Introduction
RP is the most common form of inherited retinal dystrophy, affecting 1 in 4000.
- It's x-linked variant has the worst prognosis
- Usher's syndrome is the most commonly associated systemic condition
Pathophysiology:
- Due to the rod's density being the greatest in the mid-periphery, RP typically originates in the mid-periphery
- It then spreads to the macula, with it's degeneration causing poor night vision, nyctalopia, VF collapse and central vision damage.
Dead Giveaways
By far the biggest indicator of RP is the classic diagnostic triad:
Bone Spicule Pigmentation
This occurs due to RPE pigment migration, going through the retina and clumping at perivascular regions
Forms an almost cobweb like appearance in the mid periphery
Bone spicule. On the right there is arterial attenuation
Arteriolar Attenuation
Whilst arteries are conventionally 2/3rds the size of veins, the arteries are very thin
This is potentially due to a decrease in metabolic demand due to the decrease in metabolic demand from the mid-peripheral retina due to the degeneration of PR
Waxy Pallor
It is believed that white glial tissue covers the disc, which creates a cloudy yellowish hyper-reflective appearance
Disc is fairly cloudy and yellowish instead of pinkish orange. Bone spicules and attenuated arteries can also be seen
The next biggest way of identifying RP, especially with advanced technology, is through the new diagnostic triad:
Loss of the Outer Retinal Layers (ONL and EZ)
The cystic space continues into the ONL, but it is notably very thin in the periphery. Also note the increased visibility of the choroid at the right The thinning of the ONL indicates the rod cells are beginning to die, causing it to shrink extensively
It should also be noted that just under the cystic space and above the RPE, there is a thin hyper-reflective band, which is the Ellipsoid Zone.
This ellipsoid zone fades very quickly, moving into the periphery. The disruption of homogenous hyper-reflectivity indicate photoreceptors are disrupted and are dying.
The loss of photoreceptors are linked to a loss of RPE, causing pigment migration. This increases transmission of light, allowing visualisation of the choroid.
Macular Oedema
From the previous image, the cystic spaces under the macular
Epiretinal Membrane
A membrane of glial cells and laminocytes attached to remnants of the vitreous cortex
The thin hyper-reflective membrane on top is the ERM
The last big giveaway is the fundus autofluorescence
The hyper-AF ring is present around the macular
This indicates the region where there is a region in which photoreceptors are under stress and at risk of death
The hypo-AF ring is present, represented as pigmented spicules
The hyper-AF ring around the central macula, and the pigmented hypo-AF ring
diagnostic features
Signs:
Disruption of the ONL and EZ, causing progressive thinning and loss
Pigmentary migration to form bone spicules
VF constriction
Hyper-AF ring around the macula
Waxy disc pallor
Arterial attenuation
Possible macular oedema, epiretinal membrane and posterior subcapsular cataracts
No a-wave seen, and a minimal to no b-wave seen
Symptoms:
Nyctalopia (poor night vision)
As this is a rod-cone dystrophy, the onset is around age 12 which is typically later
Cone-rod dystrophy usually has an earlier onset, and causes legal blindness by age 23
Reduced VA
Acquired colour vision deficit
Other Features:
Degenerating Scotomas
Initially, due to the mid peripheral origin, there will be scotomas in that area
Slowly, this degenerates into a ring scotoma
After this, field loss occurs at 50%/8y
Scotomas are highly symmetrical --> Colour vision defects
Kinetic perimetry can be very helpful in identifying any islands of vision that can be utilised for navigation
Atypical RP
Unilateral RP -> in one eyes
RP sine pigmento -> No bone spicule pigment
Sectorial RP -> In a segment of the eye
RP Inversa -> Affects macular not the mid-periphery, leading to central vision loss first
Abnormal ERG
As the electroretinogram measures the impulses from the photoreceptor and bipolar cells, and as the rods are greatly damaged, on a dark adapted ERG:
No a-wave and no b-wave
For photopic ERG:
May show normal a-wave and b-wave, unless macular is affected