Effect of Smoking on Orbital Fat and Muscle Volume in Graves' Orbitopathy

Abstract

Background:

Smoking adversely affects the course and severity of Graves' orbitopathy (GO). Cigarette smoke enhances adipogenesis in cultured human orbital fibroblasts. The present study tested our hypothesis that smoking is associated with increased orbital fat in GO patients.

Methods:

This was an observational case series study. In 95 consecutive patients with untreated GO, the ratios of fat volume/orbital volume (FV/OV) and muscle volume/OV (MV/OV) were calculated with validated software. The most affected orbit of each patient was chosen for analysis. Patients were divided into two groups based on smoking behavior. One group was current smokers (Sm+) and the other were those who never smoked or those who had not smoked for at least 1 year (Sm−). Patients were grouped in tertiles of FV/OV and MV/OV and contrast in OVs between the Sm+ and Sm− group. The main characteristics of GO were analyzed using Jonckheere-Terpstra trend analysis and Mann–Whitney U-test.

Results:

The proportion of current smokers was not different in GO patients when divided in tertiles according to their FV/OV. In contrast, analysis of MV/OV tertiles showed a trend to a higher prevalence of current smokers in patients with larger MVs. Smoking did not influence FV, but the Sm+ group had significantly larger MVs than the Sm− group.

Conclusion:

Smoking is associated with an increase in extraocular MV in untreated patients with GO and not with an increase in FV.

Introduction

S moking is a well-known risk factor for Graves' orbitopathy (GO) with an odds ratio of ∼4 (1). Some studies observed a dose-dependent relationship between smoke exposure and GO severity (1,2). Smokers are four times more likely than nonsmokers for developing or worsening of GO after ¹³¹I therapy for Graves' hyperthyroidism (3,4). Smokers also respond less well than nonsmokers to immunosuppressive treatment of GO (4,5). The mechanism behind the adverse effects of smoking in GO remains incompletely clarified. Orbital fibroblasts cultured under hypoxic conditions secrete larger amounts of glycosaminoglycans. These are very hydrophilic components that accumulate in orbital tissues of GO patients and contribute greatly to swelling of extraocular muscles and orbital fat in GO (6). Other studies have demonstrated upregulation of expression of HLA-DR and adhesion molecules on orbital fibroblasts upon exposure to tobacco products (7). A recent study of Cawood et al. is in this aspect particularly interesting (8). Cultured human orbital fibroblasts were exposed to cigarette smoke: a dose-dependent increase was observed in both secretion of the pro-inflammatory cytokine IL-1 and in the number of fibroblasts that had differentiated into adipocytes (8). These in vitro experiments strongly suggest that smoking may stimulate adipogenesis.

The aim of the present study was to evaluate whether, in vivo, smoking is also associated with more orbital fat. We therefore calculated orbital fat volume (FV) and extraocular muscle volume (MV) in a series of consecutive GO patients who had not received specific treatment for their eye disease and had all been rendered euthyroid at the time of volumetry. We first analyzed the proportion of smokers in relation to tertiles of orbital FV and MV and then compared these volumes between current smokers and a group of persons, some of whom had never smoked and others who had not smoked for at least 1 year.

Patients and Methods

We studied 95 consecutive patients found to have GO at first visit in a combined thyroid–eye clinic. They had not received any specific treatment for GO before volumetry and were euthyroid at the time of investigation as described previously (9). To minimize bias by ancestral differences in orbital anatomy the patients in the study were all Caucasian (10).

Clinical symptoms of GO were described and classified using the classification No signs and symptoms, Only signs, Soft tissue involvement, Proptosis, Extraocular muscle involvement, Corneal involvement and Sight loss (NOSPECS) and the Clinical Activity Score (CAS) (11 –13).

Orbital FV and MV of the four EOM (rectus muscles) were calculated from computed tomography (CT) scans with computer-assisted region growing and manual segmentation using the software Mimics^® following the protocol described previously (14,15). To eliminate anatomical differences between genders, the ratios fat volume/orbital bony cavity volume (FV/OV) and MV/OV were used (10). The most affected orbits of the 95 GO patients were used (9). Orbital CT scans were obtained in the course of routine clinical evaluation. Although the local Ethics Committee considered this study not subject to consent, all participants were asked to sign an informed consent in advance. We followed the tenets of the declaration of Helsinki.

