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Cornea/External Disease

Take a big-picture approach when dealing with corneal sensation

Remember that corneal nerves track outside the cornea, and even up into the brain.

[Topical agents] [Site of incision]

[Your turn]

December 15, 1997

BALTIMORE - When dealing with corneal sensation in cataract and refractive surgery, it is important to recognize that the neural pathways for corneal nerves track outside the cornea, even up into the brain, according to Terrence P. O'Brien, MD, of the Wilmer Eye Institute-The Johns Hopkins University.

"In the cornea, the nerves form a dense subepithelial plexus," he said. "There is rich, exquisite innervation of the cornea."

According to Dr. O'Brien, fine anterior nerves are not easily visualized at the slit-lamp most of the time. However, there are a number of conditions in which the corneal nerves thicken and become more prominent, such as multiple endocrine neoplasia type 2 and infectious diseases such as leprosy. Certain conditions also may decrease corneal sensitivity and may affect the blinking mechanism, for example ocular herpes, diabetes, thyroid disease or leprosy.

Topical anesthetic agents have an advantage with local application, according to Dr. O'Brien. He discussed the following: proparacaine, tetracaine, cocaine, lidocaine (Xylocaine, Astra) and Marcaine (bupivacaine hydrochloride, Sanofi Winthrop).

Topical agents

"Proparacaine is probably the most commonly used," Dr. O'Brien said. "It has a relatively short duration, lasting 15 minutes. It has rapid action and can be applied locally. Tetracaine is a little more toxic and irritating locally and has a slightly longer lasting action, about 20 minutes. Cocaine is even more toxic. It has a rapid onset, but the duration is shorter than proparacaine or tetracaine. It is used to loosen the corneal epithelium. It interrupts the connection between epithelial cells and leads to the breakdown of the corneal epithelium. It can even dilate pupils. Lidocaine is more potent, with rapid diffusion and penetration. It's longer acting than topical agents. Bupivacaine has a slow onset, but prolonged effect so there can be a sustained duration of effectiveness."

"To begin, topical proparacaine can be administered in the preoperative holding area," Dr. O'Brien said. "Then in the operating room, additional tetracaine or lidocaine is applied for a more sustained topical effect."

Site of incision

"The anatomic site of incision initiation can be a scleral tunnel, a limbal incision or a clear corneal incision," Dr. O'Brien said. "I prefer to make it just at the junction of the limbus and cornea where the vascular arcade enters the cornea. I like to see a little bleeding too. Incisions in this zone tend to undergo a more complete, stronger healing process."

Dr. O'Brien makes a hinge incision, creating a deep perpendicular groove 500 to 600 µm in depth. He then creates a perpendicular tunnel within the cornea, beginning at one-third of stromal depth, following the curve of the cornea 1.5 to 2 mm. Then he dimples down to create a self-sealing valve incision.

"Any of the diamond blades can be used, including three-dimensional blades which create the seal in a single pass," he said.

After surgery, a 10-0 nylon or 10-0 vicryl suture is placed to seal the incision and to prevent entry of bacteria.

"It's important to remember that free axon terminals are exposed during excimer surgery," Dr. O'Brien said. "The sub-epithelial plexus of nerves is removed in the treatment zone. But at the edges of the ablation, the plexus has free nerve endings."

Topical nonsteroidal anti-inflammatories once or twice daily are probably sufficient analgesic after surgery. Sometimes an oral compound can help palliate pain. Occasionally, a short course of non-preserved topical anesthetic may be provided (not prescribed).

PGE2 prostaglandin levels, which help alleviate pain, are increased after excimer procedures. Voltaren (diclofenac sodium, Ciba Vision) and Acular (ketorolac tromethamine, Allergan Inc., Irvine, Calif.) also can reduce the amount of pain following these procedures.

"It's important to remember the normal mechanisms of corneal sensation to provide the most comfort for our patients undergoing ophthalmic surgeries," Dr. O'Brien said.

For Your Information:

• Terrence P. O'Brien, MD, Cornea/External Disease Section Member of the Ocular Surgery News Editorial Board, and director of refractive surgery at the Wilmer Eye Institute, can be reached at 600 N. Wolfe St., Baltimore, MD 21287; (410) 955-1671; fax: (410) 614-0682. Dr. O'Brien has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.

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Copyright 1999, SLACK Incorporated. Revised 9 December 1999.