Biomedical Engineering Reference
In-Depth Information
This equation is subject to a number of assumptions and is
somewhat simplified, but it conveys the overall relationships
between the various components of aqueous humor dynamics and
the pathways by which anti-glaucoma drugs alter IOP. Methods
have been developed for directly measuring or estimating the
various components described above and these form the foundation
for determining the mechanism of action of an IOP lowering drug.
2 Measuring IOP
The only method for directly measuring IOP is manometry in which
the anterior chamber is cannulated with a fine needle which is
connected to fluid filled tubing and ultimately a calibrated pressure
transducer. This is an invasive, generally one-time procedure and in
living animals introduces numerous artifacts including inflamma-
tion (which may occlude the needle and confound results), making
it unsuitable for clinical use in which IOP needs to be sequentially
followed or in studies that are more than a few hours in duration.
Manometry is most frequently used in species in which noninvasive
tonometry is difficult, such as rats and mice, or to evaluate how well
a given tonometer performs in a specific species [
12
,
13
].
Noninvasive tonometers, however, do not actually measure
IOP but instead measure the “tone” of the eye-wall (usually
the cornea) and use a mechanical property of that tissue to estimate
actual IOP. There are a number of potential inherent errors in these
measurements, and most of these instruments are calibrated for
the human eye which generally results in an underestimation of
true IOP in animals. However, as long as this underestimation is
linear over the range of IOP being measured (as determined by
comparison with a manometer) the device is still useful in deter-
mining the effect of a given test article on IOP (Fig.
3
). This
linearity makes it generally unnecessary to convert tonometric
IOP estimates to “true” IOP values (as determined with a manom-
eter) using equations which describe the relationship between the
two instruments. It is also important that normative values be
established by tonometer, species, and tonometrist as they may
vary considerably [
13
,
14
]. In other words, IOP values with one
tonometer in one species by a given tonometrist are not necessarily
directly comparable to those acquired with a different tonometer in
a different species by a different tonometrist.
There are two principle forms of tonometry in use today:
Applanation
and
Rebound
. Applanation tonometry (as exemplified
by the Tono-Pen, Pneumatonometer, Perkins and Goldmann ton-
ometers) is based on the principle that the force required to flatten
(applanate) any portion of the surface of a sphere is directly pro-
portional to the pressure inside the sphere multiplied by the area
being flattened [
15
]. This assumes, however, that the sphere's
