retinitis pigmentosa ( rp ) is a genetically heterogeneous disease characterised by progressive retinal photoreceptor degeneration [ 1 , 2 ] .
the worldwide population affected has been estimated to be over one million individuals , whereas the frequency is approximately 1/4000 [ 3 , 4 ] .
although rp has several mutations and genetical patterns , the symptoms and histopathological findings are similar .
common symptoms are nyctalopia , impairment of visual acuity , and restriction of peripheral visual field .
characteristic findings by fundus examination include peripheral pigmented bone spicule - like lesions , retinal arteriolar attenuation , and optic disc pallor .
diagnosis is often made through a combination of clinical investigation , visual field exams , and electrodiagnostic methods such as electroretinography ( erg ) .
various histopathological studies of rp have demonstrated a reduction of rod and cone cells and thinning of the outer photoreceptor layer .
secondary to the outer retinal thinning , the inner retinal structure degenerates through suspected transneuronal damage , vascular compromise , or axonal compression [ 79 ] . previously published reports by newman et al
. demonstrated that patients of various retinal hereditary dystrophies , including rp , had ophthalmoscopically evident retinal nerve fiber defects .
the optic coherence tomography ( oct ) is a noninvasive diagnostic tool for rapid scanning and imaging of the retinal structures with high axial resolution of almost 5  m , especially in spectral domain models .
the oct scans give the clinician crucial information about the retinal nerve fiber layer ( rnfl ) , retinal pigment epithelium complex , and the junction of inner and outer segments of photoreceptors ( is - os line ) .
the new therapeutic modalities in rp , such as gene therapy [ 1114 ] or retinal stem cell transplantation [ 15 , 16 ] , are limited because they only improve the outer retinal layers .
the outcome of new therapies must focus on the pretreatment status of the inner retinal layers as well as the outer layers making oct a useful technique in the treatment prediction for such rp patients . in this study , we aimed to examine the peripapillary rnfl thickness and documented the changes in a group of rp patients .
medical records of forty - four eyes of twenty - two rp patients who were followed up in okmeydan training and research hospital , department of retina , and forty - four eyes of twenty - two healthy subjects ( control ) were enrolled into this study .
the diagnosis in all cases was made by retinal specialists using the following criteria : clinical history , fundus examination , visual field defects , and reduced amplitudes in erg .
the exclusion criteria used in our study were high refractive errors ( 6 diopters sphere ; 3 diopters cylinder ) , low degree of central fixation , significant media opacities ( e.g. , posterior subcapsular cataract ) , diabetic retinopathy , glaucoma , or cystoid macular edema .
all cases underwent a complete ocular examination , including best - corrected visual acuity ( bcva ) using snellen chart , slit lamp biomicroscopic examination , dilated fundus examination , and intraocular pressure measurement with goldmann applanation .
average and four quadrants peripapillary rnfl thickness values which were scanned by spectral - domain oct ( cirrus , carl zeiss meditec , inc . , software 5.1.1.6 ) made by the same operator were analyzed .
any rnfl layer thickness greater than 95th percentile was defined as thickening , whereas thickness lower than the 5th percentile was determined as thinning ( figure 1 ) .
the mean age of the rp patients was 37.09  2.55 years ( range 1875 ) and the mean age of control group was 39  1.70 years ( range 1854 ) .
the mean age , sex , and intraocular pressure of the two groups did not differ significantly . in the rp patients
the peripapillary rnfl was evaluated and the mean thickness was observed to be 97.57  3.2  m .
rnfl thickness in the 4 quadrants ( superior , temporal , nasal , and inferior ) was shown to be 119.18  4.47  m , 84.68  2.31  m , 75.09  3.34 , and 113.88  4.25  m , respectively .
in 20 of the total 44 eyes ( 45% ) , rnfl thinning was found in at least one quadrant . in two eyes ( 4% )
thinning was determined in 3 quadrants , in 11 cases ( 25% ) in 2 quadrants , and in 7 eyes ( 15% ) in only one quadrant . regarding the rnfl thinning of quadrants ,
the thinning was diagnosed in 12 cases ( 27% ) in superior quadrant , in 9 eyes ( 20% ) in the nasal quadrant , and in 14 eyes ( 32% ) in inferior quadrant .
temporal quadrant thinning was not found in any of the patients . in 21 ( 48% ) patients ,
five eyes ( 11% ) showed thickening in 3 quadrants , 2 ( 4% ) eyes in 2 quadrants , and 14 ( 31% ) eyes in only one quadrant .
five patients ( 11% ) had thickness in the superior quadrant , 8 patients ( 18% ) were in the nasal , 19 eyes ( 43% ) were in the temporal , and only 1 patient ( 2% ) was in the inferior quadrant . in 8 patients ,
all 4 quadrants were found within the normal values . in 5 patients there was thinning and thickening of the rnfl layer in different quadrants .
the thinning of the rnfl was most commonly observed in the inferior and moderately in the nasal and superior quadrants .
