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Cheong, Jeong, and Ahn: Density of Orbital Fat and Extraocular Muscle in Thyroid-Associated Myopathy and Idiopathic Orbital Myositis
Original Article

J Korean Ophthalmol Soc 2013;54(11):1641-1648.

Published online: 2 September 2013

DOI: https://doi.org/10.3341/jkos.2013.54.11.1641

Density of Orbital Fat and Extraocular Muscle in Thyroid-Associated Myopathy and Idiopathic Orbital Myositis

Hye Mi Cheong, MD, Woo Jin Jeong, MD, PhD, Hee Bae Ahn, MD, PhD

Department of Ophthalmology, Dong-A University College of Medicine, Busan, Korea

Address reprint requests to Hee Bae Ahn, MD, PhD, Department of Ophthalmology, Dong-A University Hospital, #26 Daesingongwon-ro, Seo-gu, Busan 602-715, Korea, Tel: 82-51-240-5227, Fax: 82-51-254-1987, E-mail: hbahn@dau.ac.kr

Received 8 March 2013       Revised 29 May 2013       Accepted 23 September 2013

Copyright © 2013 Korean Ophthalmological Society

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Purpose

To perform and compare differential diagnosis of patients with thyroid-associated myopathy, idiopathic orbital myositis and normal controls based on orbital computed tomography. Orbital fat and extraocular muscle densities were quantified using Hounsfield Unit (HU) and their characteristics were compared and analyzed.

Methods

From February 2005 to January 2013, orbital computed tomography was performed on 90 eyes of 47 thyroid-associated myopathy patients, 18 eyes of 14 idiopathic orbital myositis patients and 280 eyes of 140 normal subjects. The average values of orbital fat and extraocular muscle densities were measured and compared using HU. The density differences between the patients with thyroid-associated myopathy and the normal group were analyzed by age, clinical activity score, ocular protrusion and disease duration.

Results

In the thyroid-associated myopathy group, orbital fat and extraocular muscle densities were −87.8 ± 12.5 HU and 48.7 ± 7.1 HU, respectively. In the idiopathic orbital myositis group, the orbital fat and extraocular muscle densities were 79.9 ± 9.9 HU and 49.2 ± 9.1 HU, respectively. There was a statistically significant lower result of orbital fat in the thyroid-associated myopathy group (p = 0.002), however, the extraocular muscle density did not show a statistically significant difference (p = 0.775). The orbital fat and extraocular muscle densities of the normal group were −79.0 ± 11.2 HU and 54.3 ± 6.3 HU, respectively. There were significantly lower results in both orbital fat and extraocular muscle densities in the thyroid-associated myopathy group than normal group (p = 0.000). In active cases and those accompanied by ocular protrusion, there was no significant difference in orbital fat density (p = 0.345 and p = 0.952, respectively), while extraocular muscle density significantly decreased (p = 0.007 and p = 0.003, respectively).

Conclusions

A difference between the orbital fat and extraocular muscle densities in thyroid-associated myopathy and idiopathic orbital myositis could be quantitatively found using HU and orbital computed tomography.

Keywords: Density of extraocular muscle, Density of orbital fat, Hounsfield Unit, Idiopathic orbital myositis, Thyroid-asso ciated myopathy

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Figure 1.
Densities (in Hounsfield units as mean ± SD) calculated from a defined area (in mm2) in an axial image of a CT scan of the medial rectus muscle and orbital fat.
jkos-54-1641f1.tif
Table 1.
Characteristics of patients
Thyroid-associated myopathy group Idiopathic orbital myositis group Normal group
Study population (n, eyes) 47 (90) 14 (18) 140 (280)
Sex (male : female) 13 : 34 7 : 7 62 : 78
Age (years) 41.9 ± 17.9 52.7 ± 15.7 40.0 ± 20.1
 10-19 (%) 17 (13.8) 0 (0) 40 (14.3)
 20-29 (%) 4 (3.1) 1 (7.1) 40 (14.3)
 30-39 (%) 19 (14.6) 0 (0) 40 (14.3)
 40-49 (%) 13 (10.0) 4 (28.6) 40 (14.3)
 50-59 (%) 20 (76.6) 5 (35.7) 40 (14.3)
 60-69 (%) 11 (15.4) 3 (21.4) 40 (14.3)
 70-79 (%) 4 (3.1) 1 (7.1) 40 (14.3)

Values are presented as mean ± SD.

