CLINICAL SCIENCE

Optical ImageQuality andVisual Performance for PatientsWith Keratitis

Jose R. Jimenez, PhD, Carolina Ortiz, MD, Francisco Perez-Ocon, PhD, and Raimundo Jimenez, PhD

Purpose: To evaluate the optical image quality, using 2 optical

devices (an aberrometer and a double-pass device), and visual

performance [contrast sensitivity function (CSF)] in patients with

keratitis and after resolution of the disease.

Methods: Twenty subjects with keratitis (18 patients with unilateral

keratitis and 2 patients with bilateral keratitis) were included in this

study. We also took measurements in healthy fellow eyes for

comparison. Ten patients were studied before and after the resolution

of keratitis. Objective data were gathered by aberrometry (ocular

higher order, spherical, and coma aberrations) and the Strehl ratio,

a parameter derived from the double-pass device that included

information on diffraction, ocular aberrations, and scattering. The

CSF was also measured for all the patients.

Results: Ocular higher order, spherical, and coma aberrations in

keratitis eyes were significantly (P , 0.05) higher than those in

healthy fellow eyes. The Strehl ratio indicates a worse optical quality

for keratitis eyes being significantly (P , 0.05) lower than the Strehl

ratio for healthy fellow eyes. CSF for keratitis eyes was also

significantly lower than that for the healthy eyes (P, 0.05). Data for

keratitis eyes after the resolution of the disease showed a significant

increase in the optical function and visual performance but, after

treatment, continued to suffer significant (P , 0.05) impairment in

the optical quality of the affected eyes compared with the healthy

fellow eyes.

Conclusions: Devices to measure optical image quality and the

CSF enable the characterization of the optical quality and visual

performance in patients with keratitis, showing a significant decline

in optical quality and visual performance during keratitis and an

improvement after the resolution of the pathology.

Key Words: aberrations, contrast sensitivity, image quality, keratitis,

ocular scattering

(Cornea 2009;28:783–788)

Keratitis, the leading cause of infection-associated blind-ness in the western world, is caused by a variety of

pathogens, including viruses, bacteria, fungi, and amoeba.1–2

This pathology damages the cornea because of intraocularinflammation, epithelial defects, ulcers, and corneal scrapesand scars.3 These can deteriorate optical image quality andconsequently visual acuity.4

In recent years, a large number of different optical imagetechniques have been developed,5–9 providing useful infor-mation concerning the eye’s optical performance. The mostcommonly used device today is the aberrometer based on theHartmann–Shack sensor for measuring eye aberrations, but,more recently, other optical devices based on the double-passtechnique have been developed.5 The measurements from thedouble-pass technique give complete information on theretinal image (including ocular diffraction, aberrations, andscattering) that is not usually taken into account by mostaberrometric techniques. The use of these devices hasincreased in the last few years, although limited mostly toanalyzing surgical techniques of emmetropization. However,optical devices have neither been applied to the study ofoptical function in patients with keratitis nor used to trackchanges in optical image quality after the treatment.

The aim of this article was to measure the optical qualityand visual performance in patients with keratitis, comparingthese measurements with data from the healthy fellow eyes.Optical quality was also studied in a group of patients beforeand after resolution of the disease. For this, data from higherorder eye aberrations and the Strehl ratio5 (a parameter derivedfrom the double-pass device for the objective assessment ofthe optical function, including information on diffraction,aberrations, and ocular scattering) were taken. Concerningvisual performance, we measured the monocular contrastsensitivity function (CSF) for different spatial frequencies.CSF is an important function that characterizes the spatialvision of an observer. In this study, we gathered objective dataand measured the CSF of 20 patients, 18 of whom hadunilateral keratitis. In addition, we studied optical quality andCSF for 10 of these patients before and after the disease wasresolved.

METHODS

PatientsThis study included 20 keratitis patients (18 with

unilateral keratitis and 2 with bilateral keratitis) with a meanage of 37.7 6 13.7 years (range 14–62 years), of which 16

Received for publication June 27, 2008; revision received October 31, 2008;accepted November 23, 2008.

