Myasthenia gravis associated with thymoma
Myasthenia gravis is defined as an organ-specific autoimmune disorder. The disorder is mediated by anti-acetylcholine receptor (AChR) antibodies (Abs). However, in some cases, it may also be mediated by an anti-muscle specific receptor, tyrosine kinase (MuSK) Abs at the neurotransmitter junction. Common presentations in patients with MG-associated dysgeusia include a decrease in or loss of sweet taste perception. This may also be associated with an autoimmune mechanism.
Although studies have shown that the number of in which partial dysgeusia precedes motor symptoms is limited. In addition, no cases of increased perception of taste have been reported. This article reports the case of a 47-year-old Japanese woman who presented with a reduced perception of sweet taste and an increased perception of salty taste. The symptoms presented 3 months before the onset of her symptoms.
A 47-year-old Japanese woman presented with complaints of a sweet taste and an increased perception of a salty taste in December 2018. She otherwise had no past medical history, and her serum zinc and thyroid function were within normal range. According to the study, she presented to the hospital in April 2019 after she suffered from left-sided ptosis which worsened during the mornings. She also complained of arm weakness and her body temperature was 36.6°C. Additionally, her heart rate was 106 beats per minute, blood pressure 128/61 and 99% percutaneous oxygen saturation. On examination, the interior of her mouth and tongue were normal.
Neurological examination was significant for left-side ptosis and an impaired upward gaze of the left eye. Both her pupils were symmetric, 3mm in diameter. The manual testing score of the extremities was also normal. Ptosis in the left eye worsened, whereas the upward gaze test was normal. There was a significant improvement in the ice pack test. Furthermore, advanced ptosis was observed which suggested that her ptosis was bilateral. The patient further tested positive for the edrophonium test. Also known as the Tensilon test, the test is used to diagnose myasthenia gravis. A repetitive nerve stimulation test of the orbicular muscle of the eye, trapezius muscle and abductor muscle of the little finger at 3 Hz showed abnormal results.
The immunoassay results showed a high titer of AChR Abs. However, no MuSK Abs. Doctors further advised a chest X-ray and a CT scan of the whole body which was significant for an anterior mediastinal tumour. The findings are consistent with the diagnosis of thymoma and no ectopic thymoma. Based on these findings, the patient was diagnosed with myasthenia gravis. The patient’s quantitative MG score was 9, whereas her daily living profile of MG activities was 5. Doctors further classified the disease as IIa MG according to the criteria set by the Foundation of America.
Doctors further examined whether the cause of dysgeusia was myasthenia gravis
The study states that the “hemoglobin level was 10.0 g/dL; serum iron level, 33 μg/dL; ferritin level, 5.2 ng/dL; total iron-binding capacity, 423 μg/dL; vitamin B12 level, 241 pg/mL; and zinc level, 81 μg/dL. The thyroid function test results were in the normal range, as were the levels of serum creatinine, liver enzyme, and blood glucose; anti-SSA/Ro antibody results were negative”.
Doctors suspected that the patient may have iron deficiency anaemia and vitamin B12 deficiency. They put him on mecobalamin and iron supplementation. However, there were no signs of improvement in her dysgeusia. He was started on treatment for MG and pyridostigmine was initiated. In June 2019, the doctors performed an extended thymectomy. Histopathological analysis of the specimen confirmed the diagnosis of a type B2, noninvasive thymoma. 3 months after the surgery, her weakness worsened slightly and she lost her perception of sweet taste completely. However, the perception of sweet taste increased markedly. In September 2019, doctors added prednisolone and tacrolimus to her treatment regimen. Her partial dysgeusia showed significant improvement after this.
The case is a rare association between thymoma and myasthenia gravis. In this case, the patient developed an increased perception of salty taste and a reduced perception of sweet taste. The symptoms appeared 3 months before the onset of the motor symptoms. The increased perception of salty taste can be linked to an endocrinological mechanism. The report is suggestive of the fact that in such cases, MG should be considered a differential diagnosis in patients with partial dysgeusia in the absence of motor symptoms.
Source: American Journal of Case Reports