Patients were grouped according to tertiles FV/OV and MV/OV and to their smoking behavior. Persons assigned to the Sm+ group were current smokers at the time of the investigation. Persons assigned to the Sm− group were either those who had not smoked for at least 1 year before the visit to our thyroid-eye clinic or persons who never smoked. In the Sm− group 23 persons had not smoked for at least 1 year and 41 persons had never smoked. For ranking in tertiles and statistical calculations SPSS 16.0.2 was used. Jonckheere-Terpstra for trend analysis and Mann–Whitney U-test for comparison between two samples were used to establish statistical significance (p < 0.05 is considered to be significant).

Results

Smoking behavior was not known in 4 of the 95 patients, leaving 91 patients for analysis. There were 9 men and 82 women in the study. There were 30% current smokers, 25% ex-smokers, and 45% persons who never smoked.

Table 1 shows patient data grouped according to tertiles of FV/OV and MV/OV ratios, respectively. The proportion of current smokers did not differ as a function of FV. The larger the orbital FV, the higher the proptosis and the older the patient, but otherwise eye changes and thyroid function tests were comparable between the tertiles. When patients were grouped according to MV/OV tertiles, there was a trend (p = 0.096) to higher prevalence of smokers in the highest tertile. Indeed comparing first and third tertile the number of smokers was greater in the group with larger MVs (p = 0.05). The larger the extraocular MV, the higher the proptosis and the greater the limitation in extraocular muscle motility; larger MV was also associated with older age, higher thyroid binding inhibitory immunoglobulin (TBII), and lower thyroid-stimulating hormone. Neither fat nor MVs were related to the time interval between onset of hyperthyroidism and the date of volumetry. Therapy for hyperthyroidism consisted of antithyroid drugs + thyroxine (block and replace regimen) in 73 patients. Eighteen patients had received ¹³¹I therapy. Distribution of these 18 patients over tertiles was similar (p = 0.5 for tertiles FV/OV and p = 0.4 for tertiles MV/OV). Duration of eye complaints was shorter than 1 year in 56% of patients, and longer than 1 year in 44%; smoking habits did not differ between both groups. In Table 2 we provide patient data according to their smoking behavior. Patients in the Sm+ group had larger MV and higher proptosis values but not a larger FV.

Table 1.

Characteristics of 91 Graves' Orbitopathy Patients Divided in Tertiles According to the Ratio of Fat Volume/Orbital Volume or to the Ratio of Muscle Volume/Orbital Volume

Tertiles FV/OV ^a	First, n = 30	Second, n = 32	Third, n = 29	J-T
FV/OV	0.50 [0.44–0.52]	0.60 [0.55–0.64]	0.71 [0.66–0.71]	<0.001
MV/OV	0.21 [0.17–0.25]	0.21 [0.18–0.29]	0.22 [0.19–0.26]	0.450
Age (years)	46 [39–54]	51 [44–62]	53 [47–61]	0.033
Lidaperture (mm)	12 [11–13]	12 [11–13]	13.5 [11–15]	0.850
Proptosis^b (mm)	20 [19–21]	22 [20–24]	2 [22–24]	0.030
Abduction (degree)	50 [44–51]	45 [40–49]	46 [40–48]	0.077
Adduction (degree)	46 [44–48]	45 [42–48]	46 [40–50]	0.670
Elevation (degree)	46 [34–49]	40 [29–44]	37 [30–45]	0.071
Depression (degree)	60 [56–65]	57 [51–60]	58 [55–60]	0.082
Diplopia^c	0 [0–1]	1 [0–2]	0 [0–1]	0.480
CAS	2 [1–3]	2 [1–3]	2 [1–2]	0.530
TSH (mU/L)	0.31 [0.02–4.28]	1.8 [0.10–3.13]	1.30 [0.25–4.0]	0.290
FT4 (pmol/L)	13.6 [11.8–18.0]	15.1 [12.3–17.6]	16.0 [14.0–18.3]	0.440
Anti-TPO (kU/L)	170 [68–1003]	100 [15–630]	145 [36–1028]	0.210
TBII (U/L)	7.7 [2.6–21.6]	10.2 [4.0–16.5]	3.0 [1.0–13.9]	0.920
Onset hyper (years)^d	2 [1–7]	2 [1–4.5]	2 [1–8.5]	0.133
SM+ (n) %	(7) 23	(10) 30	(10) 32	0.62^e

