White-to-white corneal diameter, pupil diameter, central corneal thickness and thinnest corneal thickness values of emmetropic subjects

Download White-to-white corneal diameter, pupil diameter, central corneal thickness and thinnest corneal thickness values of emmetropic subjects

Post on 10-Jul-2016

212 views

Category:

Documents

0 download

TRANSCRIPT

  • ORIGINAL ARTICLE

    White-to-white corneal diameter, pupil diameter, central cornealthickness and thinnest corneal thickness values of emmetropicsubjects

    Juan A. Sanchis-Gimeno Daniel Sanchez-Zuriaga

    Francisco Martinez-Soriano

    Received: 29 November 2010 / Accepted: 11 October 2011 / Published online: 22 October 2011

    Springer-Verlag 2011

    Abstract

    Purpose This report assesses white-to-white corneal

    diameter, pupil diameter, central corneal thickness and

    thinnest corneal thickness values in a large sample of

    emmetropic subjects.

    Methods Three hundred and seventy-nine eyes of 379

    young healthy emmetropic subjects were analyzed by means

    of scanning-slit corneal topography. The age of the subjects

    ranged from 18 to 53 years (mean SD = 29 7). The

    mean of five consecutive measurements of the central

    corneal thickness, the thinnest corneal thickness, the white-

    to-white corneal diameter, and the photopic pupil diameter

    was recorded.

    Results The central corneal thickness ranged from 528 to

    588 lm; the thinnest corneal thickness ranged from 504 to574 lm; the white-to-white corneal diameter ranged from11.5 to 12.3 mm; and the pupil diameter ranged from 3.0 to

    4.7 mm. The central and the thinnest corneal thickness

    were positively correlated (r = 0.94, p \ 0.001), and thepupil diameter was significantly higher in females

    (p \ 0.001).Conclusions This study shows that there are no differ-

    ences in white-to-white corneal diameter, central corneal

    thickness, and thinnest corneal thickness between emme-

    tropic females and males. However, pupil diameters are

    greater in emmetropic females.

    Keywords Corneal thickness White-to-white diameter Pupil diameter Scanning-slit corneal topography Ocular surface

    Introduction

    An anatomists definition of the normal eye is different

    from an ophthalmologists. From an ophthalmologists

    point of view the normal eye is the non-pathological eye,

    not the emmetropic eye as accepted by anatomists [21].

    Assessment of ocular dimensions is essential for oph-

    thalmic surgeons, because it must be measured before

    scheduling excimer laser refractive surgery and cataract

    surgery [8, 16]. Cataract is the worlds leading cause of

    blindness [28], and excimer laser refractive surgery has

    been reported to be performed on more than six million

    people worldwide [10]. This is why studies of ocular

    dimensions are usually carried out in these patients and not

    in emmetropic subjects.

    The analysis of distribution of refractive errors has

    detected that emmetropia may be more prevalent than

    myopia and hyperopia in European populations [14]

    although different studies carried out in Asian populations

    have found that hyperopia and myopia are more prevalent

    than emmetropia [13, 27].

    Previous studies analyzed the ocular axial length values

    and corneal thickness values of the emmetropic and non-

    emmetropic eye [19, 20, 22, 23], but based on a biblio-

    graphic search using MEDLINE, we have found no study

    dedicated exclusively to the white-to-white corneal diam-

    eter and pupil diameter in healthy emmetropic eyes (i.e.,

    those subjects with spherical equivalent refraction

    of 0.5 diopters). Thus, currently, there is a lack of

    information on the quantitative ocular anatomy of

    J. A. Sanchis-Gimeno (&) D. Sanchez-Zuriaga F. Martinez-Soriano

    Department of Anatomy and Human Embryology,

    Faculty of Medicine, University of Valencia,

    Av. Blasco Ibanez, 15, Valencia 46010, Spain

    e-mail: juan.sanchis@uv.es

    123

    Surg Radiol Anat (2012) 34:167170

    DOI 10.1007/s00276-011-0889-4

  • emmetropic eyes, because ocular size studies are not usu-

    ally performed on these eyes. Moreover, all these subjects

    are candidates for developing cataracts in the future so it is

    important to know the quantitative ocular anatomy of the

    emmetropic eye.

