Pathogenetic mechanisms in thyroid-associated ophthalmopathy

Correlation Analysis between Intraocular Pressure and Extraocular Muscles Based on Orbital Magnetic Resonance T2 Mapping in Thyroid-Associated Ophthalmopathy Patients

Background: The correlation between intraocular pressure (IOP) and the magnetic resonance imaging (MRI) parameters in thyroid-associated ophthalmopathy (TAO) patients was explored. Methods: This study included 82 eyes in 41 TAO patients who had a large difference in the IOP between each eye. We measured the T2 relaxation time (T2RT) of the extraocular muscles (EOMs), the orbital fat, and the area of the EOMs. Results: There was a positive correlation between IOP and exophthalmos, the clinical activity score (CAS), the T2RT (of the medial rectus (MR)), the area of the MR, inferior rectus (IR) and lateral rectus, and the mean area. We established a regression model with IOP as the dependent variable, and the area of the IR was statistically significant. Conclusions: High IOP in TAO patients was positively correlated with the degree of exophthalmos and EOM inflammation (especially the inferior rectus). The state of the EOMs in an orbital MRI may partially explain high IOP and provide the necessary clinical information for subsequent high IOP treatment.

Effect on intraocular pressure of extraocular muscle surgery for thyroid-associated ophthalmopathy

PURPOSE

To study the effect of extraocular muscle surgery on intraocular pressure (IOP) in patients with thyroid-associated ophthalmopathy.

DESIGN

Retrospective, observational case series.

METHODS

The medical records of patients with restrictive myopathy secondary to thyroid-associated ophthalmopathy who underwent strabismus surgery from July 1, 1997 through July 31, 2003 were reviewed and analyzed retrospectively. Seventeen patients met the criteria and were included in this study. All patients were seen at the Thyroid Eye Center at the University of California, San Diego, a university-based tertiary referral center. The main outcome measure was IOP readings obtained before and after surgery in both primary gaze and upgaze.

RESULTS

A statistically significant decrease in IOP in upgaze was noted after extraocular muscle recession. The mean IOP before surgery was 16.6 +/- 3.78 mm Hg in primary gaze and 23.2 +/- 7.27 mm Hg in upgaze. After strabismus surgery, the mean IOP after one month was 15.7 +/- 2.36 mm Hg (P = .215) in primary gaze and 18.9 +/- 2.96 mm Hg in upgaze (P = .001).

CONCLUSIONS

Strabismus surgery resulted in a significant reduction in IOP in the early postoperative period in patients with restrictive myopathy secondary to thyroid-associated ophthalmopathy.

Botulinum A toxin injection for restrictive myopathy of thyroid-related orbitopathy: effects on intraocular pressure

PURPOSE

To study the effect of extraocular muscle injections of botulinum A toxin on intraocular pressure in patients with thyroid-related orbitopathy.

DESIGN

Retrospective observational case series.

METHODS

The medical records of eight consecutive patients with restrictive myopathy secondary to thyroid related orbitopathy (TRO) who underwent botulinum A toxin injection from December 1997 to December 1998 were reviewed and analyzed retrospectively. All patients were seen at the University of California, San Diego (UCSD) Thyroid Eye Center, a university-based tertiary referral center. The main outcome measure was intraocular pressure (IOP) readings taken before and after injection in both primary gaze and upgaze (involving one eye in seven of the patients and both eyes in one patient). Intraocular pressure readings were measured by an unmasked physician using a Goldmann applanation tonometer.

RESULTS

A statistically significant decrease in IOP in upgaze was noted 2 to 6 weeks following botulinum A toxin injection and in both fields of gaze (primary and upgaze) after 2 to 4 months. The mean IOP before injection was 21.4 +/- 3.0 mm Hg in primary gaze and 29.9 +/- 9.7 mm Hg in upgaze. The mean IOP, following injection at 2 to 6 weeks, was 19.2 +/- 4.2 mm Hg (P <.095) in primary gaze and 25.1 +/- 5.9 mm Hg (P <.023) in upgaze. At 2 to 4 months following injection, the mean IOP was 19.3 +/- 3.9 mm Hg (P <.044) in primary gaze and 27.7 +/- 8.5 mm Hg (P <.024) in upgaze. Six patients indicated improved ocular deviation, which was associated with a lowering of IOP. Two patients indicated no change in IOP or strabismic deviation following botulinum A toxin injection.

