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Congenital optic nerve head pit associated with reduced retinal nerve fibre thickness at the papillomacular bundle
  1. C H Meyer,
  2. E B Rodrigues,
  3. J C Schmidt
  1. Department of Ophthalmology, Philipps-University Marburg, Germany
  1. Correspondence to: Carsten H Meyer, MD, Department of Ophthalmology, Philipps-University Marburg, Robert-Koch-Strasse 4, 35037 Marburg, Germany; meyer_eye{at}yahoo.com

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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.

Case report

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.

Figure 1

Fundus image of the left large optic nerve head with a horizontal diameter of 2.28 mm, vertical diameter of 2.09 mm, and area of 3.42 mm2. The optic disc has a grey oval pit at the inferotemporal side and is surrounded by a hyperpigmented margin. The papillomacular bundle appeared to be darker extending from the edge of the optic nerve to the macula. In the superior and inferior quadrant the main nerve fibres are visible. The blue arrows indicate the location and direction of the corresponding linear and circular OCT scans.

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).

Figure 2

(A) Horizontal OCT scan of the temporal optic disc and the papillomacular region of the left eye. Fibroglial tissue membrane appears to overlie the vitreoretinal surface at the upper edge of the pit. The thick hyper-reflective band which is white to reddish in colour, corresponds to the retinal nerve fibre layer (RNFL). The reflectivity is elevated and the thickness is wider than normal. The standardised measurement of the RNFL determines a thickness of ≥200 μm at the edge of the optic nerve and 80 μm at 1 DD distance. (B) Horizontal OCT scan of the papillomacular region left eye. A less reflective and thinned hyper-reflective band in red and orange colours extends from the edge of the optic nerve to the fovea. The standardised measurement of the RNFL determines a thickness of 70 μm at the edge of the optic nerve and 5 μm at 1 DD distance. (C) Circular OCT scan of 3.4 mm diameter centred on the optic disc left eye. The cylindrical section is unfolded and displayed as flat cross sectional, two dimensional false colour image. The scan started nasally and measured clockwise perpendicular around the optic nerve with a diameter of 2.0 nerve head. Each b-scan consists of 100 individual A-scans (one thickness value for each 3.6′). The RNFL measurement determined a mean thickness of 182 μm in the superior quadrant, 90 μm in the temporal quadrant, 125 μm in the inferior quadrant, 64 μm in the nasal quadrant. There is a marked reduction in the RNFL to 48 μm at the 4 o’clock position consistent with the location of the optic pit.

Comment

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.

References

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Footnotes

  • Proprietary interest: none.

  • Financial support: none.

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