Tertiles MV/OV ^a	First n = 32	Second n = 30	Third n = 29	J-T
FV/OV	0.60 [0.50–0.66]	0.65 [0.54–0.71]	0.57 [0.54–0.69]	0.550
MV/OV	0.17 [0.15–0.18]	0.21 [0.20–0.23]	0.29 [0.26–0.35]	<0.001
Age (years)	45 [36–53]	54 [46–59]	51 [47–58]	0.045
Lidaperture (mm)	12 [11–13]	12 [11–13]	13 [11–15]	0.080
Proptosis^b (mm)	20 [19–23]	21 [20–22]	23 [22–25]	<0.001
Abduction (degree)	48 [45–55]	45 [40–47]	46 [39–49]	0.007
Adduction (degree)	48 [45–50]	45 [40–47]	44 [40–48]	0.009
Elevation (degree)	45 [34–52]	35 [27–46]	40 [26–44]	0.023
Depression (degree)	60 [56–63]	60 [56–61]	54 [50–60]	0.023
Diplopia^c	0 [0–0]	1 [0–2.5]	1 [0–1]	0.018
CAS	2 [1–2.5]	1 [1–2]	2.5 [2–3]	0.140
TSH (mU/L)	1.6 [0.1–5.7]	1.7 [0.04–4.2]	0.4 [0.04–2.4]	0.088
FT4 (pmol/L)	14.8 [12.1–16.9]	15.9 [12.7–17.8]	15.6 [12.5–18.5]	0.440
Anti-TPO (kU/L)	160 [40–3000]	80 [15–995]	160 [50–610]	0.480
TBII (U/L)	4.2 [1.0–9.2]	7.4 [2.1–21.7]	11.1 [6.3–29.2]	0.001
Onset hyper (years)^d	5 [1–8.5]	2 [1–4]	2 [1–3]	0.518
SM+ (n) %	(5) 17	(10) 30	(12) 39	0.096^e

Numbers are given in median with interquartile range between brackets. p < 0.05 is considered significant.

Ranking in tertiles by SPSS 16.2.

Measured with Hertel exophthalmometer.

0, no diplopia; 1, intermittenet diplopia; 2, inconstant diplopia; 3, constant diplopia.

Time between onset hyperthyroidism and first visit at Graves' orbitopathy clinics.

Chi-square test.

Anti-TPO, thyroid peroxidase antibodies; CAS, clinical activity score; FT4, free thyroxin 4, FV/OV, ratio of fat volume/orbital volume; J-T, Jonckheere-Terpstra test for trend analysis; MV/OV, ratio of muscle volume/orbital volume; SM+, group of current smokers; TBII, thyroid binding inhibitory immunoglobulin; TSH, thyroid-stimulating hormone.

Table 2.

Characteristics of the Group Never Smokers and Ex-Smokers (SM−) Versus Current Smokers (SM+)

	SM−, n = 64	SM+, n = 27	M-W
FV/OV	0.60 [0.52–0.68]	0.62 [0.54–0.73]	0.411
MV/OV	0.22 [0.18–0.26]	0.24 [0.20–0.27]	0.034
Age (years)	50 [44–57]	50 [42–55]	0.385
Lidaperture (mm)	12 [11–14]	12 [11–14]	0.654
Proptosis^a (mm)	22 [20–23]	22 [21–24]	0.058
Abduction (degree)	46 [40–49]	45 [40–48]	0.236
Adduction (degree)	45 [42–48]	45 [41–48]	0.323
Elevation (degree)	40 [30–46]	40 [29–45]	0.474
Depression (degree)	60 [53–61]	60 [54–60]	0.891
Diplopia^b	1 [0–1]	0 [0–1]	0.716
CAS	2 [1–3]	2 [1–2]	0.202
TSH (mU/L)	0.7 [0.1–4.0]	0.5 [0.0–2.5]	0.105
FT4 (pmol/L)	15.7 [12.7–17.6]	15.7 [12.0–17.5]	0.801
Anti-TPO (kU/L)	130 [35–790]	170 [73–1063]	0.261
TBII (U/L)	9.2 [2.7–21.7]	9.3 [4.1–19.7]	0.198

Values given as median with interquartile range between brackets. p < 0.05 is considered significant.

Measured with Hertel exophthalmometer.

