Optic nerve grey crescent

The optic nerve head has been defined as all areas inside the peripapillary scleral ring. Outside this ring in the parapapillary region, various features and abnormalities can be differentiated. In almost all eyes, the retinal pigment epithelium shows some histological irregularities close to the tip of Bruch’s membrane at the border of the optic disc. It is the histological equivalent of the alpha zone of parapapillary atrophy which is present in almost all normal eyes. It can usually better be detected at the temporal disc margin than in other parts of the parapapillary region. The beta zone of parapapillary atrophy, present in about 25% of normal eyes, and in a higher percentage of glaucomatous eyes, reflects a complete loss of retinal pigment epithelium cells and an almost complete loss of retinal photoreceptors. Other abnormalities or changes in the parapapillary region include a hypertrophy of the retinal pigment epithelium mainly at the temporal optic disc border in eyes with a so called conus pigmentosus of the optic nerve head; proliferations of the retinal pigment epithelium such as in retinochoroidal toxoplasmotic scars or after subretinal parapapillary haemorrhages; and melanocytic lesions of the choroid such as choroidal naevi or a malignant choroidal melanoma.¹

The grey crescent of the optic disc as originally described by Shields² is another, mostly unrecognised, feature at the border of the optic nerve head. According to Shields, it is a slate grey crescent within the peripheral tissue of the optic nerve head. In his study, 12 out of 100 consecutive black patients revealed the grey crescent. The grey crescents were usually bilateral and were most often located along the temporal or inferotemporal disc margin. The clinical importance of the grey crescent is that one may erroneously assume that the underlying tissue is not neuroretinal rim but parapapillary tissue. It will lead to a falsely small optic disc and neuroretinal rim area and, consequently, to falsely high measurements of the cup/disc diameter ratios. Additionally, it will markedly influence the assessment of the shape of the neuroretinal rim which normally follows the so called ISNT rule. The latter says that the smallest part of the rim is located in the temporal horizontal disc region, and that usually the widest part of the rim is located close to the inferior optic disc pole.¹

In their large population based study on the occurrence of the optic disc grey crescent in Iceland, published in this issue of BJO (p 36), Jonsson and colleagues found that the grey crescent was present in about 22% of the eyes examined. It was more commonly found in women, in hyperopic eyes, and in eyes without a small parapapillary atrophy. It was associated with a large optic disc, and it was usually located in the temporal region of the optic disc. The occurrence of the grey crescent was statistically unrelated to the prevalence of glaucoma. The authors have to be congratulated for their study and for renewing the interest in the grey crescent and for highlighting its importance for the morphological diagnosis of optic nerve abnormalities and diseases. One may, however, take into account the definition of the grey crescent as used by the authors. They defined the grey crescent as the “occurrence of a pigmented crescent that appeared, utilising a stereo viewer, to be located on or within the neuroretinal rim tissue—that is, inside the scleral lip of the disc whereby the scleral lip had to be clearly visible peripheral to the crescent.” Since the alpha zone of parapapillary atrophy is also characterised by an irregular pigmentation, and because the boundary between the alpha zone and the surrounding tissue usually follows a semilunar line, partially parallel to the peripapillary scleral ring (scleral lip), one must be aware not to confound the grey crescent with the alpha zone. The difference between both structures is that the alpha zone is located outside of the optic disc and may not be counted as neuroretinal rim, whereas the grey crescent is located inside of the optic disc and may partially or completely be regarded as neuroretinal rim. Not considering the differences between the alpha zone of parapapillary atrophy and the grey crescent will, therefore, markedly influence the morphological analysis of the optic disc.

The question arises what the histological equivalent of the grey crescent may be. Since the grey crescent is relatively dark, it may be associated with retinal pigment epithelium cells. These cells sitting on and forming Bruch’s membrane, may partially be located in the optic nerve head region if Bruch’s membrane extends internally to the peripapillary scleral ring. Future histological studies of the normal optic nerve head may be directed towards finding the clinical-histological correlate of the grey crescent, differentiating it histologically from the physiological alpha zone of parapapillary atrophy, and giving hints for the rate of the histological occurrence of the grey crescent in normal eyes. Optical coherence tomography may be an additional clinical method to analyse intravitreally the grey crescent and its spatial associations with the surrounding tissues.³