CONCLUSIONS

Botulinum A toxin injections cause a secondary effect to lower IOP in patients with restrictive strabismus associated with thyroid-related orbitopathy.

Thyroid-associated ophthalmopathy: clinical features, pathogenesis, and management

Thyroid-associated ophthalmopathy (TAO) is a progressive eye disorder characterized by immune-mediated inflammation of the extraocular muscles and orbital connective tissue. TAO is linked, in a unique way, with thyroid autoimmunity, in particular Graves' hyperthyroidism. Our working hypothesis for the pathogenesis of TAO is that recognition of a thyrotropin receptor (TSHR)-like protein in the orbital preadipocytes by antibodies may be the initial event leading to homing of lymphocytes into the orbital tissues. In the course of thyroid inflammation, antibodies and T cells reactive against G2s expressed in thyroid membranes cross-react with the protein in the eye muscle fiber, leading to eye muscle damage and dysfunction. Those patients with anti-G2s antibodies develop ocular myopathy. Antibodies against flavoprotein, the 64-kDa protein, which are produced in the context of eye muscle fiber damage and mitochondrial rupture, are sensitive markers of immune-mediated fiber necrosis in patients with ophthalmopathy but do not directly damage the eye muscle. Antibodies against type XIII collagen, which is localized in the plasma membranes of orbital fibroblast, may be a new marker for the congestive ophthalmopathy subtype of TAO. The measurement of antibodies against key eye muscle and orbital connective tissue autoantigens may have a role in the management of active ophthalmopathy and its prediction in patients with Graves' hyperthyroidism.

T cells and fibroblasts in affected extraocular muscles in early and late thyroid associated ophthalmopathy

AIM

To determine whether there are differences in the lymphocytic cell infiltrate present in affected extraocular muscles (EOM) during early and late stages of thyroid associated ophthalmopathy (TAO).

METHODS

17 biopsies of affected EOMs were collected from two groups of TAO patients (n=14): the first of five patients with early, active TAO, and the second of nine patients with late, inactive TAO. The control group was of EOM biopsies taken from 14 non-TAO patients undergoing squint surgery. Immunohistochemical analysis was undertaken using the relevant monoclonal antibodies and an avidin-biotin system and the three groups compared.

RESULTS

Both CD4+ and CD8+ T cells were found in the cellular infiltrate in early, active TAO specimens which were much less evident either in late, inactive stage disease or in control tissue. There was also a significant increase in both CD45RO+ and CD45RB+ cells and macrophages in early TAO compared with the others. Increased expression of HLA-DR antigen by interstitial cells including fibroblasts was detected in both early and late disease but the EOM fibres remained morphologically intact and did not express MHC class II antigens at any time.

CONCLUSION

These results demonstrate that T cells are only significantly present in early disease but increased HLA-DR antigen expression on fibroblasts is observed at all stages. This suggests that T cells are much more involved in the early than the later stages of the disease process and that early activation of fibroblasts occurs. Early intervention with immunosuppressive therapy to downregulate cytokine production by T cells may significantly influence the sequelae caused by EOM fibrosis.

Retro-orbital autoimmunity

What causes Graves' ophthalmopathy is still a mystery, but the disease process results from a complex interplay of genetic and environmental factors. Genes such as those encoding for human leukocyte antigens, cytokines or putative target antigens may determine a patient's susceptibility to the disease and the disease severity, but environmental factors may determine its course. During the last 5 years, significant progress has been made towards a more in-depth understanding of the initiating events of the orbital immune process which occur in the context of autoimmune thyroid disease. Once established, the chronic inflammatory process within the orbital tissues appears to take on a momentum of its own. The work of many investigators has recently helped to extend our knowledge about the effector and target cells, and their reciprocal interaction, in the evolution and perpetuation of the orbital immune process. This chapter's focus is on the more recent aspects of retro-orbital autoimmunity, discussing new developments concerning orbital T-cell repertoires, candidate orbital antigens, potential target and effector cells, and their role in the extrathyroidal manifestations of autoimmune thyroid disease.

