Concurrent thyrolipomatosis and thymolipoma in a patient with myasthenia gravis: a case report and review of the literature

Abstract

We present a case of a man with a background of myasthenia gravis who presented with a neck lump, which was diagnosed as thyrolipomatosis in continuity with a very large thymolipoma. Following removal of these lesions, the patient’s myaesthenic symptoms improved. While thymolipomas are often seen in the context of myasthenia gravis, thyrolipomatosis is a rare entity and to our knowledge the concurrent finding of both lesions with myasthenia gravis has never been reported. We highlight the important imaging features of both entities and the clinical importance of recognising them.

Case details

A 40 year-old man, who had been recently diagnosed with grade IV seropositive myasthenia gravis, attended the ENT department with a neck lump. Incidentally, a thyroid lesion was found on a CT study performed to look for a thymoma which can be associated with myasthenia gravis. On closer questioning he had been aware of a neck lump for some time and felt it was slowly growing. On examination, it was felt to be arising from the left lobe of the thyroid, and the patient had no clinical features to suggest thyroid dysfunction. He was taking oral pyridostigmine and prednisolone for symptom control of his myasthenia but was otherwise well.

Ultrasound of the thyroid gland demonstrated an unusual poorly delineated thyroid nodule with focal hyper-echogenicity, suggestive of adipose tissue, which involved both lobes extending across the isthmus and appeared contiguous with a structure of similar echogenicity extending inferiorly into the mediastinum (Figure 1). On review of the initial CT, the thyroid lesion was seen to contain fat (Figure 2) and a very large fat-density lesion within the chest, initially reported as a giant pericardial fat pad, was considered to represent a giant thymolipoma (Figure 3). MRI confirmed the presence of adipose tissue within both lesions (Figure 2) and a provisional diagnosis of concurrent thyrolipoma and thymolipoma was reached.

Figure 1 .

Transverse ultrasound image demonstrates a hyperechoic mass lesion in the expected location of the thyroid gland

Figure 2 .

Thyrolipomatosis on CT and MRI. Axial CT (top right) demonstrates ill-defined low density within the thyroid, and corresponding axial T2 (top right), T1 (bottom left) and STIR (bottom right) show fat signal within the same region.

Figure 3 .

Thymolipoma. Chest x-ray (left) demonstrates enlarged mediastinal shadow on the left, and axial CT image (right) confirms very large predominantly fat-density lesion within the left hemithorax with streaks of internal soft tissue density.

The case was discussed in the thyroid and lung multidisciplinary meetings. Based on previous randomised clinical trial evidence demonstrating the value of thymectomy in non-thymomatous myasthenia gravis, and given the contiguity of the thyroid lesion involving both lobes with the thymic lesion, a decision was made to remove both lesions with a total thyroidectomy and radical thymectomy.¹ After preoperative optimisation with IVIG, the procedure was performed by the ENT and cardiothoracic surgeons. A vertical midline cervical incision continuous with the sternotomy allowed a 2.4kg mass from the chest (Figure 4) to be excised. The lung on this side had completely collapsed with the pressure of the lipomatosis. He was initially managed in the intensive care unit with IVIG support, and then transferred to the ward on the fifth postoperative day. He made a full recovery and was discharged within 2 weeks.

Figure 4 .

Left: Neck dissection – thyroid mass continuous with a chest mass. Right: 2.4kg lump excised from the chest as a single lesion.

Microscopy of the thyroid nodule demonstrated thyroid follicular cells surrounded by adipose tissue, with a relatively bland appearance to both cell types (Figure 5). The chest mass was shown to be a lipomatous tumour, with mature lobules of adipose tissue separated by fibrous septae with intervening foci of mature thymic tissue. No evidence of malignancy was found in either lesion, and a diagnosis of thyrolipomatosis and thymolipoma was made.

Figure 5 .

Microscopy. Left: Thyroid follicular cells surrounded by mature adipocytes. Right: Higher magnification demonstrates the bland nature of the thyroid follicular cells and mature adipocytes.

In the postoperative period, there was complete remission of the patient’s myasthenic symptoms, and his prednisolone dose was tapered from 10mg to 6mg a day. An iatrogenic vocal cord palsy was treated with collagen medialisation of the right vocal cord. His myasthenic symptoms are currently well controlled and he has returned to full activities.

Discussion

Thyrolipomatosis and thymolipomas are both rare lesions. Thyrolipomatosis is one of a small spectrum of fat-containing thyroid entities, and to our knowledge there have been only 12 previously reported cases in the literature.² It is characterised by diffuse adipose tissue throughout the thyroid gland. It does not appear to increase the risk of malignancy, but fat-containing thyroid neoplasms require exclusion.² The radiological features centre, as in this case, around the demonstration of macroscopic fat in the thyroid gland – hyperechogenicity on ultrasound, hypodensity on CT and increased T1 and T2 signal with suppression on fat-saturated sequences on MRI.

Thymolipomas were first described in 1916 and are also rare, with a reported incidence of 0.12 per 100,000 people per year.³ They account for up to 9% of thymic tumours, and there is a reported but difficult to quantify association with myasthenia gravis and other autoimmune conditions, with some reports of clinical improvement following resection. They tend to be asymptomatic despite their potentially enormous size but can occasionally cause respiratory symptoms. The classical radiological findings are of an enlarged mediastinal shadow on chest x-ray and a fat-attenuation mass within or abutting the mediastinum on CT with streaks of soft tissue within the fat, as seen in our case. Unfortunately they cannot be definitively distinguished from malignant tumours (such as liposarcoma) radiologically and therefore require excision, but they do not tend to recur or metastasise.⁴

The combination of thyrolipomatosis and thymolipoma has not, to our knowledge, been previously described and, similarly, a contiguous lesion has not been reported. However there are previous reports of a thyrolipoma associated with a thymolipoma. Trites describes a 33-year-old male patient who also had a pharyngeal lipoma, and he posits a developmental cause as all three structures arise from the primitive foregut.⁵ Our case is unique in the added factor of myasthenia gravis, the causative relationship to which is not clear, but implies that the myasthenia gravis is likely a secondary or paraneoplastic phenomenon, particularly given the improvement of symptoms following resection.

Conclusion

This is a rare case of concurrent and contiguous thyrolipomatosis and thymolipoma, associated with myasthenia gravis. Our patient improved following resection of both tumours, and this would suggest that understanding the imaging findings of these lesions is paramount in appropriate treatment of these patients.