The three primary methods of delivery of ocular medications to the eye are topical, local ocular (ie, subconjunctival, intravitreal, retrobulbar, intracameral), and systemic. The most appropriate method of administration depends on the area of the eye to be medicated. The conjunctiva, cornea, anterior chamber, and iris usually respond well to topical therapy. The eyelids can be treated with topical therapy but more frequently require systemic therapy. The posterior segment always requires systemic therapy, because most topical medications do not penetrate to the posterior segment. Retrobulbar and orbital tissues are treated systemically.
Subconjunctival or sub-Tenon’s therapy, although not a true form of systemic medication administration, has the potential to increase both drug absorption and contact time. Medications both leak onto the cornea from the entry hole of injection and diffuse through the sclera into the globe. Drugs with low solubility such as corticosteroids may provide a repository of drug lasting days to weeks. Appropriate amounts of medication must be used. Large amounts, especially of long-acting salts, can cause a significant inflammatory reaction. For sub-Tenon’s injections, 0.5 mL per site is usually safe and effective in small animals, and ≤1 mL can be used in large animals such as horses and cows.
Retrobulbar medications are used infrequently for therapeutics. In cattle, the retrobulbar tissues can be anesthetized with local anesthetic (lidocaine/bupivicaine) for enucleation using either a Peterson block (15–20 mL) or a 4-point block of the orbit (5–10 mL/site). Whenever any medication is placed into the orbit, extreme care must be taken to ensure that the medication is not inadvertently injected into a blood vessel, the optic nerve, or one of the orbital foramen. Retrobulbar injection has a high risk of adverse effects and should not be used unless the clinician is experienced and the animal is appropriately restrained.
Systemic medication is required for posterior segment therapy and to complement topical therapy for the anterior segment. The blood-ocular barriers can limit absorption of less lipophilic drugs, but inflammation will initially allow greater drug concentrations to reach the site. As the eye starts to heal, these barriers will again become more effective and can limit further drug penetration. This should be considered when treating posterior segment disease, eg, blastomycosis in small animals with hydrophilic drugs such as itraconazole.
After topical administration, up to 80% of the applied drug(s) is absorbed systemically across the highly vascularized nasopharyngeal mucosa. Because absorption via this route bypasses the liver, there is not the large first-pass metabolism seen after oral administration. Depending on the drugs used, this can result in systemic adverse effects. Topically applied β-blockers used in the treatment of glaucoma can cause heart block, atrial tachycardia, congestive heart failure, bronchospasm, dyspnea, and decreased exercise tolerance. These drugs should be used very carefully in older animals or in animals with cardiac or respiratory disease. Cushing syndrome can be easily induced in small or medium-sized dogs with chronic use of potent topical steroids.