Most forms of CSNB manifest an ERG maximal response with a normal a-wave and selective b-wave reduction, a negative or electronegative ERG, consistent with abnormalities that are post-phototransduction. It is usually best seen in the scotopic rod-dominated maximal ERG, although it may also occur in cone-mediated ERGs under photopic conditions [46,65]. Rare forms of CSNB manifest ERG maximal response a-wave reduction consistent with disruption of rod pho-toreceptor dysfunction and are reviewed elsewhere . CSNB is genetically heterogeneous with AD, AR and X-L inheritance reported.
X-linked CSNB usually presents with nystagmus and poor vision in infancy. The night blindness usually only becomes apparent at an older age. Most children are myopic and strabismus is common. X-linked CSNB can be subdivided into complete (cCSNB) and incomplete (iCSNB) forms, a division originally based on electrophysiology and psychophysics  and now known to reflect genetically distinct disorders. The gene for cCSNB maps to Xpii.4, and results from mutation in NYX which encodes nyctalopin, believed to play a role in the development of retinal interconnections involving the ON-bipolar cells . The gene for iCSNB (CACNA1F) maps to Xpii.23 and encodes a pore-forming subunit of an L-type voltage-gated calcium channel believed to modulate glutamate release from photoreceptor presynaptic terminals .
Both X-linked forms of CSNB have markedly electronegative maximal ERGs. The cCSNB
(Figs. 9.4A, 9.2A) has an undetectable rod-specific ERG. The cone flicker and photopic (single flash) ERGs show distinctive abnormalities. The photopic ERG has a broad a-wave followed by a b-wave lacking photopic oscillatory potentials and showing a low b:a ratio. This appearance is thought to reflect loss of cone ON-bipolar contribution but preservation of the OFF- pathway found in long and medium wavelength cone systems . This is confirmed by the results of long duration ON- OFF- ERGs , which reveal a normal a-wave, a selectively diminished ON-b-wave and preservation of the OFF- d-wave, consistent with involvement of the depolarising ON-bipolar cell pathway. S-cone ERGs are also affected, confirming the defect to be post-pho-totransduction in rods and all cone types (Fig. 9.4A). The electroretinographic changes in cCSNB are identical to those in melanoma-associated retinopathy [40, 46] and although unlikely to be of relevance in paediatric practice, demonstrate the importance of always placing electrophysiological data in clinical context.
Although also showing a profoundly electronegative maximal ERG, iCSNB typically has a detectable, but subnormal or delayed, rod-specific ERG. These are accompanied by a subnormal, delayed 30-Hz flicker ERG that has a typically bifid appearance (Fig. 9.4B). The photopic single flash ERG may be markedly subnormal and occasionally has an electronegative waveform . Overall, the cone-mediated photopic ERG abnormalities in iCSNB are more apparent than those of cCSNB (Fig. 9.4B); both ON- and OFF- responses are affected .
Dominant forms of CSNB may also be associated with an electronegative ERG resulting
Fig. 9.4. Pattern ERGs, full-field ERGs, including ON-OFF- and S-cone ERGs in a normal subject (N) and in patients with complete CSNB (A), incomplete CSNB (B) and enhanced S-cone syndrome (C). In the latter case, note the S-cone-specific recordings are similar to those of the single flash photopic ERG. See text for details. Eye movement or blink artefacts, commonly present in paediatric ERGs to bright flashes, occur at about 100 ms, most prominently in the maximal ERG of patient B
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