From the Departamento de Optica, Edificio Mecenas, Facultad de Ciencias,Universidad de Granada, Granada 18071, Spain.

Supported by Ministerio de Educacion y Ciencia (Spain) grant FIS 2006-01369 and Junta de Andalucıa (Spain) grant P06-FQM-01359.

Reprints: Jose Ramon Jimenez, PhD, Departamento de Optica, EdificioMecenas, Facultad de Ciencias, Universidad de Granada, Granada 18071,Spain (e-mail: jrjimene@ugr.es).

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patients had herpes keratitis and 4 patients had bacterialkeratitis. All diagnoses were made by the same ophthalmol-ogist at the University Clinic Hospital of Granada (Spain).Patients with any other ocular diseases except for keratitis wereexcluded from the study. Eight patients were emmetropes and12 corrected ammetropes. The average refraction equivalentwas20.56 1.5 diopters (D), and none of the ammetropes hadan astigmatism higher than 0.5D. In all patients, Snellen visualacuity was measured monocularly. In the case of ammetropes,visual acuity was measured with their correction. The averagevisual acuity for keratitis eyes was 20/25 and for healthy eyes20/20. Before the examination, informed consent was given byeach patient in accordance with the Declaration of Helsinki.

For the 18 patients with unilateral keratitis, data fromoptical devices and visual performance for the active keratitiseyes were compared with the 18 healthy fellow eyes. Inaddition, 12 eyes with herpes keratitis from 10 patients werecompared during active keratitis and after resolution ofinflammatory signs. Of these 10 patients, 8 initially hadunilateral keratitis, and these 8 eyes with resolved herpeskeratitis were also compared with their 8 healthy fellow eyes.Average visual acuity for keratitis eyes of these 10 patients was20/25 before treatment and 20/20 after resolution.

Data on optical quality and visual performance for thepatients with keratitis eyes were taken before beginningtreatment. In the cases of herpes keratitis, the treatment wasacyclovir and steroids in topical administration, whereas thetherapy for bacterial keratitis involved topical antibiotics,depending on the organism’s sensitivity.

Optical Image Quality DevicesData on optical quality were taken using a Wavefront

Aberration Supported Corneal Ablation (WASCA) aberrom-eter (Carl Zeiss Meditec AG, Oberkochen, Germany), basedon the Hartmann–Shack sensor (l = 780 nm), which providesinformation on eye aberrations. The use and functioning ofthis device have been widely published.5–8 Following the indi-cations of the user’s manual, we measured aberrations usingoptical fogging to achieve a nonaccommodative state of theeye. We computed higher order ocular aberrations—total rootmean square (from third- to sixth-order Zernike coefficients)and root mean square of spherical (square root of the sum ofthe squared coefficients of Z0

4 and Z06) and coma aberrations

(square root of sum of the squared coefficients of Z13, Z

�13 , Z1

5,and Z�1

5 ). The subject was seated in front of the instrument,and an internal fixation target was presented for viewing.

We also took data from a visual quality device, OpticalQuality Analysis System (Visiometrics SL, Tarrasa, Spain),based on the double-pass technique.5,9 In this device, a pointsource (an infrared laser diode, l = 780 nm) is projected ontothe retina. The reflected light passes through ocular media, andthe double-pass image is recorded by a camera. This deviceprovides data on diffraction, ocular aberrations, and scatteringthat affect the image clarity, reducing visual quality. Theseobjective optical devices are useful in clinical applications,such as refractive and cataract surgery, where the influence ofscattering could be great. The device includes a motorizedoptometer to correct the patient’s defocus.9 No trial frame lens

was introduced during the measurements to correct cylinderbecause it was ,0.5 D for all patients.