    In the light of the above, the present paper analyzes the

    white-to-white corneal diameter, pupil diameter, central

    corneal thickness, and thinnest corneal thickness values of

    a large sample of healthy emmetropic subjects.

    Methods

    We carried out a prospective study involving 379 eyes of

    379 healthy emmetropic subjects. Inclusion criteria were

    healthy emmetropic subjects (volunteers with manifest

    sphere and manifest cylinder of 0.5 diopters) with best

    corrected visual acuity C20/20.

    Measurements of the white-to-white, pupil diameter,

    central corneal thickness, and thinnest corneal thickness

    were carried out by means of scanning-slit corneal topog-

    raphy (SSCT), as described previously [4, 19, 22], with the

    Orbscan topography system II (Orbscan, Inc., Salt Lake

    City, UT). SSCT was used on all patients, with an acoustic

    equivalent factor of 0.92 as recommended by the manu-

    facturer. All the procedures were conducted in accordance

    with the principles of the World Medical Associations

    Declaration of Helsinki. Detailed consent forms were

    obtained from each of the patients.

    Scanning-slit corneal topography makes it possible to

    determine over 9,000 data points in 1.5 s, mapping the

    entire corneal surface without touching it. With SSCT two

    scanning slit-lamps project beams at 458 to the right orleft of the instrument axis. Forty images20 with slit

    beams projected from the left and 20 from the rightare

    obtained at two intervals, each lasting 0.7 s. Surface data

    points are measured on the x, y, and z axes. It creates true

    3-D maps from the anterior segment of the eye using

    measurements based on the Scheimpflug principle [17].

    SSCT measures anterior and posterior corneal elevation

    (relative to a best-fit sphere), surface curvature, pupil

    diameter, white-to-white diameter, and corneal thickness

    values using a scanning-slit mechanism. The corneal

    thickness is calculated by measuring the distance in ele-

    vation between the anterior and posterior surfaces of the

    cornea [17]. During examination, the patients chin is

    positioned on the chin rest and the forehead against the

    forehead strap. The volunteers are asked to look at a

    blinking red light coaxial to the imaging system while the

    tracking system measures involuntary eye movements

    during the examination. The images of the cornea are

    taken using a placido disc, and are shown on the screen of

    the instrument.

    The mean of five consecutive SSCT measurements was

    obtained for each parameter. All ocular measurements were

    carried out from 10 a.m. to 1 p.m. During examination the

    temperature ranged from 18 to 228C, and the relativehumidity ranged from 38 to 45%. All the measurements

    were performed in a well-lit room, this is to say, under

    photopic conditions.

    Only one eye per subject was contemplated for the

    statistical analysis. The eye analyzed was chosen at ran-

    dom. The KolmogorovSmirnov test, Students t-test and

    Pearsons correlation coefficients were applied. P values

    less than 0.05 were considered to be statistically

    significant.

    Results

    The mean age of the subjects analyzed was 29 7 (range

    1853 years). One hundred eighty-one subjects were

    female (mean age 29 7 years, range 1849 years) and

    198 male (mean age 30 7 years, range 1853 years).

    There were no significant differences in the mean age

    between females and males (p = 0.312; Students t-test).

    Table 1 presents the white-to-white corneal diameter

    and pupil diameter values. Pupil diameter was significantly

    higher in female than in male subjects. Table 2 shows the

    central corneal thickness and thinnest corneal thickness

    values. The central corneal thickness values were signifi-

    cantly thicker than the thinnest corneal thickness values.

    Both measurements were positively correlated (r = 0.94,

    p \ 0.001).

