Biomedical Engineering Reference
In-Depth Information
1. Pharmacologic pupillary dilation
2. Darkened ambient light conditions
3. Indirect and/or direct ophthalmoscopy
4. Slit lamp biomicroscopy
Additional procedures may be included depending on the
objective of the examination. These may include, but are not lim-
ited to, corneal staining, corneal esthesiometry, pachymetry,
tonometry, fundus photography, fluorescein angiography, optical
coherence tomography (OCT), and electrophysiological assess-
ment of the visual system (e.g., electroretinography, multifocal
electroretinography, visual evoked potentials (VEP)). Sedation or
general anesthesia may or may not be required depending on the
species, the procedure being performed, and individual animal.
The routine ophthalmic examination for all animals used in
toxicologic studies should begin with the minimum database of
both biomicroscopy and indirect ophthalmoscopy. Regardless of
the species of interest, these two examination techniques are essen-
tial to ensure an accurate and complete examination of both the
anterior and posterior segments of the eye. A comparison of tech-
niques by Bellhorn found that of 100 rats with known lens abnorm-
alities diagnosed by biomicroscopy only 65/100 of the lenticular
lesions could be found using the direct ophthalmoscopy and only
35/100 were found using indirect ophthalmoscopy [
7
]. Together
these two examinations must, at a minimum, include evaluation of
the adnexal structures (eyelids and conjunctiva), anterior segment
(cornea, anterior chamber, iris, and lens), and posterior segment
(vitreous and fundus).
Ophthalmic examinations should be conducted on an eye that
has been pharmacologically dilated and should be performed in a
darkened examination room. While some have advocated the use
of 10 % phenylephrine to aid in dilation of rodents [
8
,
9
], this is
generally not required. Pharmacologic dilation is most commonly
performed using tropicamide at a concentration of 0.5 % for
rodents and 1 % for larger mammals. The ophthalmologist should
be familiar with the length of time required to achieve mydriasis
and the duration of the mydriasis for the species being examined. In
general, 10-15 min is the minimum time required to achieve
acceptable mydriasis and this may be slightly longer in heavily
pigmented eyes. The duration of mydriasis is directly related to
the amount of intraocular melanin. In albinotic rodents, mydriasis
will last no more than 1 h while in a pigmented eye of a dog or a
primate the effect will persist for 3-5 h. This information is impor-
tant so that the ophthalmologist knows when to begin dilation and
how many animals should be dilated at one time. The later will
depend on howmany animals the ophthalmologist can examine in a
given time period. For the basic examination, biomicroscopy, and
indirect ophthalmoscopy, animals are either manually restrained