For a quantitative measure of the visual quality, we tookthe Strehl ratio, a parameter commonly used for estimatingoverall optical quality,5 defined as the ratio between themodulation transfer function (MTF) area of the eye and thediffraction-limited MTF area. The MTF represents the loss ofcontrast produced by the eye’s optics on a sinusoidal grating asa function of its spatial frequency.5 The Strehl ratio rangesfrom 0 to 1. A lower value of this parameter indicates that thereis a greater contribution of the aberrations and ocular scat-tering and therefore poorer optical quality. More informationon these devices can be found in other articles.5,9

The data from aberrometry and double-pass techniquewere referred to a pupil size of 4 and 5 mm for all patients. Forboth devices, data were taken without pupil dilation. For all themeasurements taken with the aberrometer and Optical QualityAnalysis System devices, we took at least 3 measurements andaveraged them.

Contrast Sensitivity FunctionFor visual performance, the CSF was determined. This

test was conducted in a calibrated computer-controlled moni–tor with the software package Vision Works by Vision ResearchGraphics (Durham, NH). The monitor was calibrated witha PR-704 spectroradiometer (Photo Research, Chatsworth,Texas) using a 32-bit look-up table and optimizing brightnessand contrast controls.10–11 The spatial frequencies tested were1.5, 3.0, 5.9, 9.9, 14.8, 18.5, and 21.2 cycles per degree. Theaverage luminance level of the CSF test was 7 cd/m2, a valuewithin the mesopic range to guarantee a value of the pupildiameter larger than 5 mm, the maximum pupil size for whichwe refer aberrations and Strehl ratio measurements. Thepupil diameter was measured using a Colvard pupillometer(OASIS, Glendora, CA) to ensure that all patients during CSFmeasurements reached a pupil diameter greater than 5 mm.The test distance was 6 m, corresponding to a visual angle of1.3 degrees. A chin rest fixed the head position of the observer.

For each spatial frequency, the contrast threshold wasdetermined with an up–down staircase procedure with 6reversals beginning with the lowest contrast available. Thecontrast threshold was defined by the average of the last 4reversals. Spatial frequencies were tested in a random order.The CSF was computed as the reciprocal of the contrastthreshold.

At the beginning of each session, a 2-minute darkadaptation period was necessary to adapt the observer to theluminance of the stimuli. For monocular measurements, oneeye was occluded, whereas the other eye was being measured.CSF data were taken for ammetropes with their bestcorrection. Before the data were recorded, each observerunderwent several training sessions to ensure the correctunderstanding of the test.

Quantitative variables were compared using a bifactorial2 3 4 analysis of variance, with type of eye (keratitis orhealthy fellow) and optical parameters [Strehl ratio, higherorder aberration (HOA), and spherical and coma aberrations]as the main factors. In addition to the bifactorial analysis, anindividual comparison was made between the keratitis and

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Jimenez et al Cornea � Volume 28, Number 7, August 2009

their healthy fellow eyes before and after resolution of thedisease. For this, we made a post hoc comparison (the Scheffemethod, with a minimum level of significance of 0.05), whichenabled us to compare the individual measurements for all theoptical parameters considered.

RESULTS

Keratitis Eyes Before Medical Treatment andHealthy Fellow Eyes

The average data of optical parameters for keratitis andhealthy fellow eyes are presented in Table 1. For the 2 pupilsizes studied (4 and 5 mm), the average values show worseoptical quality results for keratitis eyes. According toa bifactorial analysis of variance, there were significantdifferences (P , 0.05) for 4 and 5 mm in the main factor typeof eye (keratitis or healthy eyes). For 5 mm, the Strehl ratio waslower for keratitis eyes in all patients, a post hoc comparisonrevealed a significant decrease (P, 0.05) in the Strehl ratio foreyes with keratitis in 15 (83.3%) patients. The data from thedouble-pass device indicated an impairment of the opticalquality in eyes affected with this pathology. The same trendwas found for 4 mm, the Strehl ratio being significantly (P ,0.05) lower for keratitis eyes for the same 15 patients. As anexample, Figure 1 shows the double-pass image for the healthyand keratitis (herpes) eyes of a patient for 5 mm withsignificant differences in the Strehl ratio. The effect of keratitisis visible in the double-pass image showing a worse opticalquality for the keratitis eye.

