Pathology of retinitis pigmentosa
Retinitis pigmentosa is characterised by changes in the pigment layer of the retina. The pigment layer of the retina invades and damages the nerve layer and with time it affects the blood vessels of the retina as well. The blood vessels get thinner with time and eventually may be completely atrophied. Over time the nerves are also damaged leading to atrophic changes in the optic nerve as well. This leads to loss of vision.
Rods and cones
Normally the retina contains photo receptor cells called rods and cones. Rods are responsible for black and white vision while cones are responsible for color vision.
In most cases the rods are affected first and cones are affected less commonly. However, in more aggressive and rapidly progressive types of retinitis pigmentosa cones may also be affected. The predominance of rod or cones over the other type of photoceptor thus is determined by the particular genetic defect in that patient.
Inheritance patterns of retinitis pigmentosa
Retinitis pigmentosa runs in families and is an inherited condition. The disorder can be caused by a number of genetic defects. The main risk factor is a family history of retinitis pigmentosa.
There are various inheritance patterns that determine the causation of the condition. More than 100 different genetic defects have been identified in the causation of retinitis pigmentosa. The severity of the condition and the type of retinitis pigmentosa results from inheriting one or two of these abnormal genes.
Based on inheritance patterns, the severity and course may be different. For example:-
Those with autosomal dominant inheritance are those with a milder course of the disease with late onset (often in fifth or sixth decades of life) and a slow progression. Around 30 to 40 percent have this pattern of inheritance.
Autosomal recessive inheritance is seen in 50 to 60 percent of cases.
X linked inheritance that is passed from mothers to their male offspring is seen in 5 to 15% cases. The mothers pass on the defective genes to their daughters to carry and pass it on to their sons. The X-linked form is the most severe and central vision is usually lost by the third decade.
DNA testing is available on a clinical basis for autosomal recessive (Bothnia type), autosomal dominant (RP1, RP4, RP7, RP13, RP18), CRB1 (autosomal recessive RP12, RP19 and RP20).
Other causes of retinitis pigmentosa
Non-inherited or non-genetic causes of retinitis pigmentosa are rare. This means that it is rare to find a case where there is no family history of the condition. Viral infections have been implicated but researchers believe that new mutations may be the underlying cause of such cases.