NOV 24, 2011
Graves’ orbitopathy has been subtyped according to the prevalence of fat and muscle volume (MV) increase on MRI or CT. In this prospective study, the authors tried to further characterize these subtypes in 95 consecutive Caucasian patients with presumed Graves’ orbitopathy who presented to their thyroid eye clinic. They found that four patient subgroups could be distinguished, including one with patients who show an increase in both fat and muscle volume and one with no increase in fat or muscle volume.
Classically, Graves’ orbitopathy is divided into two radiological entities: type I (lipogenic) and type II (myogenic). Type I is characterized by orbital fat increase and soft tissue edema and may result in the rare entity of globe subluxation. It is far more common in young women. Type II is characterized by extraocular muscle enlargement that may result in restrictive myopathy of less than 20 degrees and compressive neuropathy (30 percent); it is only slightly more common in males. Some add two other groups: one with no increase in either and one with an increase in both.
Using their own validated CT-based method for the calculation of orbital soft tissue volume, the authors found that 25 percent of the patients had no fat or muscle volume increase and 8 percent had an increase in both. Type II (muscle volume increase) was the most common (70 percent of patients) and was associated with older age, higher thyroid-stimulating hormone–binding inhibitory immunoglobulin TBII, more proptosis and restriction of motility. An increase in fat volume (type I) was seen in only 5 percent of patients and was characterized by proptosis.
The patients with no increase in fat or muscle volume may all have had Graves’ disease, but the authors did not mention if they all were examined for thyroid stimulating hormone or how Graves’ disease was defined. Patients with no enlargement might have had Graves’ disease without orbitopathy. Even euthyroid patients may have some specific orbital or eyelid signs, such as eyelid retraction. The authors also did not mention the clinical presentation or the course of the disease in their patients, or parameters, such as color vision and visual fields. It is also unclear when the Kruskal-Wallis statistical test was employed.
These findings raise intriguing questions regarding the pathogenesis of Graves’ disease and may shed a different light on diagnosis and treatment. However, further studies are required to find more differentiating features between the different groups.