The Bell’s phenomenon, also called the palpebral-oculogyric reflex, refers to the movement of the eyeballs in an upward direction when the eyelids are forcefully closed. It was Charles Bell, a great British anatomist, who first observed this in 1823 when trying to close the eyelids of a patient with facial palsy. Both the facial nerve paralysis and the eyeball deviation observed in this condition are named after him.
In short, this phenomenon represents a reflex in which the eyes are seen to roll up and out when both eyelids are forcibly closed. The facial nerve carries the afferent fibers for this reflex, while the efferent fibers travel via the oculomotor nerve to the superior rectus muscle that controls upper eyelid movement. In Bell’s palsy, this movement is seen because the eyelids fail to close properly.
Why does the Bell’s phenomenon happen?
While nobody knows for sure, the Bell’s phenomenon is linked to the connections between the facial nerve nucleus in the pons, which forms a part of the brain, and the nucleus of the oculomotor nerve in another part of the brain called the midbrain. The facial nerve is responsible for eyelid movement, while the oculomotor nucleus controls the eye muscles which move the eyeball.
Some scientists think that these closure movements are unified in a single movement pattern, and that such integration involves the mesencephalic reticular nucleus.
However, the Bell’s phenomenon itself is a normal reflex seen in eight out of ten people. It basically protects the eyeball (especially the cornea) against sudden contact with a potentially traumatizing agent. For instance, when a person blinks or when the eye is threatened by any object, the eyeballs roll up.
This helps to ensure that the majority of minor traumatic impacts occur on the lower part of the cornea due to the reflex rolling up of the eyelids.
However, the Bell’s phenomenon remains unnoticed until the orbicularis, which is the circular muscle around the eye, weakens. This can happen with bilateral paralysis of the facial nerve, as for example in Guillain-Barré syndrome which causes post-viral neuropathy. The point is that the phenomenon is not a sign of disease in itself.
Why is the Bell’s phenomenon important?
The Bell’s phenomenon is important in evaluating facial palsy. In normal people, the eyes can be squeezed shut, such that the eyelashes are buried between the two eyelids. However, when the facial nerve is injured, only the unaffected eye can complete this action. On the other hand, the eye on the paralyzed side fails to close properly.
In testing the facial nerve function, the strength of the orbicularis muscle is assessed by attempting to open the eyes against resistance. In this situation the Bell’s phenomenon is observed.
In some situations, an inverse Bell’s phenomenon can be seen. This includes patients in the immediate post-operative period after eyelid surgery and in patients with ptosis (drooping eyelids) - either isolated, with myopathy or with complicated ptosis. The condition is more likely after repeated surgery for residual ptosis.
The duration of such inversion depends on the exact etiology. If it is due to post-operative edema of the tissues around the eyeball (limiting in turn the eyeball movement by the superior rectus), it usually resolves within weeks without treatment.
However, if inverse Bell’s phenomenon is caused by injury to the superior rectus and oculomotor nerve, during repeated surgery on the upper muscles moving the eyeball, it may lead to abnormal eyeball movement for years. The chief risk is of exposure keratitis, which must be prevented by training the patient to use lubricating eye drops properly and to come in for check-up frequently.