Enhanced views of the cornea and anterior chamber are obtained with a slit lamp, which permits focused circular and rectangular
light beams to illuminate transparent structures, demonstrating more subtle abnormalities in the cornea. An inexpensive version
with several useful features is available from Heine USA. The best optics and optical detail require a more-sophisticated,
dedicated instrument such as the Kowa SL-15, but its cost is more prohibitive. Many direct ophthalmoscope heads have a slit
aperture, which may be used as a reasonable substitute to determine the depth of corneal lesions when observed indirectly.
In dim surroundings, the light is positioned directly in front of the globe with the slit box brought into focus on the corneal
surface, and the defect depth is assessed by observation from an oblique angle (not using the ocular). The same aperture can
be used to enhance depth perception on retinal examination, observing through the instrument eyepiece.
Corneal culture should be performed whenever focal corneal discoloration is present. The culture can be discarded later if
unnecessary, but its early acquisition ensures an unadulterated sample. Lid blockade, or topical desensitization may be used
if necessary to acquire the sample. If the culturette touches the eyelashes or skin, it should be discarded and another sample
acquired. Fungal culture requires specific requests at most laboratories: It should be ordered if cytology is suggestive of
hyphae. Sensitivity testing should be ordered concurrently with the culture.
Cytology may be acquired after corneal desensitization using a blunt instrument at the edge of the lesion. The sample should
be applied cautiously to several slides, permitting in-house review, as well as submission if indicated. Routine staining
is adequate with oil-immersion to identify the commonly encountered pathogens. Surgical blades and scissors should be used
only on the cornea with great caution. Melting ulcers can present with a prominent focal excursion from the cornea: resecting
this area may result in corneal perforation and require immediate surgical repair. In preference, cytology acquired at the
lesion edge is usually much safer and equally useful. Corneal cytology will be reviewed in greater detail in a future article.
Serology samples may be acquired in uveitis patients for Leptospira sp. and other testing. This is perhaps more relevant in recurrent episodes. The results can be useful ancillary aids but are
seldom diagnostic in their own right.
Subpalpebral lavage systemIn severe ocular disease, and nervous individuals, chronic therapy will be considerably easier, more effective and tolerable
for the patient and handler with a subpalpebral lavage system (SPL). The SPL can be placed conveniently during the same sedation
as the exam, if it is performed expeditiously. If the patient is too alert, additional sedation is desirable.
Preparation for the SPL placement includes palpebral and supraorbital (frontal) nerve blockade, local anesthesia at the intended
site of skin perforation, topical desensitization and local cleansing. The corneal and conjunctival surfaces are rinsed with
dilute povidone iodine (0.2% iodine), which may be applied from a spray bottle. This is irritating to the conjunctiva in horses,
and may be flushed with sterile saline.
Several SPL models are available, but I prefer a pre-assembled system (Lavage kit #6612 or #6612L, Mila International, 888-MILA
INT). These kits contain almost all the items necessary to place and retain the system. The University of Florida recommends
the longer, three-foot lavage line, and a more-secure administration port is assembled by inserting a 22-gauge 1.5-in. intravenous
catheter into the distal tubing, adding an injection cap and taping the device securely to a wooden tongue depressor secured
to the mane. The lavage is sutured to the skin of the skull with 2.0 nylon using the supplied plastic stops, or through zinc-oxide
tape shaped into a butterfly. Sutures are placed to avoid peripheral nerves and to establish a curvilinear track to the forelock,
then to the mane.
The lavage line is placed close to the neck skin, and braided in place, finally being covered with the mane. In polo ponies,
the lavage line may be shortened and taped to the halter. The line should lie straight but not be overly tight. The subpalpebral
lavage disc, which retains the position, is extremely unlikely to become separated unless substantial pressure is applied.