temporal rnfl thinning was not determined in any patient , whereas the temporal region was the most commonly thickened rnfl area followed by the nasal , the superior , and finally the inferior quadrant . in control group
the average rnfl thickness was 99.95  1.38  m . in the quadrant evaluation , the rnfl thickness in the superior , temporal , nasal , and inferior quadrants was 123.13  2.96  m , 66.75  2.02  m , 75.20  1.44  m , and 130.54  1.72  m , respectively .
there was no significant difference in average , superior , and nasal thickness between two groups ( p = 0.497 ; 0.463 ; 0.975 , resp . ) .
distribution of rnfl thickness between rp patients and control group is shown in figure 2 .
in rp , the photoreceptor layer progressively degenerates , followed by global changes within the inner retinal structure . in particular , the rnfl develops thinning or thickening secondary to photoreceptor cell loss .
morphometric and histological studies reported ganglion cell reduction in rp compared to the normal population [ 10 , 17 , 18 ] .
other reports suggested that vascular compromise or direct genetic effect on the ganglion cells was the reason for inner retinal layer reduction [ 18 , 19 ] . regarding the in vivo studies , walia et al .
reported using time - domain and fourier - domain oct to define rnfl thinning in 40% and 38% of rp patients [ 20 , 21 ] .
a similar result was reported by anastasakis et al .   which showed rnfl thinning in 38% of studied eyes , whereas oishi et al
.   found no significant difference between normal population and rp patient rnfl thickness using time - domain oct . in our study ,
45% of eyes showed rnfl thinning , similar to walia and anastasakis ' study results . in the previously mentioned studies , the rnfl thickening
we found that 48% of the studied eyes had peripapillary rnfl thickening , mostly in temporal quadrant .
the exact mechanism underlying this rnfl thickening is not clear but it may result from glial tissue proliferation ( which is secondary to the nerve fiber layer atrophy ) or edema of the remnant rnfl [ 23 , 24 ] .
in fact , hood et al . measured rnfl thickness in rp patients from peripheral retina and macular regions with fourier - domain oct and manual segmentation software .
they found that the rnfl was significantly thicker , especially in the horizontal meridian compared to the normal values .
in the same study , it was also suggested that the thickening might be a mechanical dysfunction , where the rnfl stretches to fill the empty space left by the photoreceptor degeneration .
the peripapillary rnfl changes in the four quadrants vary between several studies . in histopathological examination of rp patients , flannery et al .   found that the highest ratio of the photoreceptor loss was encountered in the inferonasal region , which may facilitate the rnfl thinning in this region .
reported in their studies [ 20 , 21 ] that the nasal quadrant was the most thinned , followed by inferior and superior quadrants .
observed the quadrants ' thinning sequence as inferior ( most common ) followed by the nasal and superior regions . in both xue 's et al .   and hood 's et al .  
xue et al .   found that all quadrants in their rp patient study were significantly thicker than the normal population , except the nasal quadrant , which was surprisingly thinner than the normal control group values . in our studies ,
the mostly thinned quadrant was the inferior followed by the superior and the nasal quadrants .
regarding the thickening of the rnfl , in several studies [ 2022 , 24 , 27 ] , the most commonly thickened quadrant was the temporal .
interestingly , no reports have observed thinning in temporal peripapillary area . similar to these reports
, we found that the temporal quadrant was the most commonly thickened region with no observed rnfl thinning .
this might be due to the retinal glial proliferation , which is most strongly seen between the temporal arcades .
considering future therapeutic approaches , understanding the mechanism and progression of retinal degeneration in rp will be crucial to predicting proper patient treatment .
high resolution sd - oct scans will be extremely helpful for the clinician in both aspects .
 
 aim . to evaluate the peripapillary retinal nerve fiber layer ( rnfl ) changes in retinitis pigmentosa ( rp ) patients using spectral domain optic coherence tomography ( sd - oct ) 
 . methods . 
 we retrospectively examined medical records of forty - four eyes of twenty - two rp patients . 
 the results were also compared with those of previously reported forty - four eyes of twenty - two normal subjects ( controls ) . 
 records of average and four quadrants peripapillary rnfl thickness measurements using sd - oct were assessed . 
 results . in rp patients 
 the mean rnfl thickness was 97.57  3.21  m . the rnfl in the superior , temporal , nasal , and inferior quadrants was 119.18  4.47  m , 84.68  2.31  m , 
 75.09  3.34  m , and 113.88  4.25  m , respectively . 
 while the thinning of rnfl was predominantly observed in the inferior quadrant , the thickening was mostly noted in temporal quadrant . 
 the differences between mean , superior , and nasal quadrant rnfl thicknesses were not statistically significant when compared with control group . 
 the rp patients had thinner inferior quadrant and thicker temporal quadrant than control group ( p < 0.05 ) . conclusion . 
 sd - oct is highly sensitive and effective instrument to detect rnfl changes in rp patients . 
 rnfl measurements can provide information about the progression of retinitis pigmentosa and may provide prognostic indices for future treatment modalities . 
1. Introduction
2. Methods
3. Results
4. Discussion