Corneal ulcers are generally painful and that pain is evidenced by blepharospasm (squinting), excessive tearing, and/or attempts by the patient to rub at the affected eye. The source of that pain can be due to exposure of nerve endings in the damaged cornea, reflex ciliary spasm, or a combination.
Recall that the trigeminal nerve (cranial nerve V) provides sensory innervation to the cornea. The ophthalmic branch of that nerve gives rise to the long ciliary nerves which penetrate the sclera near the limbus. They then branch out further and penetrate the cornea as free nerve endings. Of note, there are an abundance of these nerve endings in the superficial layers of the cornea, and a lesser concentration deeper in the stroma. This corresponds with what we often see clinically – that patients with superficial corneal ulcers often are more painful than those with very deep corneal ulcers or descemetoceles. Damage that extends deeper into the cornea also damages those nerve endings in the superficial layers of the stroma and, as those nerve endings are ‘destroyed,’ there’s less of a perception of corneal pain.
In addition to their sensory function, these corneal nerves tie into a reflex arc that can trigger secondary ciliary muscle spasm and miosis. Ciliary spasm triggered by corneal pain (or any cause) is, itself, painful. This is where use of a topical cycloplegic (e.g. atropine) earns its place in our treatment protocols for corneal ulcers. Atropine relaxes the ciliary body muscle, breaking that spasm-induced pain cycle.
Check out this brief discussion between two ophthalmologists about corneal ulcer pain and management options:
Regarding atropine use – when treating a simple, uncomplicated superficial corneal ulcer, a single dose may be all that's needed. Most of these types of corneal ulcers can heal within 72 hours (upwards to 5 days). A single dose of atropine can cause mydriasis (pupillary dilation) that persists for days — sometimes over a week – particularly in cats.
If, however, there's evidence of active reflex uveitis, more frequent or prolonged dosing of atropine is warranted. Clinical findings that would prompt ongoing use of atropine include:
Miosis
Aqueous flare
Hypopyon or hyphema
Lower IOP relative to the unaffected eye
Typically, in these types of cases, administering atropine BID for 2–3 days can achieve pupillary dilation, and then reducing the frequency to once a day for another 3–4 days is sufficient to maintain mydriasis. In severe cases, longer use may be indicated.
What about the formulation – atropine ointment or solution? Clinicians should factor in the ease or ability of the pet’s owner / caretaker to administer the medication at home. Aside from that consideration, the ophthalmic ointment is often preferred by many over the solution. Ophthalmic solutions can drain into the nasolacrimal apparatus and make their way into the the oropharynx. Atropine is very bitter tasting and can cause excessive salivation in pets. Salivation associated with atropine administration is more commonly seen when using a solution versus ointment.
On the other hand, and aside from owner preference, there are other specific reasons to choose the solution form of atropine (or any other ophthalmic medication) over an ointment formulation: patients with deep corneal ulcers (impending rupture) and patients who’ve recently had intraocular surgery. In these situations, it's recommended to use ophthalmic solutions and not ointments, as ointments can be extremely inflammatory and/or toxic to intraocular tissues (if they were to enter the eye).
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