Table 2.
Comparison of mean density of orbital fat between thyroid-associated myopathy and Idiopathic orbital myositis (expressed in Hounsfield units [HU])
Age (years) Thyroid- associated myopathy group Idiopathic orbital myositis group p-value*
10-19 −92.3 ± 13.9 − −
20-29 −101.6 ± 12.8 −77.3 ± 6.7 0.016†
30-39 −86.8 ± 11.3 − −
40-49 −91.8 ± 12.1 −77.9 ± 11.7 0.001†
50-59 −81.3 ± 11.9 −80.5 ± 11.3 0.862†
60-69 −89.4 ± 12.1 −76.9 ± 10.9 0.001†
70-79 −72.9 ± 7.7 −67.0 ± 8.7 0.003†
Total −87.8 ± 12.5 −79.9 ± 9.9 0.002*

Values are presented as mean ± SD.

* Student t-test;

† Mann-Whitney U test.

Table 3.
Comparison of mean density of extraocular muscles between thyroid-associated myopathy and Idiopathic orbital myositis (expressed in Hounsfield units [HU])
Age (years) Thyroid-associated myopathy group Idiopathic orbital myositis group p-value*
10-19 48.6 ± 9.3 − −
20-29 48.8 ± 8.6 48.7 ± 6.3 0.964†
30-39 50.5 ± 7.5 − −
40-49 43.6 ± 6.6 41.7 ± 12.3 0.720†
50-59 51.3 ± 4.4 51.6 ± 9.7 0.884†
60-69 49.3 ± 6.1 49.8 ± 10.2 0.819†
70-79 42.4 ± 7.8 39.3 ± 6.5 0.571†
Total 48.7 ± 8.0 49.2 ± 9.1 0.775*

Values are presented as mean ± SD.

* Student t-test;

† Mann-Whitney U test.

Table 4.
Comparison of orbital fat density between thyroid-associated myopathy and normal (expressed in Hounsfield units [HU])
Age (years) Thyroid-associated myopathy group Normal group p-value*
10-19 −92.3 ± 10.9 −79.5 ± 10.9 0.000
20-29 −101.6 ± 9.8 −73.4 ± 14.0 0.000
30-39 −86.8 ± 11.3 −76.9 ± 11.3 0.000
40-49 −91.8 ± 11.1 −79.7 ± 10.1 0.000
50-59 −81.3 ± 11.9 −80.9 ± 8.8 0.856
60-69 −89.4 ± 12.1 −83.4 ± 9.9 0.093
70-79 −72.9 ± 7.7 −77.2 ± 10.7 0.268
Total −87.8 ± 12.5 −79.0 ± 11.2 0.000

Values are presented as mean ± SD.

* Student t-test.

Table 5.
Comparison of extraocular muscles density between thyroid-associated myopathy and normal (expressed in Hounsfield units [HU])
Age group (years) Thyroid-associated myopathy group Normal group p-value*
10-19 48.6 ± 9.3 57.6 ± 5.5 0.000
20-29 48.8 ± 2.6 56.9 ± 6.8 0.000
30-39 50.5 ± 7.5 55.0 ± 7.2 0.002
40-49 43.6 ± 4.6 56.2 ± 5.6 0.000
50-59 51.3 ± 4.4 54.0 ± 7.1 0.012
60-69 49.3 ± 6.1 52.4 ± 4.7 0.156
70-79 42.4 ± 4.8 52.2 ± 5.0 0.000
Total 48.7 ± 7.1 54.3 ± 6.3 0.000

Values are presented as mean ± SD.

* Student t-test.

Table 6.
Comparison of mean density of orbital fat and extraocular muscles by Clinical activity score (CAS) in thyroid-associated myopathy (expressed in Hounsfield units [HU])
CAS ≥ 4 CAS < 4 p-value*
Fat density −86.0 ± 16.2 −88.4 ± 11.2 0.345
Muscle density 46.2 ± 6.2 49.4 ± 7.2 0.007

Values are presented as mean ± SD.

* Student t-test.

Table 7.
Comparison of Mean density of orbital fat and extraocular muscles by proptosis in thyroid-associated myopathy (expressed in Hounsfield units [HU])
Proptosis (+) Proptosis (-) p-value*
Fat density −87.9 ± 13.2 −87.8 ± 11.5 0.952
Muscle density 47.4 ± 6.7 50.6 ± 7.2 0.003

Values are presented as mean ± SD.

* Student t-test.

Table 8.
Comparison of Mean density of orbital fat and extraocular muscles by duration of Thyroid-associated myopathy (expressed in Hounsfield units [HU])
< 2 years ≥2 years p-value*
Fat density −92.0 ± 11.2 −87.2 ± 12.6 0.082
Muscle density 48.5 ± 10.7 48.7 ± 6.4 0.934

Values are presented as mean ± SD.

* Student t-test.

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