CSNB

Introduction

It is a commonly AR condition that is pseudo-non-progressive, progressing over an incredibly long time.
- It can be AD, and is usually associated with myopia
- VA can vary from 6/9 to 6/60

The disease has 4 different subtypes which depend, which have different appearances:
Normal Fundus:
- Normal CSNB
- Incomplete CSNB
Abnormal Fundus:
- Fundus Albipunctatus
- Oguchi's disease

Dead Giveaways

For both normal and incomplete CSNB, fundus examinations will be very unhelpful as it will appear normal.

Instead, the triad is used:

Abnormal Dark Adaptation
1. Requires 60-120 seconds of dark adaptation, and presentation of over 100 stimuli, so it is incredibly slow
2. For patients with CSNB, the threshold required to view a stimuli under scotopic conditions will be significantly greater than those without
Abnormal ERG
1. The defect in CSNB is in the photoreceptors, and so the photoreceptors can still process a signal, leading to a normal a-wave
2. However, it will not be able to neurotransmit that signal to the bipolar cells as effectively, leading to an abnormal b-wave, which is both outside the normal range, and smaller in amplitude than the a-wave (negative waveform)
Nyctalopia
1. Failure of neurotransmission at the bipolar cells will cause loss of information at synapses, causing poor night vision.

diagnostic features

Fundus Albipunctatus

Characteristically has numerous small white-yellow macular sparing lesions scattered around the retina
These lesions represent outer retinal atrophy, and cause VF and colour defects
The lesions represent areas of outer retinal atrophy

Oguchi Disease

AR inherited
Is a golden coloured fundus that reverts to a normal fundus after 1-2 hours of dark adaptation
- After dark adaptation, will show a normal ERG reading
- But immediately after, will not show any ERG reading, until fully adapted again as the bright flash will reset it.
Shows the golden fundus (peripheral) changing back to a normal fundus (central)

Signs:

In the retina:
- Normal if complete or incomplete
- White-yellow retinal specs dispersed but sparing macular
- Golden sheen changing to normal after long adaptation

Symptoms:

Nyctalopia
Variable VA → Associated myopia, nystagmus or strabismus