Involvement of the orbital fibroblast and TSH receptor in the pathogenesis of Graves' ophthalmopathy

Our concepts and understanding of the etiology, evolution, and propagation of Graves' ophthalmopathy have become much more sophisticated that they were 10 years ago. Given our current state of knowledge, the following scheme for the pathogenesis of Graves' ophthalmopathy can be proposed. Circulating T cells in patients with Graves' disease, directed against an antigen on thyroid follicular cells, recognize antigenic epitopes that are shared by tissues contained in the retroorbital space. Of the cell types residing in these tissues, fibroblasts are most likely to act as both target and effector cells of the retroorbital immune process. This includes those fibroblasts present in the perimysium of extraocular muscles, which do not appear to be immunologically different from fibroblasts located in the retroorbital connective tissue. By contrast, convincing evidence implicating the human extraocular myocyte itself (rather than the tissue conglomerate of extraocular muscle) as a primary target in GO remains to be demonstrated. Together with adipocytes, fibroblasts may also serve as target and effector cells in pretibial myxedema. How autoreactive T cells escape deletion by the immune system and come to be directed against a self-antigen presented by cells residing in the thyroid gland and extrathyroidal locations is unknown. T cells are recruited to and infiltrate the orbit via certain adhesion receptors, which may also play a costimulatory role in T cell activation and facilitate antigen recognition. Analysis of variable region gene usage of the T cell antigen receptors in retroorbital T cells of patients with active GO reveals limited variability, suggesting that antigen-driven selection and/or expansion of specific T cells may occur early in the evolution of GO.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of thyrotropin binding sites in orbital connective tissue: possible role in the pathogenesis of thyroid-associated ophthalmopathy

The thyrotropin (TSH) receptor is implicated in the pathogenesis of thyroid-associated ophthalmopathy, but its presence has not been demonstrated in orbital tissues. Binding of 125I-TSH to porcine orbital connective tissue and thyroid membranes was studied. At pH 7.4 125I-TSH bound to thyroid membranes with high affinity, but not to orbital connective tissue membranes. At pH 5.2, binding of 125I-TSH was evident on thyroid and orbital connective tissue membranes, although of low affinity. Low affinity binding at pH 5.2, could also be demonstrated on orbital fibroblasts; binding to membranes prepared from extraocular muscle, or from abdominal adipose tissue was not detected. Immunoglobulin G from healthy subjects (n = 17) and patients with ophthalmopathy (n = 19) were tested for inhibition of 125I-TSH binding to membranes. As anticipated patients' IgG exhibited greater inhibition than normal controls with thyroid membranes at pH 7.4 (p < 0.001). Similar results were obtained with thyroid membranes (p < 0.05), and orbital connective tissue membranes at pH 5.2 (p < 0.001). Significant correlations were found when percentage inhibition by IgG was compared between: thyroid membranes at pH 7.4 and thyroid membranes at pH 5.2, orbital connective tissue membranes at pH 5.2 and thyroid membranes at pH 7.4, and orbital connective tissue membranes at pH 5.2 and thyroid membranes at pH 5.2. A low affinity TSH-binding site is present in orbital connective tissue, and is recognized by IgG's from patients with ophthalmopathy which suggests that TSH receptor antibodies may target the orbit.

Pathogenesis of thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy is a complex disease whose pathogenesis is thought to be autoimmune. Evidence has accumulated to implicate lymphocytes, antibodies, and cytokines targeting the orbital tissues whose structural and functional characteristics are unique. Epitope sharing between the orbit and the thyroid is the likely explanation for the close association of ophthalmopathy with autoimmune thyroid disease. Environmental and other nonautoimmune factors are identified that also play a part in the initiation or perpetuation of the disease process.