Scratches, abrasions and other very superficial injuries to the clear cornea often feel worse than a gunshot wound: super intense, burning, boring discomfort. It often persists even after the eyelids close.
There is a complex layer of sensory nerves that serve the outer cornea. These tiny nerves reside right beneath the epithelial cells and are immediately exposed following any trauma to those outer protective cells. These same nerves activate the instantaneous blink reflex whenever anything approaches our eyeballs, slamming those eyelids closed like a bank vault.
Fortunately, since the nerves are so close to the surface they are very accessible to topical anesthetics. These eyedrops temporarily block the sensory fibers’ ability to conduct a pain signal back to the brain – the problem is still there, your brain just doesn’t get the word.
Attention medical history buffs: Topical anesthetics (drugs like novocaine, lidocaine and tetracaine) derive from the same chemical family as cocaine. In fact, the first reported legitimate medical use for cocaine was for performing eye surgery over a century ago. A lot of history articles depict an oral surgery procedure but the scientist behind topical local anesthesia was Dr. Karl Koller, an Austrian ophthalmologist.
Since these eyedrops numb the pain why not prescribe them for continuous use until the patient recovers? There are several wise reasons that condemn that practice.
Pain is useful to the clinician. If the patient’s symptoms persist the doctor may want to explore other diagnostic possibilities. Pain relief also signals clinical improvement.
There is no guarantee the patient will return. Why should they? Their pain is gone, at least until they run out of eyedrops. In the meantime an abrasion may become an infected ulcer and permanent visual loss may result.
Topical anesthetics can inhibit healing. Additionally, chronic use of topical anesthetic eyedrops is toxic to the eye and can lead to irreversible corneal changes.
For these reasons eye doctors do not usually dispense topical anesthetics to their patients. A comfortable eye patch with a generous amount of ointment will usually work. Savvy, suffering patients may attempt to swipe a bottle of topical anesthetic when nobody is looking. Yes, this happens all the time.
Remember, ignorance is not bliss and anesthesia is not a treatment. Numbing the cornea does little to heal the underlying problem and, in some cases, actually creates new problems.