A ten-year retrospective evaluation of acute flaccid myelitis at 5 pediatric centers in the United States, 2005–2014

Background
Acute flaccid myelitis (AFM) is a severe illness similar to paralytic poliomyelitis. It is unclear how frequently AFM occurred in U.S. children after poliovirus elimination. In 2014, an AFM cluster was identified in Colorado, prompting passive US surveillance that yielded 120 AFM cases of unconfirmed etiology. Subsequently, increased reports were received in 2016 and 2018. To help inform investigations on causality of the recent AFM outbreaks, our objective was to determine how frequently AFM had occurred before 2014, and if 2014 cases had different characteristics.
Methods
We conducted a retrospective study covering 2005–2014 at 5 pediatric centers in 3 U.S. regions. Possible AFM cases aged ≤18 years were identified by searching discharge ICD-9 codes and spinal cord MRI reports (>37,000). Neuroradiologists assessed MR images, and medical charts were reviewed; possible cases were classified as AFM, not AFM, or indeterminate.
Results
At 5 sites combined, 26 AFM cases were identified from 2005–2013 (average annual number, 3 [2.4 cases/100,000 pediatric hospitalizations]) and 18 from 2014 (12.6 cases/100,000 hospitalizations; Poisson exact p<0.0001). A cluster of 13 cases was identified in September–October 2014 (temporal scan p = 0.0001). No other temporal or seasonal trend was observed. Compared with cases from January 2005–July 2014 (n = 29), cases from August–December 2014 (n = 15) were younger (p = 0.002), more frequently had a preceding respiratory/febrile illness (p = 0.03), had only upper extremities involved (p = 0.008), and had upper extremity monoplegia (p = 0.03). The cases had higher WBC counts in cerebrospinal fluid (p = 0.013).
Conclusion
Our data support emergence of AFM in 2014 in the United States, and those cases demonstrated distinctive features compared with preceding sporadic cases.

背景
急性弛缓性脊髓炎（acute flaccid myelitis, AFM）是一种与麻痹性脊髓灰质炎相似的重症疾病。目前尚不清楚在脊髓灰质炎病毒被消灭后，美国儿童中急性弛缓性脊髓炎的发生频率。2014年，美国科罗拉多州发现了一起急性弛缓性脊髓炎聚集性病例，由此推动美国启动被动监测，共收录120例病因未明的急性弛缓性脊髓炎病例。此后，2016年与2018年收到的病例报告数量均有所上升。为助力近期急性弛缓性脊髓炎暴发的病因学调查，本研究旨在明确2014年之前急性弛缓性脊髓炎的发生频率，并对比2014年病例是否存在独特的临床特征。

方法
本研究针对美国3个地区的5家儿科医疗中心开展了2005年至2014年的回顾性研究。通过检索出院ICD-9编码与脊髓磁共振成像（magnetic resonance imaging, MRI）报告（共超37000份），筛选出年龄≤18岁的疑似急性弛缓性脊髓炎病例。由神经放射医师评估磁共振影像，并查阅病历资料；最终将疑似病例分为急性弛缓性脊髓炎确诊病例、非急性弛缓性脊髓炎病例以及疑似不明病例三类。

结果
5家医疗中心合并统计后，2005年至2013年共确诊26例急性弛缓性脊髓炎病例（年均3例，约每10万儿科住院病例中出现2.4例）；2014年共确诊18例（每10万儿科住院病例中出现12.6例，泊松精确检验p<0.0001）。2014年9月至10月期间共发现13例聚集性病例（时间扫描检验p=0.0001），未观察到其他时间或季节相关的流行趋势。与2005年1月至2014年7月的病例（n=29）相比，2014年8月至12月的病例（n=15）年龄更小（p=0.002），前驱呼吸道/发热性疾病发生率更高（p=0.03），仅累及上肢（p=0.008），且更易出现上肢单肢瘫（p=0.03）。该组病例的脑脊液（cerebrospinal fluid, CSF）白细胞计数更高（p=0.013）。

结论
本研究数据证实，美国于2014年出现急性弛缓性脊髓炎暴发，且该时段病例与此前的散发病例相比具有显著独特的临床特征。