A 35‐year‐old man was referred with progressive impairment of vision. He was previously diagnosed with anisometropic amblyobia and registered partially sighted. He had lifelong problems of night vision and described a family pedigree suggestive of X‐linked inheritance, with two generations of male members affected with nyctalopia and high myopia. No male‐to‐male transmission occurred and female relatives were unaffected. Family members could not be contacted.
Refraction showed distorted retinsocopy reflexes, with best corrected visual acuities of −18.00/+10.00 ×180 (6/60) OD and −10.00/+8.00 ×5 (6/36) OS. Bilateral steep central corneal thinning, paracentral ectasia and Vogts striae were present. His fundus appeared normal. Corneal topography disclosed 7.4 dioptres of irregular astigmatism in the central 3 mm with thinning (335 µm) characteristic of keratoconus (fig 1). There were no medical or environmental influences for his keratoconus. Electroretinography (ERG) showed no response to the dim flash in the dark, a “negative” ERG to the standard flash and attenuated tests in photopic conditions with near absence of B‐wave (fig 2).
Figure 1 Corneal topography of the right eye showing a steep central and inferior thin cornea characteristic of keratoconus.
Figure 2 Top row shows a representative normal trace with L.oculus dexter and L.oculus sinister responses of subject in rows 2 and 3.
Comment
Keratoconus has not been previously linked with CSNB1 either as a chance or true association though both show genetic predisposition. Keratoconus is characterised by progressive irregular myopic astigmatism, central corneal thinning and ectasia1 and associated with retinal diseases including retinitis pigmentosa, macular coloboma, Leber's congenital amaurosis, retrolental fibroplasia and cone dystrophy.2 The genetic bases for keratoconus is poorly defined2 although one form is associated with mutations in the retinal transcription factor VSX1 and abnormal retinal function.3,4
X‐linked CSNB is characterised by impaired night vision, myopia, nystagmus and reduced visual acuity.5 It can be subdivided into complete (CSNB1) and incomplete (CSNB2) phenotypes distinguished by ERG responses.6 In CSNB1 there is complete absence of response to a dim flash in the dark, a “negative” response to the standard flash and an altered response to the light adapted standard flash. An isolated gene in Xp11.4 (NYX, nyctalopin) is mutated in patients with CSNB1,7,8,9 which may influence the assembly of collagen fibrils.10
Although the occurrence of keratoconus and CSNB in the above patient may represent a chance association, further reports may disclose a possible pathogenetic link. It would therefore be of interest to investigate the corneal topography of patients with CNSB1 to determine if there is a possible association.
Footnotes
Competing interests: None declared.
References
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