    Table 1 White-to-white and pupil diameters values obtained in the sample of emmetropic subjects analyzed

    White-to-white corneal diameter (mm) Pupil diameter (mm)

    All Male Female All Male Female

    n 379 198 181 379 198 181

    Mean SD 11.9 0.2 11.9 0.2 11.8 0.2 3.6 0.4 3.5 0.3* 3.8 0.5*

    Minimum 11.5 11.6 11.5 3.0 3.1 3.0

    Maximum 12.3 12.2 12.3 4.7 4.5 4.7

    * Statistically significant differences between female and male (p \ 0.001; Students t-test)

    168 Surg Radiol Anat (2012) 34:167170

    123

  • No correlation was found between age and white-to-

    white corneal diameter (r = -0.06, p = 0.230), pupil

    diameter (r = 0.07, p = 0.171), central corneal thickness

    (r = 0.01, p = 0.801), or the thinnest corneal thickness

    (r = 0.01, p = 0.791).

    Discussion

    To our knowledge, this is the first study that analyzes the

    pupil diameter, the white-to-white corneal diameter, the

    central corneal thickness and the thinnest corneal thickness

    in a large sample of healthy emmetropic subjects.

    We determined the refraction by means of manifest

    refraction. It is known that manifest refraction can present

    a bias when carried out in young subjects. In fact, popu-

    lation studies [15] are carried out under cycloplegia in

    subjects aged B15 years while manifest refraction is used

    in subjects aged C15 years in order to avoid this bias. As

    all the subjects analyzed in our study were C18 years old

    we used manifest refraction.

    We used SSCT, because it makes it possible to measure

    the pupil diameter, the white-to-white corneal diameter, the

    central corneal thickness, and the thinnest corneal thickness

    measurements at the same time.

    We carried out SSCT (Orbscan pachymetry) with the

    Obscan II using the acoustic equivalent factor of 0.92 as

    recommended by the manufacturer. Nevertheless, the

    accuracy of the Orbscan when compared with the gold

    standard ultrasound pachymetry remains unclear. Cairns

    and McGhee [2] in a review study that analyzed 91 relevant

    peer-reviewed publications that used the Orbscan system

    observed that Orbscan measurements of central and

    peripheral pachymetry were determined to be 15 and

    95 lm greater thant that of ultrasound pachymetry. In orderto minimize the differences between Orbscan and ultra-

    sound measurements, the manufacturers of the Orbscan

    devices introduced the acoustic equivalent factor of 0.92 to

    transform Orbscan readings into their ultrasound equiva-

    lents. Theoretically, the differences between Orbscan and

    ultrasound pachymetry may disappear when the Orbscan

    measurements are carried out using the acoustic equivalent

    factor of 0.92. In fact, Suzuki and co-authors [24] obtained

    corneal thickness values of 548.1 33 lm with ultrasoundpachymetry and 546.9 35.4 lm with the Orbscan systemusing the acoustic equivalent factor of 0.92. Nevertheless,

    further studies indicated that the equivalent factor of 0.92

    was not sufficient to equal Orbscan and ultrasound readings

    [5], and that Orbscan tends to overestimate pachymetry

    readings in thicker corneas, and underestimate them in

    thinner corneas [11].

    The subjects analyzed had special characteristics as they

    were emmetropic and healthy. These characteristics made

    them ideal for anatomic studies. Moreover, the proportion

    of emmetropic subjects analyzed in large sample studies is

    only approximately 2% of the sample [4]. In fact, in most

    studies the results from emmetropic and non-emmetropic

    subjects are pooled without any separate analysis [4, 9, 26].

    Nevertheless, it seems that emmetropic subjects had the

    lowest corneal thickness values when compared to myopic

    and hyperopic patients [4]. In addition, Chaidaroon and

    Juwattanasomran [3] found that the mean scotopic pupil

    diameter in myopes was larger than that in emmetropes.