Concerning aberrometry, a post hoc comparisonrevealed that aberrations for 5 mm increased for all keratitiseyes, this being significant (P , 0.05) for 13 (72.2%), 16(88.8%), and 16 (88.8%) patients, for HOA and spherical andcoma aberrations, respectively. For 4 mm, these percentageswere 15 (83.3%), 15 (83.3%), and 13 (72.2%), respectively.

Figure 2 shows the results for the monocular CSFstudied for the average of the healthy and keratitis eyes for the18 patients with unilateral keratitis. This figure clearly reflectsthe deterioration in visual performance (lower contrastsensitivity) in the eyes affected by keratitis. A bifactorialanalysis shows that this deterioration for keratitis eyes wassignificant (P, 0.05). For each patient, a post hoc analysis forcomparing values found for each eye of the patients indicatedthat for the 18 patients with unilateral keratitis the decline inthe CSF of the affected eye with respect to the healthy eye wassignificant at least (P , 0.05) for 5 spatial frequencies (1.5,9.9, 14.8, 18.5, and 21.2 cycles per degree).

Also, we analyzed the CSF as a function of the opticalparameters studied (Strehl ratio, HOA, and spherical and comaaberrations) to identify any correlation between visualperformance and these 4 optical parameters. A significantcorrelation (P , 0.05) for keratitis eyes was found for 5 mm,with correlation coefficients of r = 0.50 and r = 0.52 for theCSF as a function of HOA and the Strehl ratio and r = 0.17 andr = 0.20 for the CSF as a function of spherical and comaaberrations, respectively. A significant correlation (P , 0.05)between CSF and optical parameters was also found forkeratitis eyes (4 mm) and healthy fellow eyes (4 and 5 mm).

TABLE 1. Average Data (mean and SD) for 18 Keratitis and Healthy Fellow Eyes Corresponding to Optical Parameters: Strehl Ratioand Root Mean Square of Total HOA and Spherical and Coma Aberrations

Strehl Ratio HOA (mm) Spherical (mm) Coma (mm) P

5 mm Keratitis 0.16 6 0.05 0.26 6 0.09 0.12 6 0.04 0.15 6 0.03 ,0.01

Healthy 0.23 6 0.04 0.12 6 0.05 0.07 6 0.04 0.10 6 0.03

4 mm Keratitis 0.17 6 0.04 0.21 6 0.07 0.06 6 0.01 0.10 6 0.03 ,0.003

Healthy 0.24 6 0.05 0.14 6 0.05 0.03 6 0.01 0.05 6 0.01

The P value corresponding to the bifactorial analysis for the main factor type of eye (keratitis or healthy fellow) has been included.

FIGURE 1.Double-pass image (5mm)for the healthy fellow (left image)and keratitis (right image) eyes ofa patient.

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Keratitis Eyes Before Medical Treatment andAfter Resolution of the Disease

Table 2 presents the average data for 4 and 5 mm for 12eyes corresponding to 10 patients (8 patients with unilateralkeratitis and 2 patients with bilateral keratitis) affected withkeratitis and after resolution of the disease. We can see theimprovement of optical parameters after resolution. Accordingto a bifactorial analysis of variance (main factor, type of eye:keratitis eye and eye after resolution), significant differencesappeared (P , 0.05). A post hoc comparison showed theStrehl ratio after resolution to be significantly (P , 0.05)higher for 9 of the 12 eyes (75% of the patients) for 4 and5 mm. Figure 3 shows the double-pass image for 5 mm fora keratitis eye and for the same eye after resolution withsignificant differences in the Strehl ratio. After resolution ofthe disease, better optical quality for the keratitis eye wasfound, and, according to aberrometry data, aberrationsdeclined for 4 and 5 mm. In post hoc comparisons, thisproved significant for 100% patients, except for comaaberration for 5 mm (this percentage being 90.9%).

A bifactorial analysis of variance (main factor, type ofeye: keratitis and after resolution) showed significant differ-ences (P , 0.05) and an improvement of the CSF afterresolution of the disease. A post hoc comparison for spatialfrequencies showed a significant (P , 0.05) improvement for

5 spatial frequencies for all patients (1.5, 9.9, 14.8, 18.5, and21.2 cycles per degree), the CSF being higher for these spatialfrequencies after resolution.

