Content area
Full Text
ABSTRACT
The disc-macula: disc diameter (DM:DD) ratio is the ratio of the horizontal distance between the center of the optic disc and the macula to the mean diameter of the optic disc, as evaluated from fundus photographs. The diagnosis of optic nerve hypoplasia may be difficult in cases in which the optic discs are slightly reduced In size, or when the double ring sign is absent and the condition is bilateral. The DM : DD ratio was calculated for six eyes of five children with ophthalmoscopically obvious optic nerve hypoplasia, 12 eyes of six children in whom the diagnosis of optic nerve hypoplasia was equivocal (but established by visual field examination and retinal red free photography), 13 eyes of eight children with optic atrophy, and 25 eyes of 17 children with normal optic discs. The DM:DD ratio was found to be significantly higher for both groups with optic nerve hypoplasia than for those with optic atrophy and the normal controls. For practical purposes, when the DM:DD ratio Is greater than 3.0, the diagnosis of optic nerve hypoplasia should be seriously considered.
INTRODUCTION
Optic nerve hypoplasia is increasingly being recognized as a congenital anomaly which may affect one or both optic nerves,1 and has recently been the subject of an extensive review.2 It may be a result of failure of development of the retinal ganglion cells resulting in fewer axons in the optic nerve.3,4 Degeneration of supernumerary axons occurs as part of the normal development of the visual pathways. When this degeneration is excessive, optic nerve hypoplasia ensues. A number of teratogenic influences have been implicated in the pathogenesis.2 The presence of small (hypoplastic) optic discs may be obvious immediately on ophthalmoscopy. In some cases, however, the clinical signs may be equivocal. Further investigations that support the diagnosis of optic nerve hypoplasia include the demonstration of non-progressive visual field defects,510 retinal nerve fiber loss as shown by red free fundus photography,9'11 estimation of the size of the optic disc by comparing the slit beam width with the optic disc size on contact lens biomicroscopy,12 and radiologic assessment of the diameter of the optic canal.13
The ratio of the distance between the center of the optic disc and the macula and the mean optic disc...