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  • The myasthenia scale of Millichap and Dodge

    2024-04-02

    The myasthenia scale of Millichap and Dodge was used in our study for ease of comparison of our retrospective data with previous paediatric studies. Our results, with 30% achieving complete remission and 95% achieving at least some degree of clinical improvement following thymectomy, are comparable to or better than the previous studies. The factors contributing to the generally good outcome in our series may include restricting thymectomy to children with AChR antibody-positive generalised disease, a short interval between disease onset and surgery, a transternal surgical approach that allows complete visualisation and removal of the thymus, and in some patients, the effect of additional immune-modulating agents following thymectomy. Early thymectomy has been shown to be associated with a better remission rate [27]. In a paediatric study including 85 thymectomised patients, three years after the surgery, 61% of the group who underwent thymectomy within 12months of disease onset were in remission, compared to 24% of the group who had waited longer than 12months [17]. Nonetheless, remission has also been noted in cases with delayed thymectomy [2], [28]. Patients with MuSK ARL 67156 trisodium salt or congenital myasthenic syndrome (CMS), in whom thymectomy is generally considered to be ineffective, might have been unknowingly included in some of the older studies, adversely affecting the results. ‘Late-onset’ CMS related to mutations in the Rapsyn (RAPSN) gene are known to have been initially diagnosed as ‘seronegative MG’ [29], [30] and undergone treatment with immune-modulating therapies, including thymectomy (Georgina Burke, personal communication). However, beneficial effects of thymectomy have also been reported in some seronegative patients; these patients may have carried low-affinity AChR antibodies, which are not detected by conventional assay [4]. Surgical technique may also influence disease outcome following thymectomy; incomplete removal of all thymic tissue being associated with the risk of poor disease outcome [21], necessitating repeat surgery in some instances [15]. Although less invasive techniques such as video assisted thoracoscopic thymectomy (VATS) are now resulting in comparable remission rates following thymectomy in adult MG [31], our group and others [13], [32] have consistently used a transternal approach in children to ensure complete thymic removal, which may have positively affected outcome. The effect of thymectomy is generally gradual and may take more than a year to fully influence disease severity [8], [18]. In our series, six patients received immune treatments other than steroids, five in the first 15months following thymectomy. In view of the time scale for thymectomy to have maximum effect, we would not necessarily consider a requirement for early additional immune-modulating therapy an indicator of thymectomy failure. In 3 patients, immune-modulating therapies were added within the first 6months following thymectomy. In another patient (Nr 18), treatment with high dose steroids, plasma exchange and rituximab became indicated because of severe nephrotic syndrome 3months after thymectomy; this resulted in resolution of the nephrotic syndrome and improvement in myasthenic symptoms [33]. In the remaining two patients, clinical improvement was observed following thymectomy but patients remained steroid dependent; immune-modulating therapy was added later as a steroid-sparing agent. In these patients, we recognise that the subsequent improvement could be attributed to the additional immune-modulating therapies alone, or the combined effect with thymectomy. In our series, more patients with thymic hyperplasia achieved remission compared to patients with a normal thymus. The child with thymoma had fluctuating symptoms postoperatively; azathioprine was added after 4months and he remained steroid dependent. The association between thymic hyperplasia and outcome of thymectomy is still unclear. Although it has been suggested that thymic hyperplasia may be associated with a more favourable response to thymectomy [34], this is not supported by some of the previous paediatric series [2], [17], [18], [23], [24]. Prior steroid exposure may potentially modify the thymic histopathology, further complicating histopathological interpretation. On the other hand, adults and children with thymoma have been reported to be less responsive to thymectomy [2], [3].