Keratitis Eyes After Resolution of Disease andHealthy Fellow Eyes

For the 8 patients with unilateral keratitis, eyes afterresolution of the disease were compared with healthy felloweyes to determine whether normal values of the healthy eyeswere reached by the eyes formerly affected by keratitis.Bifactorial (main factor: eyes after resolution and healthyfellow eyes) analysis of variance from data from the double-pass device and aberrometry showed significant differences(P , 0.05) for different pupil diameters (4 and 5 mm).Greater quantities of aberrations and a lower Strehl ratiovalues were found for keratitis eyes after resolution than forhealthy fellow eyes (Table 3). A post hoc analysis shows thatthe Strehl ratio was significantly (P , 0.05) lower afterresolution for 7 patients.

The data for the monocular CSF for the average of these8 patients (Fig. 4) reveal a lower CSF for keratitis eyes afterresolution of the disease. A bifactorial analysis of variance(main factor, type of eye: keratitis after resolution and healthyfellow) shows that this deterioration was significant (P ,0.05). A post hoc analysis indicated that all the patients studiedhad deteriorated CSF for the keratitis eyes after resolution withrespect to healthy eyes (P , 0.05) in at least 4 spatialfrequencies.

DISCUSSIONThe results using optical devices show a more deteri-

orated optical quality for patients suffering keratitis (Table 1and Fig. 1). Both in the case of aberrations and with thedouble-pass device, the data from these objective devicesshowed worse optical quality in the eye with keratitis than inthe healthy fellow eye, the latter registering better values bothin optical quality and in CSF (Fig. 2). However, with regard tovisual acuity, Snellen visual acuity was 1–4 lines lower(20/50–20/25) for the affected eye in 10 patients (56%), thevisual acuity being equal in both eyes (6/6) for the other8 patients (44%). This indicates higher accuracy in CSF forevaluating visual performance, although it should be indicatedthat the Snellen visual acuity test, in widespread use, hasshown less sensitivity than other visual acuity tests12 in somesituations, and the use of other visual acuity tests could haveprovided different results for visual acuity. The greater

FIGURE 2. Average CSF corresponding to the healthy fellowand keratitis eyes of the 18 patients with unilateral keratitis.Data include standard error.

TABLE 2. Average Data (mean and SD) for 12 Keratitis Eyes (10 Patients) and the Same Eyes After Resolution Corresponding toOptical Parameters: Strehl Ratio and Root Mean Square of Total HOA and Spherical and Coma Aberrations

Strehl Ratio HOA (mm) Spherical (mm) Coma (mm) P

5 mm Keratitis 0.15 6 0.04 0.30 6 0.12 0.13 6 0.06 0.17 6 0.08 ,0.003

After resolution 0.19 6 0.04 0.18 6 0.10 0.09 6 0.04 0.13 6 0.05

4 mm Keratitis 0.16 6 0.04 0.22 6 0.09 0.07 6 0.02 0.11 6 0.04 ,0.02

After resolution 0.21 6 0.04 0.18 6 0.09 0.04 6 0.02 0.08 6 0.04

The P value corresponding to the bifactorial analysis for the main factor type of eye (keratitis or after resolution) has been included.

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Jimenez et al Cornea � Volume 28, Number 7, August 2009

reduction in the CSF at high frequencies for patients withkeratitis could be expected because, when an opticaldeterioration due to aberrations or ocular scattering, the highfrequencies of the MTF (and therefore CSF) are the mostaffected frequencies.

Results for optical quality and CSF were confirmed ina second examination of 12 eyes of 10 patients (8 patients withunilateral keratitis and 2 patients with bilateral keratitis) afterdisease resolution (Fig. 4 and Table 2). For these patients, thedata revealed significant differences (P , 0.05) betweenresults for the keratitis eyes with respect to values for the sameeyes after recovery, reflecting improved visual performanceand optical quality. However, when comparing data for these8 patients after resolution with healthy fellow eyes, we foundbetter optical quality for the healthy fellow eyes.

