Acquired Optic Pits Associated with Laser-assisted In Situ Keratomileusis: A Case Series

INTRODUCTION

Acquired pits of the optic nerve (APON) are focal excavations of the neuro-retinal rim with localized breaks or disinsertion of the lamina cribrosa and depression of the pre-laminar tissue.¹ These acquired optic nerve pits typically lead to corresponding and significant retinal nerve fiber layer (RNFL) thinning with conforming paracentral visual field defects.^2–4 Additionally, eyes with APON have been shown to progress faster in relation to both visual field defects and RNFL thinning compared to other glaucomatous eyes.^5,6 While APON is more commonly associated with low tension glaucoma and female gender, susceptibility and other possible triggering events have not been well elucidated.

Large intraocular pressure (IOP) shifts can cause the lamina cribrosa to deform, typically resulting in a posterior shift in the position.^7,8 The higher and more rapid the IOP shift, the greater the displacement of laminar tissue. This displacement is greatest at the peripheral optic nerve head and exerts significant shearing forces distributed to the peripheral lamina cribrosa. Additionally, high myopia and connective tissue disorders have been noted to result in larger laminar and pre-laminar displacement than the normal population under increased IOP.⁹

During flap creation in laser-assisted in situ keratomileusis (LASIK), IOP ranges from 80 to 120 mm Hg, typically lasting less than a minute.^10,11 However, pressures have been recorded as high as 300 mm Hg in association with improper positioning and docking. This combination of high myopia plus very high pressures during the LASIK flap creation may increase the risk of APON with progressive visual field loss in susceptible individuals.^9,12

In this case series, we discuss seven patients with optic nerve pits and progressive visual field loss despite controlled IOP following LASIK.

MATERIALS AND METHODS

This study was a retrospective chart review of patients of a tertiary academic glaucoma specialist (S.M). These patients were identified by doing a search query of patients that presented for follow-up between March 1, 2018, and March 1, 2019, with a diagnosis of optic nerve pit and a history of LASIK. The study was approved by the Institutional Review Board at the University of California, Irvine. Clinical history, exam, and structural imaging (Spectralis Optical Coherence Tomography; Heidelberg Engineering), as well as functional testing (Humphrey Visual Field; Zeiss), were reviewed. After identification of patients, extended-depth optical coherence tomography (ED-OCT; Spectralis, Heidelberg Engineering) of the optic nerve head was performed.

This study was approved by the University of California Irvine Office of Research Institutional Research Board and adhered to the tenets of the Declaration of Helsinki. As this was a retrospective study, the need for informed consent was waived.

RESULTS

Seven eyes (three OD and four OS) from seven patients were identified who met inclusion criteria. Five of seven patients had >3 years of follow-up. The average age was 57 years old (range 45–73). Only three patients had a family history of glaucoma. The average pachymetry was 492 μm (range 404–612). All patients had open and unremarkable gonioscopy exams. All optic disc pits were unilateral, six of which were located inferiorly and the other centrally. Retinal nerve fiber layer thinning on OCT was noted in all seven patients with corresponding visual field defects in six patients while the last patient remained preperimetric. Extended-depth optical coherence tomography was obtained and demonstrated laminar breaks at the location of the optic disc pit seen on the clinical exam (Fig. 1). Four patients subjectively identified these visual field defects after LASIK while the other two patients were unaware of the reproducible visual field defects noted on exam following LASIK.

An ED-OCT image demonstrates a break in the lamina cribrosa inferiorly (A). Corresponding temporal and inferotemporal retina nerve fiber layer thinning (B) and a superior hemifield defect (C) were detected.

The patients had an average IOP of 17 mm Hg (range 12–33 mm Hg) and appropriate topical medical treatment was initiated and escalated as needed to control glaucomatous progression. Despite maximal medical therapy, visual field progression continued in four patients who later required surgical intervention. One patient subsequently underwent selective laser trabeculoplasty and three patients underwent trabeculectomy. In the study cohort, three patients demonstrated visual field progression in accordance with the location of their optic disc pit despite good IOP control.

DISCUSSION

To our knowledge, this is the first case series suggesting an association between LASIK and APON. This association is significant as RNFL thinning and visual field defect progression are more likely in patients with APON.² Compared to their counterparts without optic disc pits, patients with an APON are two to three times more likely to have a progression of glaucomatous visual field defects.⁶ This progression can proceed despite aggressive IOP control.

In addition, eyes that have undergone LASIK, likely have unique ocular anatomical differences that may risk further glaucoma progression. First, those with high myopia, who are more likely to seek LASIK vision correction, have greater laminar deformation during the procedure and may be at higher risk for the development or progression on APON.⁹ Second, eyes that have undergone LASIK develop decreased corneal hysteresis which may increase the risk of glaucoma under-diagnosis and under-treatment.¹³ Lastly, thinner pachymetry measurements secondary to the LASIK procedure require even lower IOP targets.

Management of glaucomatous progression in patients with these risk-factors is particularly challenging. If trabeculectomy becomes indicated, these patients are often at increased risk for complications related to hypotony.^14,15 Additionally, OCT interpretation with pathological myopia is more challenging and confounding by the anatomic abnormalities associated with high myopia.

Consideration of optic nerve pit development and increased awareness is needed given the similarity and severity of vision loss associated with the optic nerve pits in the six patients presented here. It has been proposed that APON may signal an abnormal susceptibility of the optic nerve to the damaging effects of IOP.¹⁶ The creation of a new APON or the expediting of pending visual field defects from an existing APON may be incurred by subjecting patients to transient yet significant elevations in IOP during the LASIK procedure. Furthermore, improper docking can result in even higher elevations of IOP and for longer periods than is planned, putting some patients at greater risk.⁹ Beyond the direct effect of the IOP increase during LASIK flap creation, the thinning of the cornea after ablation may also play a role.¹⁷ Decreased corneal hysteresis can alter IOP readings and may also permit more fluctuation in IOP with blinking, eye rubbing, and any other mechanical manipulation of the eye. Additionally, optic nerve pits are common in highly myopic eyes and can be difficult to identify on ophthalmoscopy, but can be visualized with OCT.⁹ Therefore, caution should be taken when performing LASIK in patients with a history of high myopia, glaucoma, or a strong family history of glaucoma. Extended-depth optical coherence tomography should be considered to identify preexisting APON in patients with high myopia or other factors before LASIK.

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