muscles and, in addition to the reflexively unconscious variant, can also take place consciously, as long as control over the facial muscles is given. Above all, the orbicularis oculi muscle and with it the seventh cranial nerve are involved in conscious and unconscious eyelid closure. The muscle is therefore irreplaceable for moisturizing and general protection of the cornea. In the form of wetting with tear fluid, the muscle prevents the eye from drying out when the eyelid is closed. The concept of eyelid closure is more associated with a conscious, non-automated eyelid closure than with the associated reflex.

Function & task

As a human visual perception system, the eyes are one of the most important perceptual entities. In the course of evolution, they have sometimes ensured human survival. For this reason, the eyes are equipped with many different protective functions. One of them is the closure of the eyelid gap. Closing the eyelids does not dry out the eyes. The eyelid-closing reflex also keeps environmental hazards away from the eye and takes place in response to stimuli that move towards the eye.

The orbicularis oculi muscle is the most important muscle for the eyelid closure function. It lies in the area of ​​the orbit opening and is also known as the eye ring muscle. It surrounds the eye in a circle, thus enclosing the fissure of the eyelids. The muscle is one of the mimic muscles and consists of three different parts. The orbital part arises on the frontal process at the maxima and the nasal part on the frontal bone. This part surrounds the fissure of the eyelid. The pars palpebralis originates from the ligamentum palpebrale mediale and the pars lacrimalis arises from the crista lacrimalis posterior, where it encloses the bags under the eyes. The orbicularis oculi muscle is innervated by the temporal branch and the zygomatic branch of the seventh cranial nerve. Because the muscle is fused with the corium, the skin follows its movements.

The reflex-like eyelid closing movement is known as the eyelid closing reflex and corresponds to a foreign reflex that does not carry afferents and efferents in the same organ. The afferent leg of the reflex is the ophthalmic nerve, if tactile stimuli trigger the eyelid closure. If, on the other hand, optical stimuli such as bright light are involved in the protective reflex, the optic nerves form the afferent limb. After switching the stimuli to the trigeminal complex, they are conducted via the superior colliculus or the ruber nucleus into the reticular formation, from where they migrate to the reflex center of the brainstem and thus reach the facial nucleus. The efferent branch of the reflex is the seventh cranial nerve which, in response to the stimuli, causes the orbicularis oculi muscle to contract. The blink reflex always takes place in both eyes. This also applies if only one of the eyes is threatened by a stimulus.

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Illnesses & ailments

The failure of the orbicularis oculi muscle is one of the most obvious complaints that can occur in connection with the eyelid closure. Such a partial or complete failure is the result of paralysis of the facial muscles and is therefore primarily caused by the failure of the facial nerve. A paralysis of this nerve is a peripheral paralysis and can occur, for example, as part of a polyneuropathy or a nerve injury. A polyneuropathy, in turn, can be due to a vitamin deficiency, a past infection, or poisoning as the primary cause.

In the case of paralysis of the seventh cranial nerve, the symptomatic picture corresponds to an incomplete eyelid closure, which is known as so-called lagophthalmos. In many cases, the incomplete eyelid closure dries out the cornea and causes what is known as xerophthalmia. Patients with incomplete eyelid closure therefore usually perceive the symptoms as a burning sensation or a foreign body sensation in the eye.

Sometimes keratitis e lagophthalmo also develops as part of an incomplete eyelid closure. This is an inflammation of the cornea that in some cases causes ulcers. These ulcers are also known as corneal ulcers. If the patient tries to close the eyelid in spite of the ulcer, Bell's phenomenon appears. The eyeball rotates upwards temporally.

In addition to peripheral paralysis of the seventh cranial nerve, scars, for example, can cause incomplete eyelid closure. With scar tissue in this area, the eyelids shorten and therefore no longer meet because they do not reach each other in length. Other causes of incomplete eyelid closure are exophthalmos, coma or ectropion. The latter clinical picture is a malalignment of the eyelid, which causes the inadequate lid closure.