Additional mutants in this pigment-encoding gene affect protein stability, disrupt mRNA integrity post-translationally, and affect the activation rates of transducin and opsin optical proteins.Additionally, animal models suggest that the retinal pigment epithelium fails to phagocytose the outer rod segment discs that have been shed, leading to an accumulation of outer rod segment debris. In mice that are homozygous recessive for retinal degeneration mutation, rod photoreceptors stop developing and undergo degeneration before cellular maturation completes. A defect in cGMP-phosphodiesterase has also been documented; this leads to toxic levels of cGMP. Diagnosis
An accurate diagnosis of retinitis pigmentosa relies on the documentation of the progressive loss of photoreceptor cell function, confirmed by a combination of visual field and visual acuity tests, fundus and optical coherence imagery, and electroretinography (ERG).Visual field and acuity tests measure and compare the size of the patients field of vision and the clarity of their visual perception with the standard visual measurements associated with healthy 20/20 vision. Clinical diagnostic features indicative of retinitis pigmentosa include a substantially small and progressively decreasing visual area in the visual field test, and compromised levels of clarity measured during the visual acuity test. Additionally, optical tomography such as fundus and retinal (optical coherence) imagery provide further diagnostic tools when determining an RP diagnosis. Photographing the back of the dilated eye allows the confirmation of bone spicule accumulation in the fundus, which presents during the later stages of RP retinal degeneration.
A variety of indirect symptoms characterize retinitis pigmentosa along with the direct effects of the initial rod photoreceptor degeneration and later cone photoreceptor decline. Phenomena such as photophobia, which describes the event in which light is perceived as an intense glare, and photopsia, the presence of blinking, swirling or shimmering lights spontaneously occurring within the visual field, often manifest during the later stages of RP. Findings related to RP have often been characterized in the fundus (back layer) of the eye as the "ophthalmic triad".