extraocular muscles were found to be longer in GO patients than in controls, decreasing to near-normal values after immunosuppression (23). Hiromatsu et al.(24) measured STIR based signal intensity ratios on MR images and observed a higher signal in GO patients than in controls. Using a cut-off value of 1.9 for the signal intensity ratio of eye muscles, he reports a þve PV of 69% and a –ve PV of 86% for the outcome of treatment with intravenous pulses of methylprednisolone. Another study in a small number of patients found the response to steroids better correlated to the clinical activity than to the T2 relaxation time (25).

We also measured longer T2 relaxation times in all extraocular muscles (except the lateral recti) of GO patients than in controls (26). The longest T2 relaxation time observed in any of six extraocular muscles (excluding the lateral muscles for technical reasons) was longer in patients than in controls (160 38 vs. 102 12 msec, p < 0.001). Using a cut-off value of 130 msec (derived from a receiver–operator characteristics curve), the longest T2 relaxation time observed in any of six extraocular muscles had a þve PV of 64% and a – ve PV of 92% for the outcome of retrobulbar irradiation (26). Our results are thus in good agreement with those of Hiromatsu et al.(24). In contrast to other parameters of disease activity in GO, MRI has a higher negative than positive PV. Apparently, orbital MRI is more reliable in detecting the inactive stage of GO than the active edematous stage. This property may be of special advantage when combined with other disease activity parameters having a high þve PV. Other advantages of MRI are its lack of ionizing irradiation and its clear delineation of orbital structures. However, the MRI scanning protocol will need adjustment in most centers in order to calculate the T2 time in eye muscles, since the method to measure the signal intensity varies with the MRI equipment.

URINARY GLYCOSAMINOGLYCANS

The cytokine-induced accumulation of hydrophilic glycosaminoglycans (GAGS) and its fractions chondroitin sulphate A,

dermatan sulphate, and hyaluronic acid in retrobulbar tissues lead to edema. An increased GAG excretion in the urine might thus reflect the activity of the eye disease. Indeed, higher levels of urinary GAG have been found in patients with GO compared to controls and to patients with Graves’ hyperthyroidism without eye changes (27). The data allowed discrimination between active and inactive GO, which, however, could not be confirmed by other investigators (28). When comparing responders and nonresponders to radiotherapy, we found no significant difference in pretreatment levels of urinary GAG excretion, or in its fractions measured by HPLC (29). We conclude that measuring urinary GAG has very little relevance in predicting outcome of immunosuppression in GO. It could be, however, that measurement of plasma GAG levels still has some prognostic value (30), although no relation exists between plasma GAG and orbital tissue GAG concentrations in GO patients (31).

SERUM CYTOKINES

Infiltrating lymphocytes of orbital tissues produce a variety of cytokines. Some (such as IFNg, TNFb, and IL-1) stimulate GAG production by fibroblasts, others (like IL-1RA) inhibit GAG production (32–34). We measured serum concentrations of IL-1RA, sIL-2R, IL-6, sIL-6R, TNFaRI, TNFaRII, and sCD30 in patients with untreated GO. All cytokine levels were significantly elevated compared with healthy controls, except for IL-1RA, which was similar in both groups (35). Cytokine levels did not correlate with duration, activity, or severity of GO. However, backward logistic regression indicated that IL-6, sCD30, and TNFaRI together had some value for predicting therapeutic outcome to orbital irradiation. As determined from a receiver–operator characteristics curve of these three cytokines combined, the area under the curve has a disappointing low-value of 0.69. The contribution of measuring these cytokines to prediction is thus low, and probably not worth the effort. Hofbauer et al. (36) reported lower IL-1RA serum concentrations in smokers than in nonsmokers

among GO patients, and higher baseline IL-1RA levels were associated with a response to radiotherapy. We could not confirm these findings.

Cytokines may also induce the expression of adhesion molecules on orbital fibroblasts (37). Serum concentrations of sICAM, sVCAM, and sELAM are indeed higher in GO patients than in controls, but again have a too low predictive value for the outcome of immunosuppression to warrant their measurements (38).

SERUM THYROID STIMULATING

IMMUNOGLOBULINS

The full-length thyrotropin (TSH) receptor is being expressed at the mRNA and at the protein level in Graves’ orbital adipose=connective tissues (39). The TSH receptor is thus a potential autoantigen in GO, and TSH receptor antibodies may play a role in the immunopathogenesis of the eye disease. Some but not all studies report a direct relationship between the serum concentration of TSH receptor antibodies and the severity of GO. In a series of patients with untreated moderately severe GO who had been euthyroid for at least 2 months, we found a highly significant correlation between the CAS and serum concentrations of TSH receptor antibodies measured either in a competitive binding assay (TBII, r ¼ 0.54) or in a cAMP assay (TSI, r ¼ 0.50) (40). In view of the observed relationship, we evaluated if TSH receptor antibodies might have predictive value for the outcome of immunosuppression. There were, however, no baseline differences in serum TBII and thyroid stimulating immunoglobulins (TSI) between responders and nonresponders to radiotherapy. The receiv- er–operator characteristics curves had a disappointingly low area under the curve (0.59 for TBII and 0.53 for TSI) (41).

CONCLUSION

It is clear that none of the discussed parameters of disease activity in GO has sufficient predictive value on its own to

warrant routine application in the selection of suitable patients for immunosuppression. Measurement of urinary GAGs, serum cytokines, or TSH receptor antibodies does not contribute to an accurate prediction. In our opinion, the disadvantages of orbital octreoscan outweigh its advantages, precluding its routine application as a predictive tool. This leaves us with the CAS and orbital ultrasonography and MRI. When applied to a large series of consecutive patients with moderately severe GO, the combination of duration of the eye disease (cut-off value < 18 months), CAS (cut-off value 4), and orbital A-mode echography (cut-off value of < 30% for the eye muscle with the lowest reflectivity) provided a rather accurate prediction for the outcome of radiotherapy with a þve PV of 79% and –ve PV of 89% (14). It is precisely in the group of patients with moderately severe GO that accurate prediction is most needed, because the patients with very severe GO characterized by optic neuropathy almost by definition will have active ophthalmopathy. It could well be that exchanging ultrasonography for quantitative MRI will further enhance the accuracy of prediction in view of the high –ve PV of MRI, whereas duration of GO and CAS have higher positive than negative PVs (26). The combination of duration, CAS, and MRI is also attractive in terms of cost-effectiveness: duration and CAS are assessed very quickly without expense, and MRI will also provide images depicting the extent of muscle and fat enlargement. Prediction, as always, remains a difficult topic, and a 100% accuracy rate will unlikely ever be reached.

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