The human face is animated by a complex network of muscles controlled by the seventh cranial nerve, also known as the facial nerve. This nerve originates in the brainstem and branches out to govern nearly all movements of facial expression. When this nerve becomes inflamed, swollen, or compressed, it results in a condition called 7th nerve palsy, the most common form of which is Bell’s palsy. This condition leads to a sudden weakness or paralysis of the muscles on one side of the face.
A primary consequence of this facial paralysis is the impact on the structures that protect the eye. When the muscles controlling the eyelid are weakened, the ability to protect the eye’s sensitive surface is compromised. This disruption can lead to ocular issues that require immediate attention to prevent lasting damage. These problems are not due to an issue within the eye itself, but a failure of the external mechanisms that shield it.
How Facial Nerve Palsy Affects the Eye
The most immediate effect of 7th nerve palsy on the eye is the inability to close the eyelid completely, a condition known as lagophthalmos. This incomplete closure is caused by the paralysis of the orbicularis oculi muscle, a circular muscle that surrounds the eye and is responsible for shutting it. Without proper function, the eyelid cannot descend fully, leaving a portion of the eye’s surface, particularly the cornea, constantly exposed to the air. This exposure disrupts the stability of the tear film that normally coats and protects the cornea.
This constant exposure is compounded by the loss of the spontaneous blink reflex. Weakness in the orbicularis oculi muscle diminishes or eliminates this reflex, meaning the tear film is not replenished as needed. Blinking normally spreads a fresh layer of tears across the eye, keeping it moist and washing away debris. The cornea relies on the tear film for oxygen and nutrients, and its exposure leads to a state of severe dryness.
A seemingly contradictory symptom that often accompanies this dryness is excessive tearing, or epiphora. This occurs not because the eye is producing too many tears, but because the system for draining them has failed. The paralysis often causes the lower eyelid to droop away from the eyeball, a condition called ectropion. This malposition prevents tears from reaching the punctum, the small drainage opening at the inner corner of the eyelid, causing them to spill over onto the cheek.
Essential Eye Protection Strategies
The primary strategy for protecting the ocular surface involves substituting the natural lubricating function of the eyelids and tears with external products. Common methods include:
- Frequent application of preservative-free artificial tears during waking hours to combat dryness. Using drops without preservatives is recommended because the high frequency of use could lead to irritation from the chemicals themselves.
- Application of a thicker lubricating ointment or gel before sleep. These formulations provide a more durable protective barrier over the cornea than artificial tears, preventing evaporation throughout the night.
- Physically closing the eyelid during sleep by gently taping it shut with hypoallergenic medical tape. The tape should be applied from the upper lid to the cheek, ensuring the eye is fully closed without pressure.
- Creating a humid environment with moisture chambers, which are small plastic shields that fit over the eye like goggles. A doctor might also suggest a temporary external eyelid weight to help the lid close with gravity.
Recovery and Long-Term Ocular Changes
For many individuals, the function of the facial nerve begins to return within a few weeks to several months, leading to a gradual resolution of eye-related symptoms. As the orbicularis oculi muscle regains strength, the ability to blink and close the eye improves, restoring the natural mechanisms of corneal protection. The recovery process varies widely depending on the underlying cause of the palsy and the extent of the nerve damage.
In some cases, as the facial nerve heals, its fibers can regrow in a disorganized way, leading to a condition called synkinesis. This “faulty wiring” results in involuntary, linked muscle movements. For instance, nerves that originally controlled the mouth might regrow to connect with the eye muscles, causing the eye to close when the person smiles, speaks, or chews.
Another phenomenon that can occur during recovery is gustatory lacrimation, sometimes called “crocodile tear syndrome.” This involves the misdirection of regenerating nerve fibers that control salivation to the lacrimal gland, which produces tears. As a result, the affected individual may find their eye tearing up whenever they eat or even think about food.
While many patients experience a full or near-full recovery, a subset may be left with persistent weakness or other long-term ocular issues. Incomplete recovery of eyelid closure can lead to chronic dry eye and exposure issues that require ongoing management. If significant problems like persistent tearing or eyelid malposition remain, further evaluation by a specialist may be necessary to explore other treatment options.