Optho:Methods of Drug administration

Ocular pharmacotherapeutics can be delivered by four methods:

• —topical instillation into the conjunctival sac, 
• —periocular injections, 
• —intraocular injections and 
• —systemic administration

a)Topical instillation 

Eye drops may be in the form of aqueous solutions (drug totally dissolved) or aqueous suspensions (drug is present as small particles kept suspended in the aqueous medium) or oily solutions

Eye ointment increases the bioavailability of the drug by increasing tissue contact time and by preventing dilution and quick absorption.

Ocusert is a system of drug delivery through a membrane. These can be placed in the upper or lower fornix up to a week and allow a drug to be released at a relatively constant rate. Pilocarpine ocuserts have been found very useful in patients with primary open-angle glaucoma; by efficiently controlling intraocular pressure with comparatively fewer side-effects.

——Soft contact lenses are also used to deliver antibiotics and antiviral drugs in patients with corneal ulcers.A pre-soaked soft contact lens in 1 percent pilocarpine has been found as effective as 4 percent pilocarpine eyedrops in patients with acute angle closure glaucoma.

b)Periocular injections

These are not infrequently employed to deliver drugs.

These include :

• —subconjunctival, 
• —sub-Tenon, 
• —retrobulbar and 
• —peribulbar injections.

Subconjunctival injections. These are commonly used to achieve higher concentration of drugs.
Further, the drugs which cannot penetrate the cornea owing to large-sized molecules can easily pass through the sclera. 

Sub-Tenon injections.These are preferred over subconjunctival injection. Anterior sub-Tenon injections are used mainly to administer steroids in the treatment of severe or resistant anterior uveitis.
Posterior sub-Tenon injections are indicated in patients with intermediate and posterior uveitis .

Retrobulbar injection: These are used to deliver drugs for optic neuritis, papillitis, posterior uveitis and also for administering retrobulbar block anaesthesia.

Peribulbar injection: These are now frequently used for injecting anaesthetic agents. Peribulbar anaesthesia has almost replaced retrobulbar and facial block anaesthesia.

c)Intraocular injections

Such injections are made in desperate cases (e.g., endophthalmitis) to deliver the drugs in maximum
concentration at the target tissue.

These include:

• —intracameral injection (into the anterior chamber), 
• — intravitreal injection (into the vitreous cavity)