Optic Neuropathy Secondary tsthetic Injection in Cataract Surgery

Direct optic nerve injury secondary to retrobulbar injection is a relatively uncommon but significant cause of blindness following cataract surgery.¹ Believed to be safer than retrobulbar or peribulbar anesthesia, sub-Tenon anesthesia nonetheless provides equally effective anesthesia and akinesia.^2-4 Use of shorter, blunt-tipped needles with a more anterior site of injection are thought to reduce or eliminate the risk of optic nerve injury.^2-4 To our knowledge, we report herein the first case of traumatic optic neuropathy secondary to sub-Tenon anesthesia and provide evidence of the mechanism of injury.

A healthy 78-year-old man noted "total blindness" in his right eye just after an "uncomplicated" cataract extraction in that eye. Preoperatively, the patient had a Snellen visual acuity of 20/200 OD with macular fibrosis, and a 3+ nuclear sclerosis cataract on the right. Potential acuity meter readings showed a visual acuity of 20/60 OD and the axial length was 23.97 mm. Phacoemulsification had been performed with a scleral-tunnel approach, with implantation of a posterior chamber intraocular lens. Anesthesia was provided as a 2-mL sub-Tenon injection of 4% lidocaine hydrochloride, 3 mm posterior to the inferonasal limbus⁴ using a Masket cannula. Supplemental anesthesia was provided by 4% topical and 1% intracameral lidocaine.

Results of postoperative anterior segment examination on the 3 days after surgery were unremarkable including the applanation tonometry that measured between 16 and 20 mmHg. Postoperative vision, first recorded on postoperative day 3, was light perception OD and 20/40 OS. Fundus examination showed previously noted macular fibrosis. Pupillary function was not tested. Subspecialty referral was made because of the concern of an optic neuropathy. Peripapillary hemorrhage and late staining of the disc by fluorescein angiography were evident 1 week later in the right eye. A magnetic resonance imaging (MRI) scan of the head and orbits was interpreted as being normal. Neuro-ophthalmic examination 2 weeks later showed no light perception OD and an afferent pupillary defect on the right. The right optic nerve was pale; peripapillary hemorrhage was not evident. Review of the MRI scan revealed 2 cuts suggestive of an abnormal signal in the immediate retrobulbar segment of the right optic nerve. Repeated MRI showed an increased T2-weighted signal of the right optic nerve with mild gadolinium enhancement (Figure 1). A diagnosis of optic neuropathy secondary to direct trauma from the sub-Tenon injection of anesthestic was made.

The average axial length of 7 cadaver eyes obtained from the New England Eye Tissue Bank, Boston, Mass, was 23 mm (range, 20-25 mm). The lengths of 3 commercially available cannulas (Masket, Eagle, and Visitec) were measured to be 23, 22, and 26 mm. The correct in situ orientation of an eye was made by considering the locations of the insertions of the extraocular muscles. Each of the cannula was advanced from the limbus along the sclera in the inferonasal quadrant, which is typically used to deliver anesthetic injections.⁴ The tip of each cannula reached the optic nerve even before the hub of the cannulas had been advanced to the limbus (Figure 2).

Unexpected poor visual outcome following cataract extraction is frequently the result of a preexisting age-related maculopathy or optic neuropathy, which might not have been recognized in advance of the removal of an opaque crystalline lens. Alternatively, cataract surgery can cause an ischemic optic neuropathy because of reduced optic nerve perfusion pressure related to perioperative fluctuations in intraocular pressure or a traumatic optic neuropathy due to direct injury from the injection needle used to deliver anesthesia.¹ Of these, only injury from an injection might be associated with an MRI scan showing abnormalities in the period shortly after the event, as occurred with our patient.

Sub-Tenon injection of anesthesia is performed by first giving topical anesthetic and then elevating and bluntly dissecting conjunctiva and Tenon fascia to 3 to 5 mm posterior to the limbus.^2-4 One of several specially designed sub-Tenon cannula is advanced against bare sclera and along the globe, typically to the point of contact of the hub of the cannula to the eye.² The cannulas are advanced to this depth to increase the likelihood that the anesthetic will diffuse sufficiently into the orbit to achieve anesthesia and akinesia.

This "blind" insertion technique is generally considered to be safe because the relatively short length of the cannula is believed to limit the potential for damage to the optic nerve, ^2-4 which is a well-recognized complication of the previously more common technique of retrobulbar injection.¹ Our case offers evidence that a sub-Tenon injection can damage the optic nerve. The contrast between our experience and the standard teaching of the relative safety of sub-Tenon injections motivated us to evaluate the design of some cannulas recommended for sub-Tenon injections. Even a cursory examination made it clear that these cannulas were long enough to cause an optic neuropathy. We believe that sub-Tenon injections are associated with a lower risk of injury to the optic nerve than retrobular injections, but the risk of a traumatic optic neuropathy is not eliminated by sub-Tenon injection.

Corresponding author and reprints: Joseph F. Rizzo III, MD, Massachusetts Eye and Ear Infirmary, 243 Charles St, Boston, MA 02114 (e-mail: jrizzo@meei.harvard.edu).