Background

Myasthenia gravis (MG) is an autoimmune disease mediated by antibodies against the acetylcholine receptor (anti-AchR), the muscle-specific kinase (anti-MuSK) or other proteins associated with the postsynaptic membrane of the neuromuscular junction. The main clinical manifestations are muscle weakness and fatigue (which worsens after repeated muscular effort) [1]. Symptoms may be exclusively ocular (ocular MG), with diplopia and/or ptosis, or generalized (generalized MG), involving weakness of the limb, trunk, dysphagia, dysphonia or respiratory insufficiency.

Several drugs, notably sedatives and some antibiotics may trigger an exacerbation of MG and hence they should be used with caution, weighing up the risks and benefits in each patient. Some over-the-counter pharmacy products may also contain substances that worsen the symptoms of MG.

Melatonin (N-acetyl-5-methoxytryptamine) is a hormone that is primarily released by the pineal gland and which is involved in regulating the sleep-wake cycle. Levels of melatonin decrease with age and this could be one of the reasons why older adults are more prone to suffer from insomnia [2].

Because exogenous melatonin binds to the same receptors as the endogenous form, it is effective as a treatment for insomnia [2,3,4,5]. Melatonin supplements are well tolerated, with few side effects (headache, dizziness, nausea and drowsiness) and no problems of tolerance. This is an important advantage over other sedative-hypnotic drugs such as antihistamines, antidepressants and benzodiazepines.

In addition to regulating the sleep-wake cycle, it is becoming increasingly clear that melatonin can also have immunomodulatory effects. There are now several reports describing the onset or exacerbation of autoimmune disorders after taking supplements containing melatonin or one of its analogues [6,7,8,9]. Here we report the first three cases of patients with a clinical exacerbation of MG after administration of melatonin, (Table 1).

Table 1 Clinical characteristics of the patients
Full size table

Case presentation

We identified three patients in our referral centre from 2014 to 2019 who presented a clinical exacerbation of myasthenia gravis after starting melatonin treatment.

The study was approved by the Ethics Committee of Bellvitge University Hospital (PR162/20).

Case 1

The patient was a 49-year-old woman with generalized MG, negative for both anti-AchR and anti-MuSK antibodies and with no signs of thymoma on chest computed tomography (CT) scan. She was being treated with prednisone 10 mg/48 h and mycophenolic acid 360 mg/12 h, and had been asymptomatic for MG for over a year (ADL: 0, MGFA: 0, MG-PIS: PR (https://myasthenia.org/What-is-MG, Appendix in Table 2, Appendix in Table 3 and Appendix in Table 4). Due to persistent insomnia she began taking an over-the-counter melatonin supplement, 1.95 mg/day. A few weeks later she presented with constant binocular diplopia, dysphonia and generalized tiredness (ADL: 5), with no other identifiable triggers. Due to the temporal relationship between her symptoms and starting the melatonin supplement she was advised to cease taking it. She did not start treatment with pyridostigmine due to gastrointestinal inotolerance. As little improvement was initially observed in the next week, the dose of prednisone was increased to 50 mg/24 h. The patient subsequently showed a progressive improvement and recovered her baseline status at five weeks.

Case 2

The patient was an 87-year-old man with generalized MG, positive for anti-AchR antibodies, without signs of thymoma on chest CT scan. The medical history included surgery for glottic carcinoma, with residual dysphagia. His MG symptoms remained stable for over six months with prednisone 15 mg/48 h (ADL 0, MGFA: I, MG-PIS: MM1), and subsequently the dose was reduced to 10 mg/48 h due to irritability. With this change the patient remained stable for six weeks. Due to difficulty falling asleep he began to take melatonin 1.9 mg each night. Two days later he presented constant ptosis during the day and increased dysphagia (ADL: 4) without evidence of glottic carcinoma recurrence. After returning to the initial dose of prednisone, plus pyridostigmine as needed, his symptoms stabilized but did not improve. He was advised to cease the melatonin, but preferred to continue taking it due to the positive effect on his insomnia.

