In 2016, CDC saw an increase in cases of acute flaccid myelitis (AFM); 144 persons in 37 states and the District of Columbia were confirmed to have AFM. After investigations in California ([@R1]) and Colorado ([@R2]) in 2014, CDC characterized AFM as an acute flaccid paralysis (AFP) distinguishable by magnetic resonance imaging (MRI) abnormalities of the gray matter of the anterior and posterior spinal cord segments, involving one or more spinal segments ([@R3]). Although certain viruses (e.g., nonpoliovirus enteroviruses, adenoviruses, and West Nile virus) can cause rare cases of AFP, and findings from the 2014 outbreak investigations indicated that enterovirus D68 (EV-D68) was temporally associated with AFM, no viral etiology for AFM has been definitively established ([@R3]). In September 2016, an acute care hospital in Arizona notified the Maricopa County Department of Public Health (MCDPH) of a suspected case of AFM and subsequent cluster of 11 children who were evaluated with similar neurologic deficits; differential diagnoses included transverse myelitis and AFM. The Maricopa County Department of Public Health, in cooperation with the Arizona Department of Health Services, CDC, the Translational Genomics Research Institute (TGen, Flagstaff, Arizona), and the acute care hospital, initiated an investigation to confirm AFM cases and identify an etiology.

The 2015 Council of State and Territorial Epidemiologists and CDC case definition for probable AFM requires acute onset of flaccid limb weakness and cerebrospinal fluid (CSF) pleocytosis (CSF white blood cell \[WBC\] count \>5/mm^3^ when corrected for red blood cells). A confirmed case must have an MRI demonstrating lesions restricted primarily to the gray matter of the spinal cord, in addition to acute onset of flaccid limb weakness ([@R4]). Based on medical chart abstraction and review of the MRI images, a CDC neurology subject matter expert verified four confirmed cases of AFM and one probable case. Among the six patients whose cases did not meet the AFM confirmed or probable case definition, two had focal limb weakness and pleocytosis (CSF WBC = 7/mm^3^ and 22/mm^3^, respectively), but MRI results indicated alternative etiologies (acute disseminated encephalomyelitis and neuromyelitis optica, respectively). The case that met the probable case definition had pleocytosis (CSF WBC = 7/mm^3^), but MRI findings were inconsistent with AFM, and no other plausible diagnosis was identified.

Onset dates for the four confirmed cases occurred during August 19--September 15, 2016. All four patients had preceding respiratory (three patients) or gastrointestinal illness (one patient), with onset dates for those illnesses occurring during August 14--September 13. Their illness began a median of 2 days (range = 2--5 days) before onset of focal limb weakness; three patients experienced tactile or measured fever preceding onset of neurologic symptoms ([Table](#T1){ref-type="table"}). Among patients with confirmed cases, focal limb weakness was present in a single limb (one case), three limbs (two cases), and four limbs (one case). Two patients with confirmed cases and one patient with a probable case had a prior medical history of asthma, and a third patient with a confirmed case reported a family history of asthma. The investigation team conducted hypothesis-generating interviews with all confirmed AFM patients and their proxies. Three of the four patients with confirmed cases were residents of Maricopa County, and no epidemiologic links were detected among the four patients. None of the patients had traveled to an area with ongoing Zika virus transmission in the month prior to symptom onset.

###### Characteristics of five pediatric patients with acute flaccid myelitis\* --- Maricopa County, Arizona, October 2016

