The third dimension—Eye surgery and stereopsis

Abstract

“Inject visco—the anterior chamber is shallowing!” During my residency, I repeatedly received these orders while performing cataract surgery. My inability to assess the need for more viscoelastic confused me. As a novice surgeon, I attributed these symptoms to my inexperienced hands and embryonic surgical skills. I presumed these would improve with practice, but that didn't seem to happen.

Later, during my LASIK training, my mentor instructed me to lift and reposition the corneal flap. I tried insinuating the instrument under the flap, but it was a struggle. A bandage contact lens eventually had to be placed to cover a corneal epithelial defect caused in the vicinity. I proceeded to the other eye, cautiously this time. I felt I had successfully raised the flap, but my instrument was floating in the air, between the patient's face and the nether end of the microscope!

“You have no depth perception.”—my mentor stated sharply. He ordered an orthoptic evaluation for me. Orthoptics. I vaguely recollected the term from my post-graduate textbooks. “Treatment for accommodation anomalies: Orthoptic exercises”—a segment I had skimmed through for my final examination. Little did I know that I would have to revisit the chapter for my treatment.

The orthoptic evaluation lasted 40 minutes. My convergence facilities were a mere 3 cycles/min, with a remote near point of convergence (22 cm) and a paltry fusional vergence (6∆D). Stereopsis measured 63 s of arc on the random dot stereo-test. That explained my intra-operative challenges. The assessment also revealed an exophoria and an uncorrected compound hypermetropic astigmatism in my left eye. Convergence insufficiency (CI) was the verdict.

I was prescribed 18 days of office-based therapy, which eventually spread across 3 months. This therapy included a diverse range of exercises, a short stint with spectacles, a good deal of patience on my part, and tremendous empathy from the orthoptists, my teachers, and, most of all, my mentor.

Therapy demanded 2 hours of focused exercises every day. An hour in the morning, followed by another grueling hour in the evening. A wide array of exercises was prescribed to correct my proximal and tonic convergence [1]. I was started on various vectographs to increase my positive fusional amplitude. A blurry airplane flying across the Chicago skyline would appear nearer, then farther, as I adjusted the slide holder on the dual polychrome illuminator trainer. To improve my eyes' ability to converge, I worked with a barrel card and brock string, fusing the beads, and trying to maintain their union [2]. The initial days were taxing—the exercises triggered a nagging headache that persisted throughout the day. Nevertheless, I adapted to the unremitting strain of continuously adducting my eyes. My near point of convergence inched closer, reducing the effort I needed for convergence. Stereograms were introduced, in the form of the cat-card test, to improve my fusional amplitudes. I could see the two figures of cats, side-by-side. The appearance of the third cat in the center was transient, with whiskers and a tail in place. Acquiring the ability to visualize a sustained image of this, often capricious, cat, was a strenuous task. However, after persistent attempts, the image of the third cat appeared clearly and stayed.

My accommodative capabilities were corrected with Hart chart exercises. These involved repetitive large-amplitude changes in the position of the stimulus, alternating quickly between the larger distant Hart chart and its smaller hand-held counterpart. Over time, I effortlessly shifted focus between the two charts and clearly visualized the print on each. Prism flippers helped increase the speed and decrease the latency of my fusional vergence.

Using the aperture rule trainer, I was made to look through the aperture slide mounted on a stand, to obtain a clear, singular image from the cards located below. The cards gradually advanced toward the aperture slide, progressively increasing the strain on my medial recti. From perceiving two separate images to fusing them, and finally adding a third dimension to them, the orthoptists helped me graduate through the three grades of binocular vision.

The success of vision therapy is monitored mainly based on improvement in presenting symptoms [3]. By the end of 3 months, there was a significant improvement in the way I visualized ocular tissue under the operating microscope (near point of convergence 6 cm; fusional vergence 14∆D; convergence facility 12 cycles/min; stereopsis 30 s of arc). The corneal flap came into view easily. The anterior chamber became a dynamic entity—it deepened and shallowed through various phases of cataract surgery—the need for viscoelastic became evident. My surgical skills improved tremendously.

Most studies assessing the efficacy of vision therapy report a high rate of success with these exercises. Among 1931 patients diagnosed with CI during the 1940s, 72% were said to have been “cured,” 19% “improved,” and a minor 9% “failed.” However, there were no standard criteria to determine which patient would fall under each category, as each set of authors had formulated their criteria [4]. Hence, I had no idea therapy would lead to such a spectacular improvement in the way I observed and performed surgery. This improvement has been maintained well over the years.

CI was described way back in 1855 by Von Graefe [5]. It is a fairly well-known condition, with a reported prevalence between 2.25% and 8.3% [6]. Eye strain, blurry vision, and headaches are documented clinical features [7]. I, however, had never experienced any of these symptoms. Nor did I have other noticeable visual problems before starting surgical training. The lack of depth perception noticed by my mentor was the only sign indicating the inadequacy of convergence in my eyes.

My experience helped me realize that other ophthalmologists with similar conditions struggle in the absence of a proper diagnosis. The need for an orthoptic evaluation for surgeons-in-training before commencing their residency was starkly obvious. Ophthalmic surgeries involve several transparent structures—the corneal layers, aqueous humor, viscoelastic, anterior capsule, cortical matter, posterior capsule, and the vitreous—depth, being their principal distinguishing factor. Visualizing these tissues requires both eyes to work in harmony, utilizing the third dimension in our vision.

An early diagnosis of binocular vision disorders would thus help immensely in developing surgical competence among young surgeons at the threshold of their professional lives, giving an invaluable boost to their confidence. Ophthalmologists give patients the precious gift of sight—why hesitate to gift ourselves the same?

Divya Trivedi: Conceptualization (equal); data curation (equal); investigation (equal); methodology (equal); visualization (equal); writing—original draft (equal); writing—review and editing (equal).

The author declares no conflicts of interest.