Only 1 - 10% of topical drugs used in the eye will be absorbed. How much is due to a number of factors: contact time with the eye (directly related to the viscosity of the preparation), tissue permeability and the concentration of the drug administered. The contact time with the eye depends largely on the type of preparation given: drops are washed away very quickly by the eye's natural tears whereas an ointment is an oily solution that remains in the eye for some time.
If several drops are required to be given at the same time of day it is recommended that 10 minutes be left between each dose. This is because the second drug will dilute the first and therefore a smaller dose of the first drug is given.
Increasing the drop size does not increase the dose delivered to the eye. Any extra fluid is washed out and will be absorbed systemically via one of several routes. Some will be absorbed into the general circulation via the capillaries in the conjunctival sac. The drops that are removed via the lacrimal system may either be absorbed into the nasal mucosa or swallowed. Topical beta-blockers can have a marked systemic effect via these mechanisms.
Tear flow decreases with age, and in certain individuals with problems such as dry eyes. These people will absorb more of any topical medication given. This is due to an increased contact time as there are fewer tears to wash away the drug and because of the smaller tear volume the drug will be less diluted on the surface of the eye.
Aqueous Solutions
Almost all multi-dose drop bottles contain preservatives. These are often abbreviated as G. (gutta). N.B. There is no shorthand for minims. Many of these preservatives are toxic and high doses of them can damage the surface of the eye. It is recommended that if a drug is required to be given more than four times a day or if there is any perforation of the eye, then a preservative free preparation should be used, e.g. minims. Minims are single use and preservative free.
Ointments
As ointments are so viscous they have a longer contact time with the eye and so have to be applied less often. For this reason ointments are often given at night to eliminate the need for multi-dosing of drops through the night. Ointments are also useful for children who it may be difficult to administer drops to. Ointment is often abbreviated as Occ (oculentum).
Injections
Injections allow higher concentrations of drugs to be achieved locally. There are several types of injections in ophthalmology:
- Subconjunctival - below the conjunctiva
- Intracameral - into the anterior chamber of the eye
- Intravitreal - into the vitreous humour
- Peribulbar - around the globe but avoiding the muscle itself
- Retrobulbar - into the muscle cone behind the eye
With intravitreal injections it is important that the fluid injected is isotonic, has a neutral pH and that it does not contain preservatives. If the solution is not isotonic it can cause osmotic fluid shifts which can alter the intra-ocular pressure. Preservatives can be toxic to the structures of the eye. Intravitreal injections are especially useful in the treatment of endophthalmitis and diabetic macular oedema.
Some drugs are available as implants that can be implanted into the vitreous.
Peribulbar and retrobulbar injections are most commonly used for anaesthetic injections to allow procedures to be carried out under local anaesthesia. Retrobulbar injections are associated with a risk of globe penetration, direct injection into the optic nerve and retrobulbar haemorrhage. For this reason it is usual practice that an anaesthetist will be on stand-by for this procedure despite it being a local anaesthetic. Peribulbar injections do not have the same risks although the degree of akinesia and anaesthesia achieved can be variable and the patient may require further injections.
Prophylactic antibiotics are often given by injection at the end of the surgical procedure. These are normally subconjunctival or intracameral.
