Inner and outer retinal volumetric and morphologic analysis of the macula with spectral domain optical coherence tomography in retinitis pigmentosa.

PURPOSE: To use spectral domain optical coherence tomography (OCT) to assess the features of the inner and outer layers of the macula in subjects with retinitis pigmentosa (RP).
DESIGN: Prospective observational comparative study.
MATERIALS AND METHODS: Subjects with RP with reduced central visual acuity and central macular thinning (RP group) underwent spectral domain OCT evaluation along with age-matched healthy control subjects (control group). Using the E-MM5 raster protocol, a 5 × 5 mm central macular cut was acquired with inner and outer macular volumes serving as the primary outcome measures. A structural analysis of the inner and outer retinal layers at the macula in the RP group was also performed using the HD cross-line protocol.
RESULTS: The RP group comprised six eyes of six RP subjects and control group comprised six eyes of six control subjects. The outer macular volume was significantly lower (P<0.05) in the RP group (2.01±0.44 mm(3)) compared to the control group (4.68±0.16 mm(3)). There was no difference in the mean inner macular volume between the RP group (2.46±0.24 mm(3)) and the control group (2.55±0.22 mm(3)). No significant structural alteration was noted in the inner retinal layers of the RP group. There was no significant correlation (r = -0.04, P>0.05) between best-corrected visual acuity and outer macular volume in the RP group.
CONCLUSIONS: Spectral domain OCT findings confirm that RP preferentially affects the outer retinal layers in the macula. Although the number of patients included in this study was small, it demonstrated relatively well-preserved volume and morphology of the inner retinal layers using spectral domain OCT.

INTRODUCTION

Retinitis pigmentosa (RP) is a heterogeneous group of retinal disorders that primarily affect the rod and cone photoreceptors. RP can be an isolated finding or associated with other systemic conditions and may be inherited as an X-linked, autosomal dominant, or autosomal recessive condition. It typically affects the rods earlier than the cones and can predispose patients to cystoid macular edema (CME), epiretinal membrane (ERM) formation, posterior subcapsular cataracts, and vitreous syneresis.1

Optical coherence tomography (OCT) is a well-recognized method of determining retinal architecture and is an accurate method of measuring retinal thickness.23 Time domain OCT has been used to evaluate RP maculopathy and studies have shown that patients with RP with visual acuity worse than 20/200 have demonstrated central macular thinning or CME on OCT examination.45 Investigators have reported that selective alterations in the structural integrity of the outer retinal layers of the macula with central outer retinal thinning on spectral domain and ultra-high resolution OCT correlates clinically with visual acuity in patients with RP.67 The inner retinal layers features in terms of morphology and thickness in RP patients have not been clearly elucidated, neither has the relationship between the outer and the inner retinal layers, especially in RP cohorts with reduced central visual acuity and central macular thinning.

In this study, we compare the volumes and the morphological alterations of the inner and outer retinal layers of the macula on spectral domain OCT in RP subjects with decreased central visual acuity and central macular thinning to healthy matched controls.

MATERIALS AND METHODS

The institutional review board approved this observational comparative cross-sectional study performed at a single center (AJEH) between August 2008 and January 2009. Written informed consent that met the tenets of the declaration of Helsinki was obtained from all subjects in the study prior to OCT imaging. Subjects with RP with reduced central visual acuity and central macular thinning (RP group) underwent measurement with spectral domain OCT and were compared to age-matched healthy control subjects (control group).

Inclusion criteria for the RP group were:

RP diagnosis based on the characteristic findings on ocular history, funduscopic appearance, visual field testing, fluorescein angiography (FA), and electroretinography (ERG) results.8

The best-corrected visual acuity (BCVA) was 20/200 or worse measured with the Snellen acuity chart.

Central foveal thickness (CFT) was less than 100 microns on spectral domain OCT HD cross-line protocol.

Subjects were at least 18 years old.

Inclusion criteria for the control group were:

The best-corrected visual acuity (BCVA) was 20/30 or better measured with the Snellen acuity chart.

Subjects were at least 18 years old.

Exclusion criteria for all eyes:

Prior history or current evidence of retinal pathology that the examiner felt would interfere with reliable macular volume measurements (includes CME, ERMs, macular holes etc.)

Prior history or current evidence of glaucoma or optic neuropathy.

Prior history of ocular trauma, intraocular surgery or laser therapy.

Refractive error greater than +6.00 D or less than –6.00 D.

Media opacity that precluded quality OCT image capture.

