Introduction
Astigmatism is a worldwide common ocular disorder. Total astigmatism is mainly driven by corneal astigmatism(CA), which occurs due to an irregular shape of the cornea. In eyes with astigmatism, retinal images can be distorted. Langenbucher et al.
Optical coherence tomography (OCT) can provide imaging of ocular structures by a noninvasive method, It is widely used in clinical and scientific ophthalmology to obtain high-resolution cross-sections of the retina images. The thickness of the retinal nerve fiber layer (RNFL) and optic nerve head (ONH) parameters can be measured by OCT. Evaluation of these parameters is essential, since the thickness of the RNFL may be effected in various diseases. For example, the RNFL becomes thinner in glaucoma and optic atrophy, whereas it is thicker in papilledema.
Many studies have reported the effect of refractive error changes induced by refractive surgery or contact lenses on RNFL thickness measured by OCT
Material and methods
Subjects
70 eyes of 35 consecutive refractive surgery candidates with spherical equivalent ≥ −6 diopters (D) were recruited for the study. Ethical approval for the study was obtained from the local medical ethics committee. All subjects were volunteers and informed consents were obtained.
Each subject underwent a full ophthalmic examination, which included measures of visual acuity, refraction, intraocular pressure (IOP) by a noncontact tonometer. Axial length measurements were obtained in each eye with the IOL Master (Carl Zeiss Meditec, Inc, Dublin, CA), CA measurements were obtained by a Topolyzer (Allegretto Wave Topolyzer, Germany), optic nerve head evaluation was performed with a 90-D lens, and peripapillary RNFL thickness and ONH parameters were measured with the Cirrus HD OCT (Cirrus HD OCT; Carl Zeiss Meditec, Dublin, CA). The peak locations of the superotemporal and inferotemporal areas were evaluated by the RNFL TSNIT curve of the Cirrus HD OCT. The peak locations, which were measured by the RNFL TSNIT curve, were translated to units of degrees by multiplying 360/256. For example, the superior peak location of 40 in the TSNIT curve was translated to 56.25 degrees (40 × 360/256 degrees). This means that the thickest superior RNFL was located at the point 56.25 degrees away from the temporal horizontal meridian. We defined α angle as the angle between the horizontal meridian and superotemporal peak location by clockwise rotation, and the angle between the horizontal meridian and inferotemporal peak locations by counterclockwise rotation were defined as β angle (Figure
An example of a measurement of the retinal nerve fiber layer characteristics in an right eye with a-6.625D of spherical equivalent, a -1.6 of CA and a 26.42mm of axial length: (A) Fundus photograph of the optic disc
An example of a measurement of the retinal nerve fiber layer characteristics in an right eye with a-6.625D of spherical equivalent, a -1.6 of CA and a 26.42mm of axial length: (a) Fundus photograph of the optic disc. Dotted line represents imaginary horizontal meridian; (b) the peak locations at the superior and inferior area were 40 and 212, respectively; (c) the peak locations were translated to units of degrees by multiplying 360/256. Angles between the horizontal meridian and the superotemporal / inferotemporal peak locations were defined as the α (superotemporal) and β (inferotemporal) angles, so RNFL peak locations of this eye were α = 40 × 360/256 = 56.25(degree), β = 360-212 × 360/256 = 61.88(degree).
The individuals were included if they had the following: best corrected visual acuity of 20/20 or better, an intraocular pressure (IOP) lower than 21 mmHg in either eye, CA as a with-the-rule (WTR) astigmatism, a healthy ONH without glaucomatous damage (i.e., no disc haemorrhage, notching or thinning of the neural rim).
Those with a history of severe ocular trauma, intraocular or refractive surgery or any ocular or neurological disease that could have affected the ONH or RNFL were excluded from the study. Subjects with evidence of macular disease or peripapillary atrophy extending more than 1.73 mm from the center of the optic disc or with glaucoma or an IOP higher than 21 mmHg in either eye were also excluded. In addition, participants with a history of systemic diseases including hypertension and diabetes were excluded.
We assigned astigmatic types as defined in Katz and Kruger
Astigmatism
CA was measured with Topolyzer (Allegretto Wave Topolyzer, Germany) and the total astigmatism was measured by a refractor keratometer (Topcon KR-8800 Auto Refractor).
