The Effect of Age, Gender, Refractive Status and Axial Length on the Measurements of Hertel Exophthalmometry

INTRODUCTION

The orbital space contains mainly the eyeball, extra-ocular musculature, retro-orbital fatty tissue, and vascular structures [1-4]. Enlargement of one or more elements of the orbital tissue or pathological conditions of the orbital walls can cause ocular protrusion [4]. The most common ethological factors for ocular protrusion are thyroid orbitopathy, orbital trauma, and tumors. High axial myopia and lid asymmetry may also result in ocular protrusion [3, 5].

Exophthalmometry is one of the routine examination methods for patients with suspected ocular protrusion [6]. There are various devices for exophthalmometric purposes; however, Hertel exophthalmometry is the most commonly used device [7]. Although the normal limits of protrusion of the human eye are accepted to be between 10 and 21mm, exophthalmometric values have been shown to vary with ethnicity, age, gender, height, weight, body mass index (BMI), orbital parameters, refraction and axial length (AL) of the eye [1, 4, 8-18].

The purpose of this study was to determine the normal exophthalmometric values of Turkish healthy adult popu-lation and the impact of age, gender, refractive status and AL on globe position.

MATERIAL AND METHODS

Healthy individuals, who were randomly selected from the patients presented to our institution, were included in the study. This study was followed the tenets of the Declaration of Helsinki. Hertel exophthalmometer (Hertel exophthalmos-meter K-0161, Inami & Co. Ltd. japan) was used to measure the degree of ocular protrusion in 236 Turkish adults. The measurements were performed under sunlight for both eyes simultaneously, but only right eye measurements were inclu-ded in this study. Two observers (OK, MOZ) have obtained all measurements by using the same Hertel exophthalmos-meter. The observers were not privy to each other’s findings and the mean of these two measurements was considered for the analysis. All subjects underwent refractive error, and interpupillary distance (IPD) measurements using an objective auto-refractometer (Topcon RK 8000PA, auto-refractometer, Topcon, Tokyo, Japan). Axial length measure-ment was carried out with a Lenstar LS 900 (HAAG-STREIT). Patients with systemic or ocular diseases, endocrine or craniofacial abnormalities, or history of orbital trauma were excluded from the study.

SPSS, version 11.6 (SPSS, Inc, Chicago, IL) was used for statistical analysis. t-test was used to compare males and females with regard to age, spherical equivalece, AL, IPD and exophthalmometric values. Chi-square t-test was used to evaluate the effect of gender on eye protrusion. Linear and multiple regression analysis was used to analyze the effect of age, spherical equivalent, and AL on exophthalmometric values.

RESULTS

A total of 236 eyes of 236 subjects were enrolled in the study. One hundred twenty-two subjects (51.7%) were male and 114 subjects (48.3%) were female. Mean age was 44.1±14.6 years (range: 18 to 87 years) in male population, and 45.9±14.3 years (range: 18-85 years) in female population (p=0.316). Mean Hertel exophthalmometric measurements for the whole population was 15.8±2.6 mm (range: 11 to 21 mm), where mean value was found as 16.1±2.6 mm and 15.5±27 mm for male and female populations, respectively (p=0.062)

Age was found to mildly affect the Hertel measurements (R²=0.098, p=0.000) to a small extent.

Spherical equivalents were negatively correlated with exophthalmometric measurements. Every 1.7 diopter increase of spherical equivalent caused a decrease of 1.0 mm in exophthalmometric values. This result was found to be statistically significant (R²= 0.028, p= 0.01).

Every 4.7 mm increase in AL was associated with a 1 mm increase in exophthalmometric value. There was a moderate correlation between AL and Hertel measurements (R²= 0.191, p=0.000).

There was no relation between IPD and Hertel measurements (R²= 0.022, p=0.000).

The age, Hertel values, spherical equivalents, AL and IPD of all subjects are shown in Table 1.

Multiple regression analysis also revealed a significant correlation between Hertel measurements and age, and spherical equivalent and AL (Table 2).

DISCUSSION

In the adult human, the total volume of the orbit is about 30 ml including the eyeball, fat tissue, muscles, and vascular structures. An excess of orbital tissues may result in eyeball protrusion known as exophthalmos [1, 4].

Exophthalmometry is a simple procedure for both the diagnosis and follow up of several orbital pathologies. Hertel exophthalmometer is a cheap, simple and portable device so it is the most commonly used device for exophthalmometric measurements worldwide. Although it gives reliable results, several studies have noticed that exophthalmometric values may vary according to various factors such as ethnicity, age, gender, height, weight, orbital parameters, and refraction [1, 4, 9-18].

Being the most important parameter, skeletal structure differences between ethnicity, age and gender affects the exophthalmometric values. Based on this knowledge, many investigators tried to determine the normative values of exophthalmometric measures in distinct populations [13, 19].

Our results demonstrate that Turkish adult males and females ocular protrusion values are 16.2 ± 2.6 mm and 15.2 ± 2.5 mm, respectively. The upper normal limits are 21 mm (male) and 20 mm (female). Our measurement results for both genders were nearly 2.5 mm higher than the previous reports on Turkish samples [4, 9]. Both studies included subjects from the north and northeast part of Turkey, respectively, whereas our study was carried out in a city, which is in the west part of Turkey. Since our country consisted of various ethnic groups, Hertel measurements may show different results in different regions.

Different studies showed various results regarding the relation between age and Hertel measurements. Several studies demonstrated a decrease in ocular protrusion values after the age of 20 years according to the involutional orbital soft tissue atrophy [16, 20, 21]. Kashkouli et al. [20] and Nath et al. [21] demonstrated that protrusion of the eyeball significantly increased with aging in children and teenagers, but decreased with aging in adults. Fledelius et al. [11] showed that the adult protrusion values are reached in late teenage years; but thereafter remain stable. Chan et al. [6] reported that measurements were not statistically different in age groups in both sexes. In our study, there was a low correlation between age and Hertel measurements.

Gender factor in ocular protrusion is controversial. Migliori et al. [1] and Dunsky et al. [22] reported a statistically significant increase in ocular protrusion values in males. Fledelius et al. [11] attributed the higher male ocular protrusion to the greater body structure. In contrast, other researchers did not find a significant gender difference in ocular protrusion [4, 20]. In our study, there was no statistically significant difference (p=0.123) between the two genders in accordance to finding in the literature.

In most studies, high myopia (< -6 D) was excluded to avoid the pseudo-proptosis effect of this condition [4, 9, 20]. In studies that evaluated the spherical equivalents and exophthalmometric value relation, no correlation between these two parameters was noted [6, 11]. In our study, we also excluded high myopia patients from the study and we found a negative correlation between spherical equivalents and exophthalmometric measurements, which is different from the previous reports.

Determining the relation between AL and Hertel meas-urements is an important issue. Chan et al. [6] demonstrated a statistically significant positive correlation between AL and ocular protrusion. They reported that every 4 mm increase in AL was associated with a 1 mm increase in exophthalmometric measurement. This correlation suggests that Hertel measurements should be corrected with regards to AL measurements. In our study, we also elucidated the relationship between ocular protrusion and AL. We found that every 4.7 mm increase in AL was associated with a 1 mm increase in exophthalmometric value. The relation of Hertel measurements and AL is still not well described and there is a need for more studies to obtain standardization of Hertel measurements and AL relation.

In summary, many factors including age, spherical equivalent and AL may have significant effects on ocular protrusion values. Consequently, larger, multiscale and prospective studies can provide much more valuable data about exophthalmometric measurements.

REFERENCES