Enhanced depth imaging optical coherence tomography of the optic nerve head improves correct diagnosis in glaucoma suspects without glaucomatous optic disc morphology

Discussion

Although visual acuity is rarely affected in eyes with ODD, insidious VFL and associated peripapillary RNFL thinning are usually present. ODD volume generally correlates with structural and functional optic nerve damage; eyes with larger ODD have more severe thinning of RNFL and VFL.3 In most cases, the VFL progresses slowly and can resemble glaucomatous defects such as a nasal step, arcuate scotomas or generalised constriction of visual field.1

Glaucomatous optic neuropathy is also characterised by functional and structural changes, such as VFL, optic disc excavation, increasing cup-to-disc ratio, rim notching and thinning of neuroretinal rim and RNFL. Therefore, reliable assessment of these parameters is important when monitoring for progression in glaucoma.4 This task becomes particularly difficult in a patient with simultaneous occurrence of both ODD and glaucoma.5 6 In these cases, it is not possible to distinguish whether progression in RNFL thinning or VFL is due to glaucoma or ODD if one does not assess the optic disc itself. In the present case, before being referred to neuro-ophthalmology, ODD had only been diagnosed by ultrasonography. However, visualisation of the anatomical ONH structure in recent years has significantly improved with the introduction of spectral-domain OCT and, in particular, EDI-OCT. This has made it possible to examine structures such as ODD in far greater detail compared with ultrasonography, which has previously been the gold standard. The result is a major expansion in our understanding of ODD and their role in ONH pathophysiology.1 In a case series from 2004 prior to the modern EDI-OCT era, data from 13 patients with glaucoma without cupping were analysed retrospectively. Two of them had concomitant ophthalmoscopically visible ODD. While ODD in some of the remaining cases were suspected, but not confirmed by ultrasonography, the examination of the ONH was mostly dependent on reviewing fundus photographs alone.6

Larger ODDs are associated with greater VFL, which is compatible with the findings in the present case.3 7 In fact, the study by Yi et al was based on four glaucoma patients with bilateral ODD, although no information was provided on the criteria used to diagnose glaucoma in these patients.7 Furthermore, it has been shown that VFL is more pronounced the more superficial ODD are located, and that global peripapillary RNFL thickness is often reduced in patients with superficial ODD compared with patients with buried ODD.8 9 This may practically explain the clinical manifestations observed in the present case. VFL in the presented patient was greatest in left eye (MD=22.3), which was also the eye with the more superficially located ODD and most RNFL thinning compared with right eye with deeper lying ODD and less RNFL thinning and VFL.

The presented patient underwent trabeculectomy in both eyes. Most recently in right eye, as IOP increased following discontinuation of topical IOP-lowering treatment followed by a slight worsening of VFL. It should be mentioned that VFL is more likely to occur in ODD patients with ocular hypertension compared with normotensive ODD eyes, regardless of ODD degree.10 Ultimately, the clinical progression could well be attributed to mechanisms regarding ODD and concomitant elevated IOP.

The close monitoring of this patient by the glaucoma unit is understandable. First, because of genetic predisposition, as our patient had three siblings with POAG. Second, IOP tended to be in the lower 20s despite dual IOP-lowering therapy and history of both ALT and SLT. One must also remember the severe constricted visual fields of this patient. However, the visual fields had been stable for many years, although IOP fluctuated between approximately 14 mm Hg and 24 mm Hg.

Having severe ODD almost exclusively buried in the ONH at the age of 83 was very unexpected. Buried ODDs are predominantly present in children and are believed to become ophthalmoscopically visible when undergoing a process of gradual enlargement in the two first decades of life.1 11 Despite the fact that ODDs were diagnosed early on by ultrasonography in the present case, its significance has most likely been underestimated due to the rather unremarkable optic disc presentation on ophthalmoscopy. The multiple large, buried ODD, which occupies most of the prelaminar region, were only discovered when fundus autofluorescence examination and especially EDI-OCT was performed, leading to questioning of the reliability of the POAG diagnosis in the presented patient. Glaucomatous excavation of the optic disc would also be expected in case of concomitant POAG and buried ODD.

Although there is no best answer to clinical practice, we highly stress examining for ODD in detail using OCT, when patients with a history of glaucoma present with an optic disc with no apparent cupping, and especially if ODD have already been diagnosed by other means, such as ultrasound. EDI-OCT has been proven to be more reliable compared with B-scan ultrasonography and enables detailed visualisation of the entire ONH, adding significant understanding to the clinical presentation. Furthermore, compared with ultrasound and fundus autofluorescence, OCT has the additional advantage of providing the ability to quantitatively correlate retinal measures of neuroaxonal structural damage with ODD characteristics. Therefore, additional medical and surgical IOP-lowering intervention might be avoided in other ODD patients in whom glaucoma is suspected.