    We found that pupil diameter was significantly higher in

    women, which agrees with classical studies [1], and that

    there is no correlation between age and pupil diameter.

    Several studies have found not only an inverse correlation

    between both parameters, but also no differences in pupil

    diameter between genders [3, 4, 25, 26]. Nevertheless, in

    such studies Pearson r coefficients were very low (ranging

    from -0.3 to -0.5), and emmetropic and non-emmetropic

    subjects were pooled for their analysis of the correlations

    between age, gender, and pupil diameter. This was also the

    case of the studies which found a negative correlation

    between age and white-to-white corneal diameter [4, 7, 18]

    which is not shown by our results.

    We found that the thinnest corneal thickness was sig-

    nificantly lower than the central corneal thickness in

    emmetropic eyes, our results being similar to those

    obtained previously by our group [19].

    Finally, it must be noted that we examined a racially

    homogeneous population made up of young Europeans,

    although it is known that there are differences in corneal

    thickness values between populations from a different

    Table 2 Central corneal thickness and thinnest corneal thickness values obtained in the sample of emmetropic subjects analyzed

    Central corneal thickness (lm) Thinnest corneal thickness (lm)

    All Male Female All Male Female

    n 379 198 181 379 198 181

    Mean SD 559 18* 560 17 558 18 543 17* 544 17 542 17

    Minimum 528 528 531 504 511 504

    Maximum 588 588 585 574 569 574

    * Statistically significant differences between the central and the thinnest corneal thickness (p \ 0.001; Students t-test)

    Surg Radiol Anat (2012) 34:167170 169

    123

  • ethnic origin [6, 12]. According to these studies, different

    results would probably be obtained if another population of

    emmetropic subjects was analyzed.

    In conclusion, the present study has revealed the quan-

    titative anatomy of the white-to-white corneal diameter,

    pupil diameter, central corneal thickness, and thinnest

    corneal thickness of healthy emmetropic subjects, which

    shows that the photopic pupil diameter is greater in

    emmetropic women. Nevertheless, further research is

    necessary to detect if there are differences in such values

    between emmetropic, myopic, and hyperopic subjects, and

    between populations with different ethnic origins.

    Acknowledgments This study was supported by a grant from theUniversity of Valencia (UV-3691).

    Conflict of interest None.

    References

    1. Alexandridis E (1985) The Pupil. Springer-Verlag, New York,

    p 117

    2. Cairns G, McGhee CN (2005) Orbscan computerized topography:

    attributes, applications, and limitations. J Cataract Refract Surg

    31:205220 (Review)

    3. Chaidaroon W, Juwattanasomran W (2002) Colvard pupillometer

    measurement of scotopic pupil diameter in emmetropes and

    myopes. Jpn J Ophthalmol 46:640644

    4. Cosar BC, Sener BA (2003) Orbscan corneal topography system

    in evaluating the anterior structures of the human eye. Cornea

    22:118121

    5. Doughty MJ, Jonuscheit S (2010) The orbscan acoustic (correc-

    tion) factor for central corneal thickness measures of normal

    human corneas. Eye Contact Lens 36:106115

    6. Doughty MJ, Zaman ML (2000) Human corneal thickness and its

    impact on intraocular pressure measures: a review and meta-

    analysis approach. Surv Ophthalmol 44:367408

    7. Fea AM, Annetta F, Cirillo S, Campanella D, De Giuseppe M,

    Regge D, Grignolo FM (2005) Magnetic resonance imaging and

    Orbscan assessment of the anterior chamber. J Cataract Refract

    Surg 31:17131718

    8. Findl O (2005) Biometry and intraocular lens power calculation.

    Curr Opin Ophthalmol 16:6164 (Review)

    9. Goldsmith JA, Li Y, Chalita MR, Westphal V, Patil CA, Rollins

    AM, Izatt JA, Huang D (2005) Anterior chamber width mea-

    surement by high-speed optical co...

Recommended

View more >