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Congenital pits of the optic nerve head result from an imperfect closure of the superior edge of the embryonic fissure. An unequal growth on both sides causes a delayed closure of the fissure at approximately 5 weeks of gestation. Optic pits appear as crater-like indentations of the surface of the optic nerve head usually with a steep temporal wall.1
Anatomically the most anterior component of the optic nerve head contains the retinal nerve fibre layer (RNFL), composed of approximately 1.2 million unmyelinised retinal ganglion cell axons extending from all regions of the retina. The outgrowth of axons from certain ganglion cells may be incomplete so that the primitive epithelial papilla is built up with aberrant nerve fibres.2 Histological sections of optic pits define defects in the lamina cribrosa associated with rudimentary retinal tissue, resembling pigmented tissue and aberrant nerve fibres. These anomalous papillomacular nerve fibre bundles may be less resistant,1 predisposing this sector to spontaneous schisis-like retinal detachments during later life.3
We present a young patient with a unilateral optic pit and a clinically significant temporal nerve fibre loss. In vivo measurements by optical coherent tomography (OCT) determined the thickness of RNFL at the side of the pit and the corresponding papillomacular bundle.
A 27 year old white woman presented with a 9 month history of blurred vision; her best corrected visual acuity was 20/20 right eye and 20/25 left eye. On Goldmann perimetry in both eyes, there were no visual field defects, arcuate or paracentral scotomas. On slit lamp examination the anterior segment appeared normal in both eyes. Fundus biomicroscopy of the left eye revealed a large optic nerve head with a grey oval pit at the temporal margin and a brownish rim at the temporal side. The papillomacular bundle appeared to be darker, extending from the edge of the optic nerve to the macula, compared to the superior and inferior quadrant corresponding with severe RNFL loss according to the semiquatitative assessment of Niessen et al4 (Fig 1). Fundus examination of the right eye was unremarkable.
Linear OCT disclosed a significantly thickened RNFL in the superior quadrant (Fig 2A) and thinned RNFL at the temporal quadrant of the optic nerve in the left eye. There were no signs of a schisis-like retinal detachment (Fig 2B). Circular OCT demonstrated a significantly reduced thickness of the RNFL in all quadrants but predominantly in the temporal (90 μm) quadrant (Fig 2C).
The oval depression of the optic nerve head in optic pits may relate to an enlarged optic nerve head, an incomplete closure of the embryonic fissure and a reduced RNFL. The size of the optic disc was significantly larger when compared to the mean size (1.76 mm) in normal eyes, reducing the mean RNFL density.4 In addition, an incomplete closure of the embryonic fissure seams to prevent a proper fusion of the temporal RNFL. OCT confirmed both a reduced mean RNFL and significant loss especially in the papillomacular bundle. Whereas normal subjects have a mean RNFL of 153 μm and 126 μm in the temporal quadrant,5 our patient with an optic pit had a mean RNFL of 115 μm and 90 μm in the temporal quadrant. Glaucomatous eyes with a mean RNFL below 103 μm frequently develop visual field defects,6 whereas in our patient with a mean RNFL of 115 μm none became apparent.
OCT precisely measured the retinal thickness with micrometer scale and provided additional evidence for pronounced reduced thickness of the RNFL in the temporal quadrant of the optic pit. Fundus photography confirmed the enlarged optic disc and temporal RNFL damage. The imperfect closure and lack of papillomacular nerve fibre bundles represent a “loco minoris resistenciae” in optic pits, the development of a spontaneous schisis-like detachment during ageing.
Proprietary interest: none.
Financial support: none.