0, no diplopia; 1, intermittenet diplopia; 2, inconstant diplopia; 3, constant diplopia.

M-W, Mann–Whitney nonparametric test.

Discussion

The present study suggests that smoking is related to larger extraocular MVs in GO patients and that smoking is not related to orbital FV. The analysis comparing tertiles of MVs showed a trend to higher volumes in current smokers, but failed to reach statistical significance, likely due to the relatively low number of smokers in each group. The analysis comparing current smokers and (never+ex) smokers in the whole group clearly showed larger MVs in Sm+ than in Sm−, whereas FVs were not related to smoking behavior.

Higher MVs are associated with more severe GO (higher proptosis values, greater degree of limitations in eye muscle ductions, and more severe diplopia) in keeping with the notion that smoking is associated with more severe GO (5). Higher TBII levels were also observed in the highest tertile of MVs, but TBII was not significantly higher in current smokers relative to never-smokers + ex-smokers. Smoking had no effect on the clinical activity score.

A possible confounding factor in the observed relationship between smoking and FV/MV is age. Higher age is associated with higher FV and MV in GO patients in line with literature data that older age is associated with more severe GO (16,17). However, in Table 2 a significant difference in MV was observed between (never+ ex) smokers versus current smokers (p = 0.034) without an age difference between both groups, thereby eliminating age as a confounding factor.

It thus appeared that smoking is associated with more severe GO related to larger extraocular MV but not larger FV. This is in contrast with in vitro findings indicating increased adipogenesis of cultured orbital fibroblasts upon smoke exposure (8). The reasons for the discrepancy, between in vitro and in vivo findings, could be that, in vivo, orbital fibroblasts are not exposed to cigarette smoke extract in concentrations as applied in vitro. Another explanation can be that adipogenesis is a relatively late phenomenon in the natural course of GO. In this cross-sectional study we compared volumes between GO patients. It will be interesting to evaluate longitudinal changes in volumes of the same patients in a follow-up study.

Our results do not confirm the findings of a previous study on the relation of smoking and fat/muscle enlargement in GO patients by Szucs-Farkas et al. (18). They found that connective tissue volumes were influenced by smoking history, whereas MVs were not affected. Ex-smokers had a larger amount of extraorbital connective tissue than current smokers. The discrepancy between their and our study might be explained by differences in the applied methodology. In the study of Szucs-Farkas intraorbital connective tissue was defined as intraorbital tissues presenting high signal intensity on T1-weighted MR images, including bloodvessels and smaller nerves, but not the optic nerve. Tissue components were outlined on the monitor. In contrast, the applied method in our study measures the proper FV, which does not include blood vessels and smaller nerves. Fat tissue is identified by CT densities with the use of a software program. They also studied only eight ex-smokers and six never smokers, so their patient numbers were smaller.

An explanation for the increase in MV caused by smoking can be hypoxia. Smoking induces chronic hypoxia by preventing hemoglobin to carry oxygen through CO binding. In an orbit already compromised by decreased blood flow the oxygen deficit of the muscles could worsen. As reported by Yu Wai Man et al., EOM have a large amount of mitochondria and are very high consumers of oxygen (19 –24). Lack of oxygen can lead to impaired muscle function and probably also swelling of the muscle fibers.

All possible mechanisms are of course speculative but provide us with the challenge to investigate these.

Conclusions

Smoking is associated with an increase in extraocular MV in untreated patients with GO. In current smokers the MVs are larger, the ductions are worse, and the disease is more severe. We found no influence of smoking on the FV in GO patients.

Footnotes

Disclosure Statement

The authors declare that no competing financial interests exit.

References

Vestergaard

, Rejnmark

, Weeke

, Hoeck

, Nielsen

, Rungby

, Laurberg

, Mosekilde . 2002. Smoking as a risk factor for Graves' disease, toxic nodular goiter, and autoimmune hypothyroidism. Thyroid, 12:69–75.

Hegedius

, Brix

, Vestergaard

. 2004. Relationship between cigarette smoking and Graves' ophthalmopathy. J Endocrinol Invest, 27:265–271.

Bartalena

, Marcocci

, Tanda

, Manetti

, Dell'Unto

, Bartolomei

, Nardi

, Martino

, Pinchera

. 1998. Cigarette smoking and treatment outcomes in Graves ophthalmopathy. Ann Intern Med, 129:632–650.

