Reappraisal of the ratio of disc to macula/disc diameter in optic nerve hypoplasia.

Abstract

The ratio of disc to macula/disc diameter is characteristically increased in eyes with optic nerve hypoplasia. We present the largest reported series of patients with a definitive diagnosis of optic nerve hypoplasia for whom this ratio has been determined. Ali measurements were made by an independent masked observer. Our Underdevelopment of one or both optic nerves is a common anomaly.' The aetiology is unknown. Optic nerve hypoplasia (ONH) is a diagnosis which should be considered in any person with poor vision for which there is no immediately apparent explanation. Although the small optic disc of ONH is easily detected in severe cases, more subtle forms of the disorder may present more difficulty. The differential diagnosis includes the apparently small optic nerve head of hypermetropia. Subtle forms of the disorder may be difficult to distinguish from normal appearances. Red-free fundus photography can aid in this differentiation by detecting evidence of nerve fibre layer defects which correspond with that of the pattern of visual field loss.2 However, many cases of ONH do not show such focal defects and are manifest solely as a uniformly thin nerve fibre layer within affected areas, which makes the condition more difficult to define than focal defects.3Franceschetti and Bock4 measured the optic disc diameter by means of focal illumination of the fundus, using contact lens biomicroscopy and a micrometer scale. Estimation of the size of the optic disc by comparing the slit beam width on the Hagg-Streit 900 slit-lamp with the optic disc diameter on contact lens biomicroscopy has also been advocated.5 These relative measurements are useful in comparing the diameter of the optic disc relative to the slit-lamp beam or the scale, because their magnification when projected on the optic disc is assumed to be the same as the magnification of the optic disc, as the slit beam and microscope optics are parfocal and both pass through the optical system of the eye.Other methods which have been described include comparison of the size of the optic disc with the size of the whole area of the fundus as seen in the standard fundus photograph,' measurement with a scale over the frontal lens during indirect ophthalmoscopy,7 direct measurements of the orbital part of the optic nerve by CT scanning,8 A-scan ultrasonography,8 and B-scan ultrasonography.9Variations in the size of the eye with concomitant differences in dioptric power even in emmetropic eyes render absolute measurement of intraocular structures difficult.'0" Interference fringes provide a means of accurate measurement' but necessitate sophisticated equipment and a co-operative patient. The alternative is to make relative measurements from fundus photographs. The apparent size of the optic disc in fundus photographs is influenced by several factors, including the axial length, corneal curvature, and the shape of the fundus,"3 and by refractive errors and optical aberrations, but because all photographed structures are equally influenced by anatomical and re...

Retinal function in infants with optic nerve hypoplasia: electroretinograms to large patterns and photopic flash

“…Both neuro-radiological studies and anthropomorphic measurements aid in establishing the diagnosis and prognosis for young infants with ONH. [9][10][11][12][13][14][15][16][17][18] Visual evoked potential (VEP) testing can give a functional measure of vision early in life. In ONH, the amplitude of VEPs to flash stimulation and the presence of VEPs to pattern stimulation are correlated with visual function.…”

Congenital disorders of the optic nerve: excavations and hypoplasia

“…In one study, a ratio of 2.94 provided a one-tailed upper population limit of 95%. 74 For practical purposes, a DM : DD ratio of 3.00 should lead to serious consideration of the diagnosis 73 and a value of 4.00 probably accords a definitive diagnosis. 75 The visual acuity is associated with the DM : DD ratio.…”

“…76 In another study, however, in 75% of bilateral cases, the eye with the relatively smaller optic disc was paradoxically found to have a better visual acuity than the fellow eye. 74 Both the degree of afferent pupillary dysfunction 76 and reduction in brightness sense 77 are also related to the level of acuity.…”

Optic nerve axons: life and death before birth

“…Eye diameter ratio 125,126 ( Figure 14). MRI may be better than photographic analysis when compared with clinical examination.…”

“…133 Visual acuity is determined largely by the size of the papillomacular bundle, not with the overall size of the disc; there is not a very close correlation between the size of the optic disc and the visual function. 126 The final vision may to an extent be predicted from assessment of the optic disc size, 134 the initial acuity card assessment and VEPs. 129,135 Reassessment is important in infants as Figure 13 A double ring was found in both eyes in this asymptomatic, healthy child with -0.1 logMAR acuity and normal colour vision bilaterally.…”