Case 3

The patient was a 35-year-old man diagnosed with generalized MG, positive for anti-AchR antibodies. Chest CT scan showed no thymoma, the patient having undergone a thymectomy (thymus hyperplasia). He was being treated with pyridostigmine 60 mg/4 h, mycophenolic acid 540 mg/day in two divided doses and prednisone 15 mg/48 h. With this treatment his MG symptoms were well controlled for over a year, and he only presented intermittent diplopia during the day (ADL: 2, MGFA I; MGFA-PIS: MM3). After a period of six months during which he suffered from insomnia, he began treatment with doxylamine, with no worsening of his MG, but this treatment was suspended due to ineffectiveness. One month after cessation of doxylamine, and due to the persistence of his insomnia, he began taking an over-the-counter supplement containing melatonin 1.95 mg/day. One week later he presented a progressive worsening of his MG, with no other identifiable trigger. Neurological examination revealed facial muscle weakness, difficulty chewing and appendicular weakness and fatigability (ADL: 7 points). He ceased taking melatonin, but in the absence of improvement over the next few days the dose of prednisone was increased to 30 mg/24 h. The patient’s ADL score remained at 6 for the following month and it was thus decided to administer intravenous immunoglobulins at a dose of 0.4 g/kg/day for 5 days. Two weeks after this treatment the patient experienced a marked clinical improvement, and at one month he had returned to his baseline status.

Discussion and conclusions

Melatonin and its agonists are widely used in the treatment of insomnia, which has a prevalence in the general population of 30–50% [10, 11]. In some European countries, melatonin supplements may be purchased over the counter (without prescription) in pharmacies, provided that the tablets contain less than 2 mg of the active substance. Importantly, the use of melatonin among the general population has increased in recent years due to it being indicated as a treatment for REM sleep disorders [12], jet lag, chronic migraine, cluster headache [13] and chronic pain [14].

Another less well known function of melatonin is as an immune system regulator, it modulates both the innate and specific immune response. Here we describe three patients with MG whose symptoms were well controlled with a low maintenance dose of corticosteroids (plus, in two cases, with mycophenolic acid as a steroid-sparing agent) and who presented a worsening of their condition within days or weeks of starting melatonin. The fact that the exacerbation of MG symptoms manifested some days after starting melatonin suggests a mechanism of action other than its direct sedative effect, which would have occurred within an hour of taking it [12].

All patients were asked about recent infectious diseases, increased stress, surgery, changes in their usual treatments or other supplement intake. Although in Patient 1 there was no other identifiable trigger, possible triggers were present in the other two cases: a reduction in the maintenance dose of prednisone in Patient 2 and prescription of doxylamine in Patient 3. However, Patient 2 remained stable for six weeks with the reduced dose of glucocorticoid, while Patient 3 showed no change in symptoms in the month following cessation of doxylamine. By contrast, both patients presented neurological deterioration several days after beginning to take a melatonin supplement, hence the suspicion that melatonin had a role in their clinical exacerbation.

In none of the three patients did an increased dose of corticosteroids lead to meaningful clinical improvement in the next month. Patient 1 took five weeks to achieve symptom remission following cessation of melatonin, while Patient 3 was administered intravenous immunoglobulins four weeks after ceasing melatonin because his daily activities were still limited as a result of the symptom exacerbation. Patient 2 only experienced a worsening of his ocular symptoms and otherwise remained stable despite continuing to take the melatonin supplement. This may be because his MG symptoms were previously well controlled with a low dose of prednisone, whereas the other two patients (1 and 3) were also receiving an immunosuppressant, whose action was likely to have been affected by the introduction of melatonin.

The immunomodulatory effect of melatonin has become better understood in recent year. On the one hand melatonin seems to downregulate the overreaction of the innate immune response and promotes the adaptive immune activity. It enhance B lymphocytes production and stimulates T helper (Th) cell activity [6, 15] that play an important role in MG pathogenesis. On the other hand, it also increases the production of proinflammatory interleukin (IL). IL2 can induce resistance to glucocorticoids (both endogenous and exogenous) in Th cells [16] and may explain why these patients did not respond to an increased dose of glucocorticoids, when previously they had done. This effect is likely to persist beyond the plasma elimination [17] of exogenous melatonin, which would also account for why these patients did not improve until several weeks after cessation of melatonin.

The role of melatonin in autoimmune disorders such as rheumatoid arthritis and Crohn’s disease has previously been reported [15, 16, 18]. In the literature there are three case reports of autoimmune hepatitis after treatment with melatonin: two involved patients with no prior autoimmune disease [8, 9], while the other was a patient with primary sclerosing cholangitis and ulcerative colitis whose treatment included mycophenolate mofetil [7]. The three cases described here are the first in which melatonin administration was associated with symptom exacerbation in patients with MG who were being treated with immunosuppressants.

The use of melatonin in patients with MG, whether ocular or generalized, may trigger exacerbations of the disease, probably due to an upregulation of adaptative immune response and an interaction with treatment involving corticosteroids and other immunosuppressants. We consider that melatonin should be administered with caution in patients with MG, especially those who are being treated with immunosuppressants.