An Injection Primer, Part 2
BY WILLIAM L. MILLER, OD, PHD, FAAO
In September I discussed the essentials for preparing for any ocular injection and I particularly discussed how to perform intralesional injections. This month I'll outline the steps for perforsubconjunctival (SC) and subtenons (ST) injections. Both are typically the last line of treatment after topical and oral routes have proven unsuccessful, but in either case you should undertake a careful review of necessary Occupational Safety and Health Administration (OSHA) require ments and universal precautions.
Differentiating SC From ST
Indications for SC injections include delivery of steroids for recalcitrant anterior and posterior uveitides. You may also use it to administer antibiotics in cases of recalcitrant or severe corneal infections or to deliver antimetabolites such as mitomycin C in trabeculectomies.
Indications for ST injections include steroid injections for posterior uveitis and vitritis as well as cystoid macular edema.
Contraindications to SC and ST include cases of scleritis. Just as you would with any injectable procedure, weigh the benefits with any risks that may occur.
Make Them Comfortable
I suggest administering a topical anesthetic to the ocular surface to eliminate discomfort. Still, some patients will be apprehensive even with the anesthetic, which may necessitate a prophylactic dose of 5mg diazepam (Valium) p.o. It is important to minimize patient anxiety to prevent any unexpected head or eye movements during the procedure.
Perfecting the Details
The first step for SC and ST injections is to retrieve the medication using a large-diameter, 20-gauge needle because most medications are viscous. In cases of anti-inflammatory therapy, fill the syringe with 0.2cc to 0.5cc of the steroid.
Replace the 20-gauge needle with a 27-gauge needle and advance it tangential to the globe. Lift the superior or inferior temporal conjunctiva with a tissue forcep, avoiding extraocular muscle insertions and creating a triangular structure tented above the globe.
Advance the needle near the bottom of the triangle with the bevel of the needle facing down toward the globe to minimize chances of globe perforation.
Once the needle penetrates the conjunctiva you may need to aspirate to prevent inadvertent application of medication directly into a blood vessel. Now inject the medication into the subconjunctival area, slowly pushing on the plunger to form a small, bulbous-like cyst reservoir of steroid.
You perform an anterior ST injection in much the same way you do an SC injection. One does not offer a significant benefit over the other. You deliver a posterior ST injection, on the other hand, in the inferotemporal quadrant just posterior to the fornix.
Move the needle laterally back and forth to verify that the needle has not entered the sclera. As with other injection procedures, pull back on the plunger and aspirate to prevent intravascular application of the medication.
Come What May
Complications that may occur include globe perforation (rare) and subconjunctival hemorrhages -- the latter representing a much more common reaction. Some practitioners recommend an initial drop of a topical vasoconstrictor to minimize hemorrhage formation. You can arrange follow-up visits for two to three days after the injection procedure.
Dr. Miller is on the faculty at the University of Houston College of Optometry. He is a member of the America Optometric Association and serves on its Journal Review Board. You can reach him at email@example.com.