Retinitis Pigmentosa (RP) is a group of inherited disorders which are known by progressive dysfunction, cell loss, and finally atrophy of retinal tissue.
Initial disturbance of photoreceptors causes subsequent damage to inner retinal cells. After a while, there is a widespread atrophy of several layers of retina (In some cases most layers are involved.). Night blindness and visual field loss are the initial consequents of RP. The final consequent of RP manifests itself as central visual dysfunction (1).
Narrowed retinal vessels, mottling and granularity of the retinal pigment epithelium and migration of pigments from the disrupted retinal pigment epithelium in to the retina to form clumps of various sizes or bone Spicule formation is the significant changes which are occurred in the fundus.
Also, atrophy of the retinal pigment epithelium and choriocapillaris are the final changes made in the fundus (2, 3). Unilateral retinal degeneration is a misnomer which can be described more properly by the term unilateral retinitis pigmentosa. Although functional and ophthalmoscopic changes are obvious in the affected eye, there is no symptom and ERG (Electroretinogram) abnormalities in the fellow eye during years (4). To diagnose the primitive URP, the Francois and Verriesr criteria are needed:
1. Exclusion of all infective etiologies.
2. Checking the clinical signs of retinitis presented in
the affected eye.
3. Making sure there is no symptom in the fellow eye.
4. Using a long follow-up period (at least five years)(2).
Determining if ERG and EOG (Electro-oculogram) will be the two reliable and useful examinations to diagnose the disorder, if they are performed according to ISCEV (International Society for Clinical Electrophysiology Vision) standards (5, 6).
A 29-year-old female with a long history of visual disturbance in the right eye underwent an eye examination eight years ago. Her chief complaint was night blindness without photophobia and colour vision problem. She did not have any history of ocular trauma or surgery. At the time of the first visit, the Best Corrected Visual Acuity (BCVA) in the Right Eye (RE) was 20/22 and in the Left Eye (LE) was 20/20. The biomicroscopic anterior segment examination was normal in both eyes. Although bone spicule degeneration appearance in the midperiphery area, waxy disc palor and vessel attenuation were obvious in the fundus examination of RE, The fundus examination of LE was completely normal (Figure1).
The patient was diagnosed as a suspected unilateral RP case. Serological surveys to rule out all the infectious agents – which might cause the similar retinal appearance – were negative. There was no affected person in the patient’s family. Also, the patient’s mother did not have any history of infection during pregnancy. Wide-field computerized perimetry, ERG and EOG were performed. Despite the generalized sensitivity reduction and restricted visual field with central island in the RE computerized field examination, the LE was totally normal (Figure2).
While the standard photopic and scotopic ERGs were normal in the LE, they were markedly reduced in the RE (Figure3).
The result of the standard EOG was normal in the LE and subnormal in the RE (Figure4).
With respect to probable diagnosis as a suspected unilateral RP case and the lack of treatment, she was just followed up. After an eight-year follow-up, the BCVA of RE decreased to 20/28; however, the BCVA of LE remained 20/20. Although the pigmentary degenerationof midperipheral retina was worsened in the RE, no changes were seen in the LE. Serial field examination could not find any abnormality in patient’s left eye.
However, a progressive visual field defect has been the main characteristic of these assessments in the right eye.
While RP appears to be an inherited disorder- either as an autosomal or X-linked trait- URP is more frequently sporadic and in adult ages (7,8).
The etiologies are usually not known, but there are some documented causes which can lead to this disorder such as transient ophthalmic artery occlusion, perforating injury to the eye without direct trauma to the retina (9), and structural defects like pit of the optic disc with prolonged serous retinal detachment (4).
Despite the large number of bilateral RP cases, the URP cases are much more infrequent and rare because of its progression in two eyes during years (10). The rarity of URP has clarified the importance of electrofunctional examinations (5, 6). To diagnose better in this case, a long follow-up period is needed in order to find the clinical signs in the affected eye and no related changes of RP in the fellow eye (2).
The case presented in this article conforms to the four proposed criteria to justify the presumptive URP (11): Long disease duration (over five years), presence of functional changes and an ophthalmoscopic appearance of typical primary pigmentary degeneration in affected eye, no clinical and electrophysiological change in unaffected eye, and exclusion of an inflammatory cause. In some studies, it is shown that the ERG abnormalities are seen even without any ophthalmoscopic fundus involvement (12).
In addition to making the diagnosis, the severity of ERG abnormalities is proportional to the extent of the involved area which is beneficial to monitor either the progression of the disorder or absence of the disorder (13). EOG is also useful to confirm the unilaterality of RP, as ERG is helpful to exclude the involvement of the fellow eye or the possibility of different progression of bilateral RP (2). Unfortunately, there is not any treatment for RP at present, but definite diagnosis is crucial because other diseases in the differential diagnosis of RP have effective proved treatment.
To conclude, characteristic clinical signs and symptoms with ipsilateral abnormal ERG and EOG with long term follow-up are very helpful to diagnose and confirm the case of unilateral RP.