Neurology



Short Reports

Increasing incidence of elderly-onset patients with myasthenia gravis in athe local area of, Japan

Naoko Matsui M. D. 1, Shunya Nakane M. D. 1*, Yasushi Nakagawa M. D.2,

Kazuya Kondo M. D. 2, Takao Mitsui M. D. 1, Toshio Matsumoto M. D. 3,

Kokichi Arisawa M. D. 4, Ryuji Kaji M. D. 1

1 Department of Neurology, Institute of Health Bioscience,

Tokushima University Graduate School of Medicine, Tokushima, Japan

2 Department of Oncological and Regenerative Surgery, Institute of Health Bioscience,

Tokushima University Graduate School of Medicine, Tokushima, Japan

3 Department of Medicine and Bioregulatory Sciences, Institute of Health Bioscience,

Tokushima University Graduate School of Medicine, Tokushima, Japan

4 Department of Preventive Medicine, Institute of Health Bioscience,

Tokushima University Graduate School of Medicine, Tokushima, Japan

* Department of Neurology, Nagasaki Medical Center of Neurology,

Nagasaki, Japan, presently

Correspondence:

Shunya Nakane; 3-18-15 Kuramoto, Tokushima City2005-1 Shimogumi-go, Kawatana-cho, Higashisonogi-gun, TokushimaNagasaki, 770-8503859-3615, JAPAN; Phone: +81 (0880956) 633-720782-3121; Fax: +81 (0880956) 63383-72083710;

E-mail: spvn6ax9@sand.ocn.ne.jpsnakane@clin.med.tokushima-u.ac.jp

Keywords: Myasthenia gravis, retrospective study, elderly, incidence, thymectomy

Word count: 1640960

Figures: 1; Tables: 1

Supplementary data: appendix and table 1

Abbreviations: AChR: acetylcholine receptor; CSR: complete stable remission; CT: computed tomography; EOMG: elderly-onset myasthenia gravis; IVIg: intravenous immunoglobulin; IVMP: intravenous methylprednisolone; MG: myasthenia gravis; MG-ADL: Myasthenia gravis activities of daily living score; MGFA: Myasthenia Gravis Foundation of America; MM: minimal manifestations; MOMG: middle-aged onset myasthenia gravis; PI status: MGFA postintervention status; PR: pharmacological remission; TFH: thymic lymphofollicular hyperplasia; U: unchanged; YOMG: young-onset myasthenia gravis

Introduction

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction characterized by increased fatigability and weakness of skeletal muscles. It is usually caused by antibodies against the nicotinic acetylcholine receptor (AChR), which impair neuromuscular transmission.1 Previously, several groups have reported an increasing incidence of elderly-onset patients with MG in the United States and Europe.2-5 The increased number of elderly-onset MG patients in Japan has also recently been reported, but the a detailed epidemiological analysis has not been made.6

To review the clinical characteristics of elderly-onset MG patients, and to investigate an appropriate treatment approach for these patients in Japan, we conducted a retrospective, hospital-based epidemiological study on this disease during the years 1986 through 2006 in Tokushima University Hospital, Japan. In the present study, we surveyed the incidence with respect to the onset age over decades in Tokushima, Japan in Tokushima, Japan. To explore the significance of onset age among young-onset, middle-aged onset and elderly-onset MG patients, we evaluated clinical characteristics, therapy, and prognosis in each group.

Study area

Tokushima prefecture, located in the eastern part of Shikoku Island in Japan, covers an area with approximately 0.8 million inhabitants. Tokushima University Hospital is the only neuromuscular center in this area staffed by neurologists, and therefore, virtually all the MG patients have been referred to theis hospital. because of geographical reasons.

Patients and Methods

We retrospectively studied the clinical records of 112 consecutive patients with a confirmed diagnosis of MG, treated and followed in our institution from 1986 to 2006. The diagnosis of MG was based on the following findings: clinical features, neurological examination, and positive response to cholinesterase inhibitors, . This was further corroborated by abnormal results on electrophysiological tests (repetitive stimulation or single fiber electromyogram study) or detection of serum anti-AChR antibody.1 Data were registered from all cases of MG with onset of the disorder from 1971 through 2006 within the geographic area of Tokushima Prefecture. The age and sex distribution of this study population is virtually identical with that of the whole country. The background population by age and sex was based on the population census of ????.

We obtained name, gender and age at onset, and then classified patients into three groups, young-onset MG (YOMG, onset age ≤ 39), middle-aged onset MG (MOMG, onset age = 40-59), and elderly-onset MG (EOMG, onset age ≥ 60), based on a previous study in Japan.7 To investigate changes in the incidence of MG among the elderly population, the patients were divided into one of the following four periods according to the date of onset: 1971-1980, 1981-1990, 1991-2000, and 2001-2006. We calculated the onset-adjusted age-specific average incidence rate (IR = cases/100,000 person-years) and 95% confidence interval (CI) of the whole populationcases of MG (Whole MG) and the three groups (YOMG, MOMG, and EOMG) in each period by assuming a Poisson distribution. To determine whether there was a recent increase of the EOMG, the relative risk (RR) and 95% CI were also computed for the Whole MG and the three groups, using 2001-2006 as a reference.

We compared clinical characteristics and therapeutic outcomes among the three groups. For all patients, we recorded the disease severity at onset age, the titer of anti-AChR antibody, extrathymic malignancy, the presence of an autoimmune disease, and occurrence of thymoma were recorded. Disease severity and distribution were expressed according to the classification of the Myasthenia Gravis Foundation of America (MGFA) 8 and scored by the Myasthenia gravis activities of daily living score (MG-ADL). 9 The presence of anti-AChR antibody in serum was determined using a standard radioimmunoassay kit (RSR Limited, Cardiff, U.K.) according to the manufacturer’s instructions. Chest CT scans were performed upon diagnosis of MG to screen for thymomas. The mean value of anti-AChR antibody was obtained solely from seropositive patients. Histological analyses of thymus were performed if the patients underwent thymectomy. In this study, In this study, diagnoses of thymoma, thymic lymphofollicular hyperplasia (TFH), thymic atrophy and adipose tissue were made by pathologists, certified by the Japanese Society of Pathology, in our institute certified by the Japanese Society of Pathology.

Surveys were conducted toWe determined which therapies the patients received, including cholinesterase inhibitors, corticosteroids, immunosuppressive drugs, immunoglobulin, plasmapheresis, and thymectomy. We evaluated the clinical effects of thymectomy using the MGFA-Post-Intervention (PI) Status developed by the MG Task Force for Severity and Treatment Standards.8

Commercially available statistics software was used for data analysis (SigmaPlot® and STATA®). Data that were normally distributed were analyzed by one-way ANOVA. Pairwise comparisons were evaluated by Student-Newman-Keuls methods. For data that were not normally distributed, the one-way ANOVA on ranks was employed. Comparisons with P ................
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