Prior to enrollment, all subjects underwent a complete ophthalmologic examination including BCVA testing, 24-2 Humphrey visual field testing, Goldmann applanation tonometry, slit lamp biomicroscopy, and indirect ophthalmoscopy. Snellen BCVA was converted to the logarithm of the minimum angle of resolution (log MAR) developed by Ferris et al., for statistical analysis.9

Using the E-MM5 raster protocol and HD cross-line protocol of the RTVue (Optovue Inc., California, USA) spectral domain OCT, a 5 × 5 mm cut of the central macula and vertical and horizontal line sections through the fovea were acquired for each eye through a dilated pupil. OCT images were converted to grayscale for better contrast of the individual layers. The reference line, pre-selected by the computer software, was used as the demarcation between the outer and inner retina on grayscale images. The ganglion cell complex (GCC) comprising the inner plexiform, ganglion cell, and nerve fiber layers was compared between groups for structural integrity using the HD cross-line protocol [Figure 1]. Retinal volume was measured by the area covered under the 5 × 5 mm cut.

Figure 1

Spectral domain optical coherence tomography of the macula in a healthy control showing the distinct retinal layers, outer macula and inner macula with their demarcation line. RPE CC: Retinal pigment epithelium chorio capillary complex, OS: Outer segment, IS: Inner segment, ELM: External limiting membrane, ONL: Outer nuclear layer, OPL: Outer plexiform layer, INL: Inner nuclear layer, IPL: Inner plexiform layer, GCL: Ganglion cell layer, NFL: Nerve fiber layer, ILM: Internal limiting membrane

The primary outcome measure was the RTVue inner and outer macular volumes of both groups. Statistical analysis was performed using computer software (StatXact-4, Cytel Software, Cambridge, MA). The means were compared using the unpaired T-test, and correlations were calculated with the Pearson correlation coefficient. All tests were two-tailed with a statistically significant P value less than 0.05.

RESULTS

Six eyes of six RP patients comprised the RP group and six eyes of six healthy patients comprised the control group. The mean age was 45 years in the RP group and 46.83 years in the control group (P<0.05). The mean BCVA was 1.15 ± 0.14 (log MAR ± SD) in the RP group and 0.0 in the control group.

The outer macular volume was 2.01 ± 0.44 (mm3 ± SD) in the RP group and 4.68 ± 0.16 (mm3 ± SD) in the control group. The inner macular volume was 2.46 ± 0.24 (mm3 ± SD) in the RP group and 2.55 ± 0.22 (mm3 ± SD) in the control group. The outer macular volume was significantly lower in the RP group when compared to controls (2.6, 2.17 - 2.99, P<0.05). There was no difference in the inner macular volume between groups (-0.09, -0.38 - 0.2, P>0.05) [Tables 1 and 2, Figure 2].

Table 1

Study eyes: Demographics, Best-corrected visual acuity, Macular Volumes

Table 2

Control eyes: Demographics, Best-corrected visual acuity, Macular Volumes

Figure 2

A 5 × 5 raster cut on the E-MM5 protocol showing the measurement of outer and inner macular volume in a case with advanced retinitis pigmentosa with central foveal thickness less than 100 μm

There was no significant correlation (r=-0.04, P>0.05) between BCVA and outer macular volume in the RP group. The layers of the GCC were distinct in both study and control groups. The inner segment/outer segment (IS/OS) junction and external limiting membrane (ELM) were disorganized in all RP patients when compared to controls.

DISCUSSION

Histopathologic studies demonstrate photoreceptor layer disruption/shortening in most cases of RP.1011 This finding has been further corroborated by in vivo spectral domain or ultra-high resolution OCT studies of the retina. Witkin et al.,7 noted significant photoreceptor layer shortening in RP cases compared to matched controls, while the same difference was not noted between their CFTs. Our study supports this observation, in that all eyes with RP had significantly reduced outer macular volumes compared to age-matched controls, while the same difference was not noted in the inner macular volumes. Additionally, inner retinal layer morphology at the macula was well-preserved. This preferential change of the outer retina with relative preservation of the inner retina, at least structurally, could help determine the level of intervention in the order of the neuronal pathways with regards to impulse generation and transmission by artificial means.

Aizawa et al.,6 categorized the inner segment/outer segment (IS/OS) junction on spectral domain OCT into three grades according to the clarity of the demarcation line. They found a significant correlation between IS/OS junction grade and BCVA as well as CFT, suggesting that BCVA and CFT was significantly better in RP eyes with distinct, highly reflective IS/OS junctions. Witkin et al.,7 noted excellent correlation between BCVA and photoreceptor layer shortening in RP eyes. Our study did not register a significant correlation between BCVA and outer macular volume. Perhaps a small sample size with selective inclusion of RP cases with reduced central visual acuity and central macular thinning could have accounted for the above. Nevertheless, a statistically significant difference in the outer macular volumes between the study and control groups was measured, and this was the primary aim of our study. Another weakness of this study besides the small number of cases involved is the fact that the variability of measuring the inner and outer retina in RP patients by the automated segmentation technique is not currently known.

In conclusion, the spectral domain OCT findings in this study indicate that RP is a disease preferentially affecting the outer retinal layers. The RP cases included in this study demonstrated relatively well-preserved volume and morphology of the inner retinal layers on spectral domain OCT.