Bell phenomenon

At the Bell phenomenon The eyeballs roll upwards, as they do in the context of the blink reflex. The so-called faialis nerve is primarily involved in the reflex movement, so that facial palsy is often associated with a failure to close the eyelid. If the eyelid is not closed completely, the Bell phenomenon reveals the whiteness of the eyeball.

What is the Bell Phenomenon?

The Bell phenomenon is characterized by an upward rolling of the eyeballs. This movement takes place within the framework of the so-called eyelid-closing reflex or blink reflex. This phenomenon is a reflective protective movement of the eyes in which the lid closes automatically and involuntarily.

As an innate external reflex, the efferent and afferent fibers of the eyelid closing reflex are not in the same organ. The eyelid closure is more likely triggered by the interconnection of several consecutive synapses. After the mechanical irritation of the cornea or the skin in the immediate vicinity of the eye, the reflex movement causes the eyelids to close and is accompanied by the upward rolling of the eyeballs.

The Bell phenomenon primarily means that this upward movement, and thus the white eyeball, becomes visible when the eyelid closes less. In this form, the phenomenon has disease value and occurs as a symptom above all in the context of facial nerve paralysis.

The namesake of the Bell phenomenon is the British physiologist Charles Bell, who first observed the phenomenon in the 19th century.

Function & task

The eyelid closing reflex is a physiological protective reflex that is intended to protect the human organ of vision and the cornea from mechanical injuries, from drying out and from foreign bodies. The receptor of the reflex arc is the cornea. After this reflector is stimulated, the stimulus is transmitted in the form of an action potential via the afferent leg and thus the nasociliary nerve and the first trigeminal branch of the ophthalmic nerve to the trigeminal ganglion.

The excitation reaches the sensitive fibers from which central ganglion cell processes extend to the core of the trigeminal nerve. In the nucleus spinalis nervi trigemini the stimulus is switched, migrates via the colliculus superior into the formatio reticularis and reaches the nucleus nervi facialialis, where the efferent leg of the reflex movement begins.

The fibers of the nucleus nervi facial are attached to the fibers of other facial nuclei and together with them form the nervus facial or also the 7th cranial nerve. The visceromotor fibers of this facial nerve innervate the orbicularis oculi muscle. When the excitement reaches this muscle, it contracts and causes the eyelid to close. The eyelid closing reflex is a consensual reflex whose afferents lie on the ipsilateral and contralateral facial nuclei.

The upward movement of the eyeballs occurs physiologically at the same time as the reflex movement and has no disease value per se. Rather, the physiological movement itself is a protective reflex and corresponds, for example, to the position of the eyeballs during sleep. However, if the movement is visible and the whites of the eyeballs can be seen while the eyelids are closed, then the Bell phenomenon can be described as pathological.

The blink reflex and the eyeball roll always take place simultaneously in both eyes. Activation of only one eye is not possible due to the interconnection. The Bell phenomenon can, however, also only be present in one eye and thus occur, for example, in the context of a one-sided facial paralysis that blocks the lid closure in one of the two eyes.

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Illnesses & ailments

A facial nerve paralysis is technically referred to as a facial paralysis and corresponds to a paralysis of the facial nerve. Facial nerve palsy can be congenital or acquired. They are caused by either peripheral or central nerve damage.

The incomplete lid closure and thus the Bell phenomenon is characteristic of paresis. While the eyelid closure reflex does not occur, the Bell phenomenon persists even with lagophthalmos, i.e. incomplete eyelid closure. Facial paralysis can also be associated with drooping corners of the mouth. Weakened or eliminated frown behavior can also be symptomatic.

There are a number of reasons for the incomplete eyelid closure due to facial paralysis. Infections such as borreliosis can be the cause, as can head trauma, tumors or inflammation and strokes.

The incomplete eyelid closure and the Bell phenomenon are sometimes associated with the expression of Bell's palsy, in which a unilateral facial paralysis is present. The cause of the paralysis is unknown in the case of Bell's palsy. Presumably, the paresis is caused by a compression of the facial nerve associated with inflammatory processes. In most cases, Bell's palsy resolves on its own within a few weeks or goes away with treatment with corticosteroids. It rarely causes permanent damage. However, complete paralysis of the half of the face should be treated well in order to achieve healing with complete remission of the symptoms.

Both the Bell phenomenon and Bell’s palsy are usually a matter of neurology. Incomplete eyelid closure, in particular, is often symptomatic of a primary disease such as multiple sclerosis. This autoimmune disease causes episodes of immunological inflammation in the central nervous system and thus demyelinates the central nervous tissue. The conductivity of the affected tissue is therefore often permanently impaired.

The Bell phenomenon and the blink reflex not only play a role in medicine in connection with diseases of the nervous system, but are also important parameters for assessing the depth of anesthesia in anesthesia.