Congenital Stationary Night Blindness

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Main Features

Night Blindness, variable vision loss, largely normal fundus
May have nystagmus and strabismus

Night blindness
Normal Appearing Fundus
High Myopia
- may have tilted discs and nerve pallor
ERG
- “Negative ERG”
  - Photoreceptor derived a-wave with a bright flash is normal
  - Reduction of the inner nuclear derived b-wave from inner retinal dysfunction
- No detectable Rod specific ERG- dark adapted dim flash

Figure 45.2 Taylor and Hoyt Congenital stationary night blindness. The left-hand column traces (A) show data from a patient with “incomplete” CSNB (iCSNB); the center traces (B) are from a patient with “complete” CSNB (cCSNB); the right-hand column traces (C) are from a representative normal subject. In iCSNB the rod ERG (DA 0.01) is mildly subnormal. The bright flash response (DA 10.0) is electronegative, with a normal a-wave confirming normal photoreceptor function, but a profoundly reduced b-wave. The 30 Hz flicker ERG (LA 30 Hz) is markedly subnormal and clearly shows the delayed double peak characteristically seen in iCSNB. The photopic single flash ERG (LA 3.0) shows marked reduction in the b:a ratio with simplification of the waveform and loss of the photopic oscillatory potentials, shown on ON/OFF response recording (200 ms orange stimulus on a green background) to reflect involvement of both ON (depolarizing) and OFF (hyperpolarizing) cone bipolar cell pathways. The pattern electroretinogram (PERG) is mildly subnormal in keeping with mild macular dysfunction. In cCSNB there is no detectable DA 0.01 response and the profoundly electronegative DA 10.0 ERG confirms the site of the dysfunction to be post-phototransduction. The LA 3.0 response shows a distinctive broadened a-wave and a sharply rising b-wave with a reduced b:a ratio and lack of photopic oscillatory potentials. This appearance indicates marked dysfunction of cone ON bipolar cell pathways but preservation of the OFF pathways. The profoundly negative ON response, with preservation of the ON a-wave and loss of the ON b-wave, accompanied by a normal OFF response is confirmatory. The broadened trough of the 30 Hz flicker ERG with a sharply rising peak is a manifestation of the same phenomenon. The PERG is almost undetectable. Overall, the findings in cCSNB are those of loss of ON pathway function in both rod and cone systems. Michel Michaelides; Graham E Holder; Anthony T Moore, 45, 526-549

Autosomal Dominant
- Genes that affect rod-specific phototransduction (RHO, GNAT1 , cGMP, PDE6β)
- Normal or mildly reduced visual acuity
Autosomal recessive
- Biallelic disease causing variants of GRM6, TRPM1, GPR179 or LRIT3 which are all involved in photoreceptor-bipolar cell synaptic development or function.
- Variants of CABP4, a calcium binding protein
  - Associated with “incomplete” forms of the disease
- Variants in the SLC24A1 gene
  - give CSNB without a negative ERG response. Both a- and b-waves are equally reduced
  - SLC24A1 is a member of the solute carrier protein superfamily located in inner segments, outer and inner nuclear layers, and ganglion cells
- Mild to moderate central vision loss
- Nystagmus and strabismus
X-Linked
- CACNA1F gene mutations
  - encodes retina specific subunit of voltage-gated L-type calcium channel
  - involved in “incomplete” forms of the disease
- NYX gene mutations
  - encodes leucine-rich proteoglycan nyctalopin which is expressed in photoreceptor inner segments, outer and inner nuclear layers and ganglion cells.
  - Involved in the “complete” forms of the disease
- Mild to moderate central vision loss
- Nystagmus , strabismus