Unilateral situs inversus of optic disc associated with reduced binocularity and stereoacuity resembling monofixation syndrome

Optic Disc Dysplasia and Situs Inversus

This case report discusses a diagnosis of optic disc dysplasia made on routine examination of a man aged 33 years.

Short foveo-disc distance in situs inversus of optic disc

Situs inversus of optic disc (SIOD) is thought to be a congenital optic disc abnormality that is caused by dysversion of optic nerve insertion. SIOD, however, has many additional features that cannot be explained by abnormal optic-nerve-insertion directionality. In this study, we measured the distance between the fovea and disc in 22 eyes of 15 SIOD patients. For comparison, two control eyes were matched with each SIOD eye by age and axial length. The vertical distance between the temporal vascular arcades also was measured. The foveo-disc distance was shorter in the SIOD eyes than in the control eyes, while the inter-arcade distance did not differ. Further, we measured the circumpapillary retinal nerve fiber layer thickness, which showed nasal crowding of two humps in the SIOD eyes. This nasal crowding disappeared when we shifted the circle scan by the mean difference (465 μm) of the foveal-disc distance between the two groups. Our findings suggest that the optic disc was located closer to the fovea than it would have been normally. Thus, SIOD might reflect incomplete expansion of the posterior pole in the direction of the fovea-disc axis.

Peripapillary retinal nerve fiber layer and optic nerve head characteristics in eyes with situs inversus of the optic disc

PURPOSE

This study was performed to investigate the peripapillary retinal nerve fiber layer (RNFL) and optic nerve head (ONH) characteristics, as determined using a spectral-domain optical coherence tomography (OCT), in eyes with situs inversus of the optic disc.

METHODS

The peripapillary RNFL and the ONH were assessed in 12 eyes belonging to 6 subjects with situs inversus of the optic disc (situs inversus group) and 24 eyes in 12 age-matched, sex-matched, and refractive error-matched healthy subjects (control group) by using OCT. The average, quadrant, and clock-hour RNFL thicknesses (clock-hour 9 on the scan represented the temporal side of the optic disc in both eyes), the superior/inferior RNFL peak locations, and ONH characteristics such as disc area, rim area, cup-to-disc ratio, vertical cup-to-disc ratio, and cup volume were obtained. The differences in RNFL and ONH characteristics between the 2 groups were analyzed.

RESULTS

The situs inversus group had a thicker RNFL in the clock-hour sectors 3 and 4, a thinner RNFL in the clock-hour sectors 7, 8, and 11, and more nasally located superior and inferior RNFL peak locations than the control group (P≤0.001). The situs inversus group had a smaller cup-to-disc area ratio, smaller vertical cup-to-disc ratio, and a lesser cup volume than the control group (P<0.01).

CONCLUSIONS

Eyes with situs inversus of the optic disc showed different peripapillary RNFL and ONH characteristics from those without this abnormality. These findings should be considered when assessing eyes with situs inversus of the optic disc.

Unusual association of keratoconus with situs inversus and micronystagmus

Keratoconus is a bilateral non-inflammatory ectasia of the cornea and the presentation is usually as an isolated condition. A few ocular and a number of systemic associations have been described. Situs inversus of retinal vessels is a rare developmental anomaly due to anomalous insertion of optic stalk into the optic vesicle. In this case report, we describe an unusual association of keratoconus with situs inversus and micronystagmus in a young man. An additional feature, a blue dot cataract, was also present. To the best of our knowledge, such a conglomeration of multiple ocular features has not been described previously.

Glaucoma in an eye with situs inversus of the optic disc

Situs inversus of the optic disc is a congenital abnormality characterized by dysversion of the optic nerve head and surrounding retinal vessels. Here, we report the retinal nerve fiber layer (RNFL) characteristics of eyes with situs inversus of the optic discs with and without glaucoma, as imaged using spectral-domain optical coherence tomography (OCT). OCT findings of an eye with situs inversus of optic disc and uveitic glaucoma revealed abnormal RNFL thinning. After six months of treatment, progressive RNFL thinning in the superior-nasal and inferior-nasal areas was found. The fellow eye with situs inversus of the optic disc without glaucoma showed a thicker nasal RNFL, a thinner temporal RNFL, and more nasally located RNFL peak locations compared with normative database. RNFL thickness characteristics in eyes with situs inversus of the optic discs, presented here, should be considered when assessing glaucoma in eyes with this abnormality.