Krassas

, Wiersinga

. 2006. Smoking and autoimmune thyroid disease: the plot thickens. Eur J Endocrinol, 154:777–780.

Thornton

, Kelly

, Harrison

, Edwards

. 2007. Cigarette smoking and thyroid eye disease: a systematic review. Eye, 21:1135–1145.

Kahaly

, Forster

, Hansen

. 1998. Glycosaminoglycans in thyroid eye disease. Thyroid, 8:429–432.

Mack

, Stasior

, Cao

, Stasior

, Smith

. 1999. The effect of cigarette smoke constituents on the expression of HLA-DR in orbital fibroblasts derived from patients with Graves ophthalmopathy. Ophthal Plast Reconstr Surg, 15:260–271.

Cawood

, Moriarty

, O'Farrelly

, O'Shea

. 2007. Smoking and thyroid-associated ophthalmopathy: a novel explanation of the biological link. J Clin Endocrinol Metab, 92:59–64.

Regensburg

, Wiersinga

, Berendschot

TJJM

, Potgieser

, Mourits

. 2010. Do subtypes of Graves' orbitopathy exists. Ophthalmology, Jul 27[Epub ahead of print]. PMID: 20673587.

10.

Regensburg

, Wiersinga

, van Velthoven

MEJ

, Berendschot

TTJM

, Zonneveld

, Baldeschi

, Saeed

, Mourits

. 2010. Age-, gender specific reference values of orbital fat-, muscle volumes in Caucasians. Br J Ophthalmol, Jun 7[Epub ahead of print]. PMID: 19955201.

11.

Wiersinga

, Prummel

, Mourits

, Koornneef

, Buller

. 1991. Classification of the eye changes of Graves' disease. Thyroid, 1:357–360.

12.

Werner

. 1977. Modification of the classification of the eye changes of Graves' disease. Am J Ophthalmol, 83:725–270.

13.

Mourits

, Prummel

, Wiersinga

, Koornneef

. 1997. Clinical activity score as a guide in the management of patients with Graves' ophthalmopathy. Clin Endocrinol (Oxf), 47:9–14.

14.

Regensburg

, Kok

, Zonneveld

, Baldeschi

, Saeed

, Wiersinga

, Mourits

. 2008. A new and validated ct-based method for the calculation of orbital soft tissue volumes. Invest Ophthalmol Vis Sci, 49:1758–1762.

15.

Materialise, Leuven, Belgium. Mimics(version 10.2)2007.

16.

Perros

, Crombie

, Matthews

, Kendall-Taylor

. 1993. Age and gender influence the severity of thyroid-associated ophthalmopathy: a study of 101 patients attending a combined thyroid-eye clinic. Clin Endocrinol (Oxf), 38:367–372.

17.

Kendler

, Lippa

, Rootman

. 1993. The initial clinical characteristics of Graves' orbitopathy vary with age and sex. Arch Ophthalmol, 111:197–201.

18.

Szucs-Farkas

, Toth

, Kollar

, Galuska

, Burman

, Boda

, Leovey

, Varga

, Ujhelyi

, Szabo

, Berta

, Nagy

. 2005. Volume changes in intra-, extraorbital compartments in patients with Graves' ophthalmopathy: effect of smoking. Thyroid, 15:146–151.

19.

Yu Wai Man

, Chinnery

, Griffiths

. 2005. Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders. Neuromuscul Disord, 15:17–23.

20.

Spencer

, Porter

. 1988. Structural organization of the extraocular muscles. Rev Oculomot Res, 2:33–79.

21.

Wooten

, Reis

. 1972. Blood flow in extraocular muscle of cat. Arch Neurol, 26:350–352.

22.

Carry

, Ringel

, Starcevich

. 1986. Mitochondrial morphometrics of histochemically identified human extraocular muscle fibres. Anat Rec, 214:8–16.

23.

Chang

, Johns

, Walker

, de la Cruz

, Maumence

, Green

. 1993. Ocular clinicopathologic study of the mitochondrial encephalomyopathy overlap syndromes. Arch Ophthalmol, 111:1254–1262.

24.

Fischer

, Gorospe

, Felder

, Bogdanovich

, Pedrosa-Domellof

, Ahima

, Rubinstein

, Hoffman

, Khurana

. 2002. Expression profiling reveals metabolic and structural components of extraocular muscles. Physiol Genomics, 9:71–84.