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Routine scan flags GCC and focal, global loss volumes
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A 44-year-old man came in for an annual contact lens examination with no particular complaints. He had been a patient in the practice for many years. His current medications included Tylenol PM (acetaminophen and diphenhydramine, McNeil) for reported sleep difficulties. Further review of systems and family medical history were noncontributory with the exception of an allergy to tetracycline.
The patient’s best-corrected acuity was 20/20 in both eyes, and intraocular pressure was 16 mm Hg OU as measured at 8:30 a.m. His pupils were normal by RAPDx (Konan Medical). The anterior chamber evaluation was unremarkable for disease. Upon entry into the practice an iWellness scan (Optovue Inc.) was performed as part of the routine work-up.
The iWellness scan displayed no apparent problem with the B-scan cross sections, but the ganglion cell complex (GCC) data demonstrated an overall caution (yellow flag) on thinning. In addition, the focal loss volume (FLV) is flagged in the left eye, and the global loss volume is flagged in both eyes. This caution indicated the need for further medical investigation. The patient’s fundus appeared clear, and he had cup-to-disc ratios of 0.3.
Central corneal thickness was 545 microns OD and 549 microns OS, with angles measured by spectral domain optical coherence tomography (SD-OCT) as 45 degrees OD and 41 degrees OS. A full medical SD-OCT was run based on the findings of the iWellness scan. Three-D optic nerve head assessment revealed tilted discs and relative thinning of the retinal nerve fiber layer (RNFL). The optic nerve head (ONH)/GCC OU report demonstrated both RNFL and GCC thinning.
The question then became: Had this patient developed a variation of normal-tension glaucoma or ischemic optic neuropathy associated with sleep apnea? Or, had this been a long-standing issue that was nonpathological and was never uncovered without the iWellness test?
A visual field test revealed fairly significant constriction that implied a pathological defect. Visually evoked cortical potentials demonstrated a difference in latency and amplitude between the right and left eyes.
What, then, caused the visual field defects, the thinned RNFL, the thinned GCC and the altered visually evoked potential?
The ONH/GCC OU report indicated both RNFL and GCC thinning.
Images: McCall JA
Three-D optic nerve head assessment revealed tilted discs and relative thinning of the retinal
nerve fiber layer.
The visual fields revealed fairly significant constriction, which implied a pathological defect.
The iWellness scan displayed no apparent problem with the B-scan cross sections, but the ganglion cell complex (GCC) data demonstrated an overall caution (yellow flag) on thinning. In addition, the focal loss volume (FLV) is flagged in the left eye, and the global loss volume (GLV)
is flagged in both eyes.
All OCT devices give the clinician much more information than is ever assessed. In this case, the answer appears in a table in the ONH/GCC report, in the disc area and cup-to-disc ratio.
The disc areas were 1.39 mm2 OD and 1.44 mm2 OS, with very small cup-to-disc ratios of 0.04 and 0.04. The expected ranges for this normative database with the iVue for a male Caucasian put disc area at an average of 2.030 mm2 with a standard deviation of 0.374. At one standard deviation below the average in the male category, this patient’s lower-end number would be 1.682 mm2. This patient is more than one standard deviation below the expected ranges in both eyes.
Larry J. Alexander
John A. McCall Jr.
The measurements, therefore, put this patient in the congenital microdisc classification with consequent fewer nerve fibers, explaining the reduced RNFL, the thinned GCC and the functional visual field compromise. Numerous studies have elucidated expected disc sizes in different populations (Hoffman and colleagues, Jonas and colleagues, Tam and colleagues).
It is important to realize that the microdiscs can put any patient at risk for nonarteritic ischemic optic neuropathy (NAION) and low-tension glaucoma, especially when coupled with the symptoms of sleep apnea. NAION is most commonly encountered in small nerves with little to no cupping. As it relates to NAION, this anatomical configuration is frequently referred to as a “disc at risk.”
Many systemic conditions and certain medications are associated with NAION, but obstructive sleep apnea syndrome (OSAS) has risen to the top of the list. This, then, alerts the practitioner to the necessity for careful follow-up and trend analysis as well as the recommendation for a consultation for the symptoms of sleep apnea. Also, the clinician must pay attention to the association with floppy eyelid syndrome.
Lin and colleagues have reported a proportional decrease in the RNFL measurement of patients with OSAS. Lin and colleagues and Steindel and colleagues also reported that moderate to severe OSAS is more likely to attenuate RNFL. Coincident with the attenuated RNFL is a concurrent reduction in visual field sensitivity (Tsang and colleagues and Preechawat and colleagues).
This analysis does not imply that there are no other issues in this case, but rather that any analysis must be based upon the fact that this patient has congenitally small optic nerve head areas.
A sleep study was ordered for this patient, with careful follow-up of all findings. Remember that in patients younger than 50 years there is also a strong link between NAION and an underlying anomalous or small disc (92%) and an increased likelihood of involvement of the fellow eye (Preechawat and colleagues).