OCT imaging
After pupillary dilation to a minimum diameter of 5 mm, the eyes of the subjects that satisfied the study criteria were scanned using the Cirrus HD-OCT system with software version 5.0. All the scans had signal strength of at least 6 and all measurements were taken by a single, well-trained examiner. The superior clock hour was 12 o’clock and the others were assigned accordingly in a clockwise manner in the right eye and counterclockwise in the left eye.
Statistical analysis
Statistical analyses were performed with commercially available software (SPSS ver. 17.0; SPSS Inc, Chicago, IL). The total average and mean clock hour RNFL measurements were compared between the two groups with an independent
Results
Seventy high myopic eyes of 35 subjects were analyzed. The mean age was 26.42 ± 6.95 years (range 18 to 39 years). The mean spherical equivalent, axial length, CA, and total astigmatism were −8.08 ± 1.77 D (range-6.00 to-15.00D), 26.86 ± 1.04 mm (range 24.24 to 29.84 mm), -1.17 ± 0.64D (range −0.20 to −3.30D), and −0.83 ± 0.65D (range 0.00 to −2.50 D), respectively.
Of the 70 subjects, 36 eyes were classified as high astigmatism (≤ − 1 D of CA; mean-1.65 ± 0.49D), and 34 eyes were classified as normal astigmatism (> − 1D of CA; mean-0.67 ± 0.28D). Characteristics of the two groups are listed in Table
Table
RNFL profiles of normal astigmatism group (n = 34) and high CA group (n = 36)
RNFL profiles of normal astigmatism group (n = 34) and high CA group (n = 36). Significant differences were found at 2 o’clock, 9 o’clock, and 10 o’clock, * indicates
Correlation analysis demonstrated significant correlations between CA and the nasal RNFL thickness, temporal RNFL thickness, α angle and β angle. The correlation coefficients for normal astigmatism were
Discussion
The optical coherence tomographer is a modern imaging device designed to measure the RNFL and ONH parameters in a noncontact and noninvasive manner. RNFL measurements have been reliable and reproducible, and newer versions of optical coherence tomographers based on spectral domain technology that provide higher resolution and faster scanning speeds have been developed
It has been reported that many factors, including refractive error, axial length, myopic optic disc tilt, eccentric scan location, and head tilt during the examination can affect the OCT measurements
Our study showed that CA influenced spectral-domain OCT measurements of both RNFL thickness and ONH parameters. Eyes with higher CA had a larger disc area and rim area, thinner temporal RNFL thickness and farther temporally positioned superotemporal and inferotemporal peak locations of RNFL thickness. The high CA group had significantly thinner RNFL thickness than the normal astigmatism group in the 2 o’clock, 9 o’clock and 10 o’clock sectors (Figure
Our results showed an intriguing finding that had not been reported previously. To date, the mechanism for changes in RNFL thickness and ONH parameters induced by astigmatism is not clear, however, possible explanations are as the followings: In high myopes, the optic disc is usually inserts obliquely. Once the optic disc tilts temporally, the nasal half of the optic disc elevates anteriorly, and the temporal half of the optic disc depresses posteriorly
These findings are ascribable to CA induced ocular magnification. The relationship between the measurement of the OCT image and the size of the actual fundus dimension can be expressed as
In this study, the sample size may be inadequate to reveal a statistically significant correlation between total astigmatism and the temporal / nasal quadrant average RNFL thickness. Further studies are needed to clarify this point.
In conclusion, we found that high corneal astigmatism with the rule influences the measurements of both RNFL thickness and ONH parameters by the Cirrus HD OCT. Eyes with higher corneal astigmatism had a larger disc area and rim area, thinner temporal RNFL thickness and farther temporally positioned superotemporal and inferotemporal peak locations of the RNFL in high myopes. Therefore, the degree of corneal astigmatism with the rule influences should be considered when interpreting the ONH parameters and peripapillary RNFL thickness measured by the Cirrus HD OCT in high myopes.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
LL participated in the study design, reviewed the literature, collected the clinical data, and drafted the manuscript. JZ provided the conception and design of the study and reviewing the manuscript. HH collected the clinical data and selected the material. J-gY took part in the study design and performed the statistical analysis. S-rC participated in collected the clinical data. All authors have read and approved the manuscript.