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Anterior segment OCT can guide posterior sub-Tenon drug delivery
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 Amar Agarwal |
Corticosteroids have been used in the treatment of a number of ocular conditions, such as diabetic macular edema, cystoid macular edema after cataract surgery, and ocular inflammation, especially uveitis. Depending on the severity of the disease, it is administered topically, orally or periocularly.
Sub-Tenon injection is the preferred drug delivery route in uveitis resistant to topical steroids and inflammation principally located in the posterior segment and/or for cystoid macular edema. The sub-Tenon space is preferred due to its prolonged effect and minimal systemic side effects. Although sub-Tenon injections have been given by various methods, the position of the cannula or the drug has not been imaged in vivo until now. We have used high-speed anterior segment optical coherence tomography for imaging posterior sub-Tenon drug delivery in vivo. This idea was conceived by Dr. Dhivya A. Kumar.
Technique
All patients received a posterior sub-Tenon injection of triamcinolone acetonide using the following technique. The periocular skin was cleaned with povidone iodine 5%, and a few drops of it were instilled into the conjunctival sac. A drop of proparacaine 0.5% was then instilled in the eye. A sterile cotton-tipped applicator soaked in 4% lidocaine was placed over the superotemporal quadrant for 2 minutes as the patient looked inferonasally. Under sterile conditions, Kenalog-40 suspension (triamcinolone acetonide, Bristol-Myers-Squibb) was shaken, and 1 cc (40 mg/mL) was drawn into a tuberculin syringe. The eyelids were retracted with a wire speculum. The patient was made to sit in a comfortable position in a sliding chair and asked to fixate on an inferonasal target.
Under sterile conditions, the superotemporal conjunctiva was lifted with forceps, and an intravenous polytetrafluoroethylene 22-gauge cannula, 0.8 mm in diameter and 25 mm in length, was introduced into the superotemporal quadrant, as described by an earlier method. When half of the cannula was inside the ocular coat, the patient’s head was positioned on the chin rest in the OCT machine. The patient was asked to fixate on the target at the inferonasal side. Cross-sectional imaging of the eye centered on the cannula was taken with a high-speed anterior segment OCT machine from Carl Zeiss Meditec. Corneal high-resolution single scan mode was used. Axis 180-0 across the cross section of the cannula was imaged (Figures 1 to 3). When the cannula was confirmed to be inside the sub-Tenon space, the drug was injected. After injection the patient was given a prophylactic topical antibiotic steroid combination for a week.
 Figure 1. Anterior segment OCT image showing superior (SS) and inferior (IS) sub-Tenon space in relation to the cannula (Cn) below the conjunctiva-Tenon (CT) complex. Images: Agarwal A |
 Figure 2. Anterior segment OCT image (left) and the clinical photograph of the patient (right) showing the drug in the sub-Tenon (ST) space immediately after posterior sub-Tenon injection (S: sclera; CT: conjunctiva-Tenon complex). |
 Figure 3. Anterior segment OCT image showing the difference between subconjunctival (left) and sub-Tenon (right) drug position. Note: The sclera is seen, and no bleb is formed in sub-Tenon injection. |
Discussion
Posterior sub-Tenon steroid injections are a standard drug delivery method used for the treatment of chronic uveitis of the posterior segment. The goal is to reach a sufficient concentration at the site of inflammation. The sub-Tenon space is preferred for drug administration due to its prolonged effect, which lasts for about 3 to 4 weeks. Moreover, the maximum local concentration is obtained with minimum side effects.
The conventional technique of posterior sub-Tenon injection involves the use of a sharp tipped 26-gauge, 5/8-inch needle that is inserted up to its hub to obtain adequate placement of the drug into the posterior sub-Tenon space. With this technique, the risk of perforation of the globe, although minimal, remains a potential complication. An intravenous cannula made of polytetrafluoroethylene for injection of corticosteroids into the posterior sub-Tenon space has been used, which allows safer delivery of the drug. In whatever method we follow, it is important that the drug is administered in the preferred space for its maximum effect. When the sub-Tenon drug is not administered properly into the potential space, the desired effect on the ocular inflammation is not attained.
OCT is a non-contact optical imaging modality that applies low-coherence interferometry and uses an image mapping process to display high-resolution cross-sectional imaging of the ocular microstructure. OCT is the optical analogue to ultrasound B-mode imaging, except reflections of low-coherence light are detected rather than sound. However, OCT-guided posterior sub-Tenon drug delivery has not been reported previously. We have used time-domain OCT with an axial resolution of 18 µm and transverse resolution of 60 µm for imaging. This is a non-contact and noninvasive imaging of the anterior ocular structures.
From our experience, it has been seen that OCT helps in direct visualization of the drug in the potential space during posterior sub-Tenon drug delivery. It can be used for periocular delivery of any drug in the future.
- Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Eye Hospital and Eye Research Centre. Prof. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; email: dragarwal@vsnl.com; website: www.dragarwal.com.
- Disclosure: The authors have no relevant financial disclosures.