Normative Measurements of Orbital Structures in the Hong Kong Population

 

 

 

We sought to establish normative measurements of orbital structures in the Hong Kong population using computed tomography (CT), and to evaluate for differences according to sex and age.

 

 

CT paranasal sinus examinations were retrospectively reviewed. Differences according to sex and age were analysed using Student’s t test and Pearson’s correlation, respectively.

 

 

Data of 256 patients (F:M=141:115, age range, 21-91 years) were included. Normal measurements of the diameters of the extraocular muscles (EOMs) [mean ± 2 standard deviations] were: medial rectus 3.5 ± 1.3 mm; lateral rectus 3.2 ± 1.3 mm; superior rectus and levator palpebrae superioris (superior muscle group) 3.4 ± 1.3 mm; and inferior rectus 3.8 ± 1.7 mm. The normal ranges for other orbital structures were as follows: globe diameter 25.8 ± 2.8 mm; optic nerve diameter 4.4 ± 1.7 mm; and interzygomatic line 97.7 ± 7.8 mm. The normal position of the globe was 16.7 ± 5.2 mm. Sex differences were noted in the means of EOM thickness, globe size, interzygomatic lines (p = 0.0002, p = 0.0002, p < 0.0001 respectively), and anterior globe position (p = 0.0045). A positive correlation was found between age and the sum of the mean diameters of the EOMs (r = 0.17, p = 0.0072).

 

 

Our results may help radiologists and clinicians to interpret CT scans of Hong Kong patients with suspected EOM enlargement, proptosis, or other changes in orbital morphology more quantitatively.

Key Words: Anatomy; Asian continental ancestry group; Exophthalmos; Extraocular muscles; Orbit

 

 

 

 

 

The EOMs were found to be thicker in men than in women (p = 0.0002) [Table 2]. With the exception of the medial rectus muscle, all measurements were significantly different between men and women (all p < 0.05). The globe sizes and interzygomatic line distances were larger in men than in women (p = 0.0002 and p < 0.0001 respectively). The globe position values were also larger in men (p = 0.0045), meaning that the globes were more anteriorly positioned in men than in women. No significant difference was noted between the measurements of optic nerve sheath complexes in men and those in women (p = 0.2806).

 

Table 2. Sex differences in sizes of orbital structures.

 

No consistent correlation with age was found in any of the individual measured structures (Table 3). Comparing age with the sum of all EOMs, a significant positive correlation was noted (r = 0.17, p = 0.0072).

 

Table 3. Correlations of sizes of orbital structures with age.

 

The EOMs are long fusiform structures that stretch from the orbital apex to the globe. Enlargement of the EOMs is observed in multiple disease entities, including Grave’s ophthalmopathy, primary and metastatic neoplasms, inflammatory conditions, and trauma.

 

Varying the window settings has an impact on the measurements of EOMs.[6] In the present study, a standard ‘soft tissue’ window of width 300 HU and level 40 HU was used in accordance with the routine practice of the authors’ centre.

 

Compared with other studies conducted in Asia (Turkey[6], Korea[7], and Thailand[8]), the mean measured diameters of the rectus muscles in the present study were slightly thinner for all of the individual muscles (Table 4). Such differences could be due to underlying genetic, socioeconomic, and nutritional factors. Some of the studies also measured the EOMs in the axial plane instead of the coronal plane as in this study. Differences in window settings may have also contributed. Caution must therefore be maintained regarding window settings when referring to the measurements of the current study in clinical practice.

 

Table 4. Normal measurements of sizes of orbital structures compared with other studies.

 

In the present study, men were found to have significantly larger globe sizes, interzygomatic line distances, and sums of EOM thickness. This result is in accordance with other similar studies. In another study, however,[5] it was noted that the ratio of the interzygomatic line distance to the diameters of the EOMs showed no statistically significant difference between women and men. It was therefore proposed that differences between the diameters of EOMs depend on head size rather than sex.

 

One of the focuses of the current study was to determine the normative value of the globe position as an aid in determining the presence or absence of proptosis. It is known that the normal values of globe positions vary with ethnicity.[2] [3] With the difference in terms of facial structure such as deeper eye sockets in Caucasians, normative measurements of globe positions in Western populations may not be directly applicable to the Hong Kong population. Two common options to gauge the globe position are to measure the distances between the anterior or posterior margins of the globe and the length of the interzygomatic line.[5] [7] [8] [9] We have chosen to measure the distances between the anterior margins of the globe for the following reasons. First, the distance from the anterior margin is often longer than the distance between the posterior margins, resulting in a lower margin of error in measurements. Second, abnormalities such as staphyloma more often affect the posterior margin of the globe, which may reduce the accuracy of measurements. The commonly quoted figures for the definition of proptosis from Western sources are >21 to 23 mm anterior to the interzygomatic line.[10] [11] [12] In the present study, the upper limit of the normal range was 21.9 mm, which falls within the range for proptosis. This could be due to factors other than facial structure affecting the measurement, for example the high prevalence of myopia in the Asian population, which affects the shape of the globe.[13] [14]

 

In our findings, men had more anteriorly positioned globes compared with women (p = 0.0045). The normal range for men was 17.2 ± 5.8 mm; for women it was 16.3 ± 4.3 mm. Studies that measure degrees of proptosis using exophthalmometry have shown conflicting results, with some showing sex differences and other showing none.[2] [15] Other studies using CT to measure the position as the distance between the interzygomatic lines and the posterior margins of the globe noted no significant sex differences.[5] [7] For a more direct comparison, we have converted our measurements by subtracting the distance between the interzygomatic lines and the anterior margins of the globe from the globe size in individual orbits (Table 5, Figures 2 and 3). When measuring the globe position in this way, our findings also showed no significant difference between women and men (p = 0.4169), suggesting that larger globe sizes in men most likely contribute to the more anterior globe margins in men than in women.

 

Table 5. Sex differences in normal position of the globe as measured by the distance between the interzygomatic line and the anterior or posterior margin of the globe.

 

Previous studies have shown a tendency for the EOMs to slightly enlarge with age,[5] [7] but the differences did not attain statistical significance. The same observation was made in the current study, with most individual EOMs showing positive weak correlation (r < 0.2). Furthermore, no correlation was found between age and globe position (r = –0.06).

 

The limitation of our study is that the data were collected retrospectively, and the eye movement/gaze could not be standardised. It is noted that eye movements during imaging may affect the diameters of the EOMs, with some studies asking the patient to maintain a forward gaze and gentle eye closure during scanning. However, this effect was minimised in our study with a mixture of various gazes and a considerable population size. In addition, we believe that including cases with some side gaze is important in establishing normative values, since it may not be always possible to obtain a primary gaze in daily practice (eg, in patients with inability to comprehend or cooperate).

 

In conclusion, the normative measurements of orbital structures established in our study of the Hong Kong Chinese population may help radiologists and clinicians to quantitatively interpret CT scans of patients with suspected EOM enlargement, proptosis, or other changes in the orbital morphology.