MG is a relatively uncommon autoimmune disease characterized by muscle weakness. The thymus gland plays an important role in the pathogenesis of MG. Approximately 90% of patients with MG display thymic abnormalities; namely TLH (70 %) and thymoma (20%) [13].
The differentiation of thymoma from TLH is critical in the evaluation of surgical treatment. Thymectomy is strongly recommended in all thymoma cases, whereas the surgical indication in hyperplasia cases should be only considered when conservative treatments are ineffective [14].
Thymoma is seen as a focal soft tissue mass, while, thymic hyperplasia shows a diffuse symmetric enlargement of the gland. However, it is difficult to differentiate the two conditions on CT because of high interrater variation [15].
TLH may display a focal soft tissue mass; in contrast, thymoma may demonstrate diffuse enlargement in both lobes. In these cases, CT results in indeterminate findings, whereas chemical shift MRI can differentiate the two pathologies by detecting fat in tissue showing signal intensity loss on opposed-phase imaging compared to in-phase imaging [16]. Moreover, it can specifically outline the boundaries of thymoma and its relationship with surrounding tissues [17].
Demographics of our study were similar to those reported in several previous studies as the mean age was mostly in the third and fourth decade of life [18].
The results of our study showed that ocular muscles was the commonest affected group followed by bulbar muscles. The current evidence in literature points out that ocular muscles are the first group of muscle to be affected in patients with MG presenting with squint or ptosis, followed by bulbar muscles and limb muscles [19, 20].
Our findings showed that there was no significant difference in age, gender, or affected muscles based on histopathology. These results were inconsistent with the results of many studies which stated that patients with thymoma are significantly older than patients with TLH [21].
In our study, histopathology revealed that 60% of the included patients had thymoma and 40% were diagnosed with TLH. This finding was similar to literature as it is estimated that prevalence of thymoma in patients with MG is 62 % versus 38% for TLH [21].
Our MRI findings showed that the most common shape of thymus gland in MG patients was convex gland, followed by round, oval, irregular, and lobulated while pyramidal shape was the least common. We also found that TLH was more common to be convex in shape and thymoma was more likely to be round or irregular.
Our findings were supported by results of similar studies. Inaoka et al demonstrated in a study of 41 patients with MG (23 with TLH and 18 with thymoma) that in the hyperplasia group, there was convex enlargement of the gland without lobulation in 73% and with lobulation in 26%. In the thymoma group, the thymus gland was round in 83.3 % of the cases, had diffuse enlargement without lobulation in 11.1% and had an irregular shape in one patient [12].
All patients in the thymoma group in our study showed no decrease in the signal intensity on the out-phase image compared to the in phase (Figs. 3, 4, 5). Accordingly, we found that chemical shift MRI had 90% sensitivity and 100% specificity in detecting thymoma with overall diagnostic accuracy of 93.3%. These findings are similar to the ones reported by Tuan et al., who stated that opposed phase imaging showed sensitivity 97.0% and 90% specificity in detection of thymoma in MG patients [21].
Such high reliability of chemical shift MRI in detecting fat in tissue and discriminating thymoma from TLH in our study is comparable to that of several other studies. Popa et al. also demonstrated that none of the patients in the thymoma group showed a decrease in the signal intensity of the thymus gland [22].
In our study, all the patients in the hyperplasia group demonstrated a homogeneous decrease in the signal intensity of the thymus gland on the opposed-phase image relative to the in-phase image (Figs. 6, 7) except for two cases; an 18-year-old male and a 21-year-old female. Their chemical shift MRI showed no signal drop on the opposed phase images and their CSR values were 1.01 and 1.10, respectively. These cases were proven to be TLH with minimal fat infiltration on histopathology. These findings were consistent with Ackman et al. study illustrating a pathologically proven case of normal thymus in a 21-year-old woman that displayed no fat replacement on the opposed-phase chemical shift MRI with CSR = 1.1 [23].
Priola et al. also reported a true hyperplasia case in a 60-year-old female being treated with corticosteroids without fat infiltration on chemical shift MRI [24]. Furthermore, Phung et al. reported a case of a 22-year-old woman with MG that showed no drop in signal intensity on the opposed phase images and was diagnosed as thymic tumor. Post-operative histopathological findings proved TLH with only a few fat cells, which was not sufficient to detect the decrease of signal intensity on chemical shift MRI [13].
In our study, CSR showed 100% sensitivity by using >0.85 as a cut off value for diagnosis of thymoma with specificity 83.3%. Our results were consistent to Priola et al. study which included 83 patients diagnosed as MG who underwent surgical intervention and were assessed using MRI preoperatively; their results showed that MRI had a sensitivity 100% and specificity 96.7% at cutoff point >0.85 CSR [25].
Our findings also more or less agreed with a cross-sectional study of 53 participants conducted in Vietnam where 53 MG patients were included; comparison between CT and MRI findings showed that MRI had sensitivity 100% and specificity 95% when using >0.75 as a cutoff value of MRI chemical shift [21].
Limitations of our study included the small sample size. A larger number of patients is necessary to clarify the utility of the chemical shift MRI imaging for differentiating thymic hyperplasia from tumors of the thymus gland in MG patients [
25
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Also, in few cases especially in young patients and early adulthood, chemical shift MRI may be not solely enough for differentiation between TLH and thymoma. In such cases, complementary Diffusion weighted MRI would be helpful to allow proper diagnosis. Furthermore, overlapping CSR values between normal or hyperplastic thymus and tumors can be expected in early adulthood as cases of lipid-poor normal or hyperplastic thymus may occur.
