FOR PATIENTS

Eyes with anesthetic corneas or those with reduced sensation (hypoesthesia) are classified in the group of ocular surface disease. The human cornea has the highest density of sensory nerves and is the most sensitive tissue of the human body. In addition to its obvious protective function, the integrity of the cornea's nerves is critical to the normal healing process. Aging, exposure to air and the friction of blinking result in damage and death of some corneal surface cells. These are rapidly replaced by the reparative process. The normal balance between death and renewal of corneal cells maintains the integrity of the cornea. When the cornea has no sensation, this balance is upset. Anesthetic corneas are almost always associated with reduced tear production. The reduced lubrication of dry eyes increases the damage caused by drying during exposure to air between blinks. To make matters worse, corneal healing is impaired when its nerve supply is compromised. As a result, these corneas are vulnerable to the development of spontaneous ulcers that can cause permanent scarring and, in rare cases, perforate requiring emergency corneal transplant surgery to save the integrity of the eyes. Surgery rarely restores functional vision and transplants in these eyes often fail to heal completely.

Causes of corneal anesthesia

Anesthetic corneas may be present at birth (congenital) as one of multiple disorders (syndrome). It can also be inherited such as in familial dysautonomia. These infants are at higher risk of profound and permanent vision impairment because of inadvertent self mutilation, a consequence of the absence of corneal sensation and their inability to recognize a change in their visual status or correlate cause and effect. Compounding this danger is the fact that corneal ulcerations in these eyes are often not accompanied by other clues such as eye redness. All too often the first sign of this condition is a white spot (ulcer or scar) on the cornea that has already impaired vision. When vision is compromised at an early age, the development of the brain's ability to process images of that eye is interrupted (amblyopia) and this can be permanent if vision is not rapidly restored and the affected eye is not forced to work by intermittent patching of the good eye in unilateral cases.

In most cases, the damage to the sensory nerve of the cornea (trigeminal nerve) at some point in its pathway anesthetic corneas is acquired. It can be a complication of acoustic neuroma (benign tumor of the hearing nerve) surgery, procedures designed to destroy the trigeminal ganglion ( trigeminal ganglionectomy or rhyzotomy) for the intractable pain of trigeminal neuralgia), herpes simplex infections of the cornea, shingles (herpes zoster) of the cornea and diabetic neuropathy (extremely rare).

Management of neurotrophic corneas

The principal goal is to protect the cornea from drying and the friction of blinking through the frequent use of lubricating agents and slowing tear drainage through the insertion of punctual plugs. This often suffices to prevent complications. However, when corneal ulcers do develop and fail to heal or recur, more drastic measures are needed to limit the scarring (that reduces vision permanently) and prevent the cornea from perforating. Since corneal transplants in these eyes rarely heal completely, surgery is not an option except as a last resort to seal a perforation. Partial, extensive or complete suturing of the lids together (tarsorrhaphy) has been the most effective treatment when all other measures fail. Because of patient resistance, tarsorrhaphy is often delayed and this can result in permanent scarring of the cornea. Although it can be easily reversed, doing so in some cases may lead to recurrences of the corneal ulcers. Moreover, its use in infants and children is a last resort because the interruption of vision at an early age will stunt the vision potential of that eye.

The Boston Sclera Lens device adds a new dimension to the management of neurotrophic corneas. By enclosing the corneal surface in a protective reservoir of artificial tears, it is often effective in preventing recurrences of ulcers. When used as a prophylactic measure, the device is worn during waking hours and the eye is protected during sleep by instilling a lubricating ointment at bedtime and, in some cases, taping the lids closed. The Boston Scleral Lens can also be effective in healing corneal surface defects when all other measures fail. This usually requires the lens to be worn continuously and removed every 48 hours in the corneal specialist's office in order for the eye to be examined. Close monitoring is important because continuous wear increases the risk of developing a devastating infected ulcer.

When is the Boston Scleral Lens Device indicated?

This device should be considered when there are signs of corneal erosions or a history of ulcer despite the aggressive use of lubricants. Whenever possible, this treatment should be initiated early before scarring has occurred or to prevent it from becoming worse since this complication is permanent. The development of a large erosion is more ominous since it is far more difficult for it to heal, generally leaves a denser scar and may require continuous wear of the device that increases the risk of infection. Therefore, when it is indicated, fitting the BSL should be done early. In general, the BSL is an effective alternative to tarsorrhaphy and offers the advantage of not compromising vision, being more cosmetically acceptable and enabling the ophthalmologist to monitor the corneal status more effectively.

There are many anesthetic corneas that do not suffer from complications and therefore do not require the BSL. However, infants and young children are an exception to this rule since inadvertent self-mutilation and ulceration of their insensitive corneas is common and even temporary interruption of vision can have permanent consequences.

Are you (or your child) a candidate for the BSL?

Because the process for custom-fitting this device is costly and time-consuming and our resources are limited, we do not accept patients without a referral from their ophthalmologists that includes a detailed report describing the condition of the patient's eyes and documentation that all other available treatments have been unsuccessful.

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