Introduction
This disease is fairly rare:
- Affects 1 in 50,000
- Noted to onset in the 1st to 2nd decade onset
- Nyctalopia and VF constrictions noted around 3rd decade
- Affects the central vision around age 40-50
- As it is X-linked, it is more likely to affect males
Pathophysiology:
- The CHM gene which is affected primarily involved with the membrane transport proteins and RPE
- Is also associated with PR degen and significant vision loss.
- It is unsure if choroideraemia affects the choroid, PR or RPE first, but progressive degeneration of inner choroid and outer retina is the result.
Dead Giveaways
The tells depend the fundus examination and OCT. However, there are many differential diagnosis, so may require family history taking.
Fundus
Similar to RP, there is pigment migration, mottling and clumping. However the clumps are not bone spicule like, but are blob like
Shows the blobs, as well as increased transparency of the RPE. Choroidal vessels become more visible Can degrade to the point that the sclera becomes visible (see the main image)
OCT:
There is an associated loss of the choriocapillaris and choroidal layers, as well as the outer retinal layers
There is randomised sparing which can occur, but most of the time, is just thinned or lost.
Relative sparing, but on either side there is intense thinning of the choroidal layer and outer retinal layer. RPE, EZ and ONL can barely be seen On the same image, the left side displays two hyper-reflective dots. These are tubulations
Associated with late stage degeneration of the ONL
It is composed of degenerating photoreceptors
On the same image, the right side displays a clump that casts a shadow --> pigment clump
Autofluorescence Loss
The fundus has relative hypo-AF
Spared patches are hyper-AF. The ONH has a pseudo-hypo-AF appearance due to the increased scleral transparency
diagnostic features
Carrier's Choroideraemia
Female carriers (males cannot be carriers) will show features of the choroideraemia
It bypasses lionisation
When recessive genes are turned off permanently and allow the other gene to function normally
Pigment mottling also occur, but VA and VF are normal
Fundus AF also normal, along with OCT, VA and there is no nyctalopia
In ERG is relatively normal, but may have changed amplitudes
ERG:
Very variable, but progresses to be worse overtime
Early stage:
Scotopic ERG shows similar defects in the a/b-wave similar to RP
PR degenerates
Late stage:
The photopic ERG also begin to show defects as the cones begin to be involved
Signs:
In the retina:
Early pigment mottling followed by pigment clumping
Increased choroid visibility due to RPE degeneration
Late stage scleral visibility
In OCT:
Thinning and loss of PR, RPE, Choriocapillaris, and eventually choroid
Outer retinal tubulations for later stage retinal disease
In FAF:
Loss of background autofluorescence
Hypo-AF in areas of clumping
ONH hypo-AF due to scleral visibility
Symptoms:
Nyctalopia and VF constriction
Loss of central vision