Absorption, distribution, metabolism, excretion in otology
Only the external ear is easily accessible for direct drug administration. The main problem secondary to their application comes from skin irritation
Tympanic membrane separates external ear from middle ear. The middle ear is only susceptible to drug effect if incision made in tympanic membrane i.e. injected drug.
Inner ear not accessible for direct drug administration either. Drugs administered to the middle ear may be taken up by inner ear.
No capacity for drug metabolism within inner ear.
Drugs administered systemically to act on inner and middle ear must be able to cross blood inner ear barrier
Earwax is slightly acidic to prevent growth of microbes. pH of drops is a consideration to reduce microbial growth (more acidic).
Ear Drops: easy to administer but likely to run out of the ear. Increasing the viscosity of ear drops makes them more likely to stay in place. Can add cotton wool as a physical barrier to stop the drops running out
Pregnancy & lactation
No risk with topical ear drops – no systemic absorption (if no perforated ear drum). If using in perforation, theoretical risk of foetal ototoxicity and nephrotoxicity if systemic absorption.
Lactation: minimal absorption into breast milk from topical administration. If it can be used in paediatrics, it would be generally safe in lactation. Avoid quinolones.