Dimensions of Human, Guinea pig and Mouse ears derived from 3-D reconstructions
When it comes to drug distribution in the ear the length of the spirallly-wound tubes is of prime importance. This is because the fluids in the ear are not moving (they do not flow) so that drug movement along the ear is dominated by diffusion.
The plot below shows the time it takes for a small drug to diffuse different distances
It takes just 3 Hours to diffuse along the 4.7 mm length of the mouse cochlea.
It takes 2 Days to diffuse along the 17.1 mm length of the guinea pig cochlea.
It takes 6 Days to diffuse along the 28.5 mm length of the human cochlea.
This is why drugs applied intratympanically often do not reach apical cochlear regions.
The situation gets more complex when elimination from the ear is considered.
The above picture shows scala tympani of the human cochlea "unrolled". The approximate regions coding different sound frequencies are marked. When drug is applied to the middle ear (intratympanically) it enters scala tympani through the round window membrane, shown green at the left of the picture. If a drug is lost from perilymph as it diffuses (such as by elimination to the vasculature) , it may never reach the apical, low-frequency regions of the cochlea. This means that some drugs applied intratympanically may never reach the parts of the ear they are intended to treat.