The optical deterioration during keratitis could bebecause of the fact that, during the inflammatory process,changes occurring in the cornea could have affected its opticalproperties, such as variations in the trajectory of the lightreaching the cornea (eg, because of changes in cornealthickness) and/or the refraction index in different zones of thecornea affected by keratitis. These changes in optical propertiesmodify both the aberrations and the ocular scattering.

An extensive analysis on the influence that the type ofkeratitis (or other corneal or clinical variables) after resolutionexerts on optical quality and visual performance would needfurther studies with more patients of each type of keratitis orclinical variable.

It bears mentioning that the aberrometer and the doublepass showed the same trends in the results for opticaldeterioration in the keratitis. The only difference was thatin the correlation between the CSF and the optical variables,the values of the correlation coefficient for 2 of the aberra-tions studied (spherical and coma) were lower than those forthe Strehl ratio and HOA, but it should be pointed out thatthe study of the metrics to characterize visual quality andperformance is an open area and the results differ accordingto the metric used.13 Our results illustrate this, given thatthe correlation varied according to the optical parameterused, although the degree of correlation found was not high.

In any case, a full explanation of these results is notpossible because the optical parameters were not measuredbefore the development of keratitis and, therefore, prekeratitisdata cannot be compared with those during the disease andafter its resolution. Consequently, it was possible to comparethe results for the healthy eye only with those for the fellow eyeduring and after keratitis, taking into account that thiscomparison may be influenced by the effect of interoculardifferences. In the case of the aberrations, it is known thatinterocular differences in aberrations appear at least in healthyeyes.14,15 Patients in the study on interocular differences withboth healthy eyes15 show interocular differences in aberrationsalthough interocular differences in monocular CSF are notsignificant. However, in the present study, significant differ-ences appeared between monocular CSF corresponding to thehealthy fellow eyes and those after resolution. Therefore, it

FIGURE 3.Double-pass image (5 mm)for a keratitis eye (left image) andthe same eye after resolution(right image).

TABLE 3. Average Data (mean and SD) for 8 Keratitis Eyes After Resolution and the Healthy Fellow Eyes Corresponding to OpticalParameters: Strehl Ratio and Root Mean Square of Total HOA and Spherical and Coma Aberrations

Strehl Ratio HOA (mm) Spherical (mm) Coma (mm) P

5 mm After resolution 0.18 6 0.04 0.21 6 0.14 0.10 6 0.05 0.13 6 0.11 ,0.002

Healthy fellow 0.22 6 0.03 0.14 6 0.08 0.08 6 0.06 0.09 6 0.06

4 mm After resolution 0.19 6 0.05 0.21 6 0.07 0.05 6 0.03 0.09 6 0.06 ,0.05

Healthy fellow 0.25 6 0.07 0.15 6 0.04 0.03 6 0.02 0.05 6 0.02

The P value corresponding to the bifactorial analysis for the main factor type of eye (after resolution or healthy fellow) has been included.

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cannot be dismissed that after the resolution of inflammatorysigns and even with visual acuity reaching prekeratitis values,some optical deterioration persists in the cornea, influencingvisual performance (CSF).

In summary, the results of the present work show thatoptical image–quality devices can provide relevant informa-tion on an important visual pathology such as keratitis,extending the use of such devices beyond emmetropizationtechniques such as refractive and cataract surgery. The resultsfor aberrometry and the double-pass device show thedeterioration of optical quality for keratitis eyes in comparisonwith healthy fellow eyes, with a deterioration in visualperformance also being found, although we should add thatoptical image tests consume far less time than do psycho-physical tests and do not require intensive cooperation fromthe patients, which can be especially difficult at an advancedage. The experimental data for patients after resolution ofthe disease pose the possibility that optical quality (aberra-tions and scattering) and the CSF remain deterioratedalthough visual acuity reaches values equivalent to those ofthe healthy eye.

ACKNOWLEDGMENTSThe authors wish to thank David Nesbitt for translating

the text into English.

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FIGURE 4. Average CSF corresponding to the healthy fellowand keratitis eyes after resolution of the disease for 8 patientswith unilateral keratitis. Data include standard error.

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