Page 1167 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1142 CHAPTER 11

VetBooks.ir horse using tape and skin sutures. Some clinicians as previously described. An 8-Fr, 1-metre (42-inch)
feeding tube or polyethylene tubing is introduced
tunnel the tubing under the skin in multiple sites over
the face in lieu of tape ‘butterflies’ and skin sutures
the nasolacrimal sac. Tape is placed around the tub-
(Fig. 11.34). The delicate silicone lavage tubing can into the nasal punctum and passed to the level of
be reinforced by encasing it in intravenous drip-line ing at the point of entry into the nasal punctum and
tubing which is split longitudinally and wrapped secured to the nose before the tubing is run up the
around the silicone tubing. An injection port, for head and along the neck and attached in a similar
manual injection, can then be attached to the free manner to the SPL system. Larger volumes of drug
end of the tubing, secured to a tongue depressor and are required in this type of delivery system, mak-
wrapped in gauze and tape to make it less likely that ing systemic absorption more likely. The tubing may
the end will bend or kink (Fig. 11.35). also slip or kink and, as the tubing does not com-
Manual delivery involves injecting approximately pletely occlude the nasolacrimal duct, the medica-
0.15–0.2 ml of medication into the tubing system at tion may drain down around the tubing.
the injection port and flushing the drug into the eye
using 3 ml of air. The air must be injected slowly to Topical drug reservoirs
avoid irritation to the corneal or conjunctival sur- Drug-impregnated collagen shields and contact
face, thereby decreasing the likelihood of discom- lenses can also be used as drug delivery devices, pro-
fort. Although manual injection is most often used viding prolonged therapeutic drug levels but avoid-
to deliver the treatment solution through the SPL ing frequent administration (Figs. 11.36, 11.37).
system, continuous drip or pump systems have also Collagen shields and contact lenses should be pre-
been employed. soaked in the chosen drug for a minimum of 10 min-
Complications associated with SPL systems utes and 30 minutes (some studies describe 24-hour
include infection of the eyelid, loss of the footplate soaking), respectively, to ensure saturation with the
in the eyelid (although no long-term problems have medication. Initial corneal drug levels may be high,
been reported), conjunctival granuloma, endo- but they deplete rapidly, thereby limiting the appar-
phthalmitis, iatrogenic trauma to the globe during ent benefit over topical therapy. Some drugs are less
insertion, plugging or breakage of the tubing, tube bioavailable from soaked contact lenses (e.g. ofloxa-
displacement or premature removal by the horse, cin) so drug choice for this approach is important.
suture loss and injection port damage or loss. Poorly
placed tubing or tube slippage can quickly produce Subconjunctival administration
corneal irritation or ulceration or allow topical med- Subconjunctival injection can be facilitated by the
ications to leak into the subcutaneous tissue, lead- use of topical anaesthesia and an auriculopalpe-
ing rapidly to eyelid swelling (chemosis) and severe bral nerve block. A maximum volume of 1 ml may
inflammation. SPL systems should be checked daily be injected under the bulbar conjunctiva, using a
for complications associated with their use and to 25–27-gauge needle. The bevel of the needle should
ensure patency. SPL systems are generally eas- remain up and the hand holding the syringe should
ily placed and well-tolerated for extended periods rest on the horse’s head when injecting in order to
of time. Topical ophthalmic suspensions should be decrease the risk of inadvertent globe perforation
used with care in an SPL system because they may or trauma (Fig. 11.38). The dorsolateral quadrant
precipitate and block the tubing. Ointments should is the easiest place to inject; however, it is important
not be used in an SPL system. to inject as close to the lesion site as possible because
drug levels are highest in the region immediately
Nasolacrimal lavage system adjacent to the injection site. Subconjunctival injec-
A nasolacrimal lavage system has also been described tion may establish much higher medication levels
for delivering drugs to the eye. This system may also in tissues for a longer period of time than those
be placed under sedation and local anaesthesia, using attained with occasional topical application. This
a technique similar to NLS cannulation and lavage, technique is beneficial in emergency situations