  Characteristic                                                    Patient 1                                                                                     Patient 2                                                                                                       Patient 3                                                             Patient 4                                                                Patient 5
  ----------------------------------------------------------------- --------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------- ------------------------------------------------------------------------ ---------------------------------------------------------
  Case status                                                       Confirmed                                                                                     Confirmed                                                                                                       Confirmed                                                             Confirmed                                                                Probable
  Age at onset (yrs)                                                3.5                                                                                           10                                                                                                              4                                                                     9                                                                        12
  Sex                                                               Boy                                                                                           Girl                                                                                                            Girl                                                                  Girl                                                                     Girl
  Onset of focal limb weakness                                      August 23, 2016                                                                               August 19, 2016                                                                                                 September 15, 2016                                                    September 8, 2016                                                        August 27, 2016
  Onset of preceding respiratory or gastrointestinal illness        August 21, 2016 (respiratory)                                                                 August 14, 2016 (respiratory)                                                                                   September 13, 2016 (respiratory)                                      September 6, 2016 (gastrointestinal)                                     August 17, 2016 (respiratory)
  Presence of fever (tactile^†^ or measured)                        Yes                                                                                           No                                                                                                              Yes                                                                   Yes                                                                      No
  Limbs affected (region)                                           1 (LUE)                                                                                       4                                                                                                               3 (BUE, RLE)                                                          4                                                                        1 (LUE)
  Cranial nerve features and timing                                 None                                                                                          Facial droop subsequent to onset of limb weakness                                                               Facial droop subsequent to onset of limb weakness                     Facial droop before onset of limb weakness                               Diplopia concurrent with limb weakness
  Patient and family history of asthma                              Asthma and family history of asthma                                                           Asthma                                                                                                          None                                                                  Family history of asthma                                                 Mild asthma, seasonal allergies, food allergies, eczema
  Corticosteroid history                                            Maintenance inhaled fluticasone; oral budesonide for asthma exacerbation August 15--19        Maintenance inhaled fluticasone; oral prednisolone for asthma exacerbation beginning August 14                  None                                                                  Oral prednisolone for treatment of Bell's palsy beginning September 5    Maintenance inhaled fluticasone
  Magnetic resonance imaging (MRI) findings                         T2 signal abnormalities in anterior and posterior columns of the central gray cervical cord   T2 signal abnormality with anterior and posterior involvement, contiguous through multiple levels of the cord   T2 signal abnormality in the anterior horn of the central gray cord   Anterior horn signal abnormality extending four cervical levels          Normal MRI result
  Cerebrospinal fluid/white blood cell/mm^3^                        50                                                                                            150                                                                                                             207                                                                   115                                                                      7
  Nasopharyngeal swab polymerase chain reaction results from TGen   Positive for EV-D68                                                                           Positive for EV-D68                                                                                             Positive for EV-D68                                                   Unavailable                                                              Unavailable
  Stool specimen testing results                                    Negative enterovirus/parechovirus by RT-PCR                                                   Negative viral culture and enterovirus/parechovirus by RT-PCR                                                   Negative viral culture and enterovirus/parechovirus by RT-PCR         Positive for coxsackievirus A10 by Sanger sequencing of the VP1 region   Unavailable

**Abbreviations**: BUE = bilateral upper extremities; EV = enterovirus; LUE = left upper extremity; RLE = right lower extremity; RT-PCR = reverse transcription--polymerase chain reaction; T2 = T2 weighted image; TGen = Translational Genomics Research Institute (Flagstaff, Arizona); VP1 = viral protein 1.

\* Four with confirmed cases and one with a probable case.

^†^ Felt warm to the touch, according to parent.

CSF was collected from all four patients with confirmed AFM. Median CSF WBC count was 133/mm^3^ (range = 50--207), and initial viral testing at the hospital included CSF reverse transcription--polymerase chain reaction (RT-PCR) assays for enterovirus (three patients) and West Nile virus (WNV) (two patients), polymerase chain reaction (PCR) assay for herpes simplex virus (two patients), and enzyme immunoassay to detect immunoglobulin M (IgM) or immunoglobulin G (IgG) for WNV (three patients). All results were negative. All four CSF specimens were negative on TGen amplicon sequencing assay ([@R5],[@R6]) using primers based on the 2014 circulating EV-D68 virus. Results of microbiome analysis by metagenomic sequencing of RNA and 16S rRNA gene sequencing of DNA extracted from CSF revealed bacterial sequencing dominated by *Propionibacterium*, which is a normal component of the skin flora and most likely represents a contaminant (*7*). Serum collected from one patient at initial evaluation was negative for WNV IgM and IgG on a hospital immunoassay; serum collected from the same patient 47 days after onset of focal limb weakness and from two additional patients (19 and 26 days after onset of focal limb weakness) were negative for WNV IgM and St. Louis encephalitis IgM at the Arizona State Public Health Laboratory.

Three of the four patients had nasopharyngeal (NP) swabs available from initial evaluation that were forwarded to CDC; one specimen was positive for enterovirus/rhinovirus on a panviral respiratory PCR panel at the admitting hospital laboratory and for EV-D68 at CDC. RNA extracted from NP swabs from all three patients was positive by the TGen amplicon sequencing test for EV-D68 (GenBank Bioproject); an NP specimen from a patient who did not meet the AFM confirmed or probable case definitions also was positive for EV-D68 by the same assay.

Stool specimens were collected from two patients at the time of initial evaluation; vital cultures of these specimens were negative on viral. One available specimen and three additional specimens, collected 28, 47, and 63 days after onset of focal limb weakness, were sent to CDC for four enterovirus/parechovirus RT-PCR assays. A stool specimen, collected at day 28 from the patient who did not have an NP swab available, was positive for coxsackievirus A10.

This cluster of AFM at one children's acute care hospital is the largest cluster identified to date in Arizona and is part of a nationally identified increase in AFM cases. Although no statewide surveillance system specific to AFM is available, this cluster was detected by physician reporting, highlighting the need for physicians to remain vigilant for this emerging disease and to report cases that fit the AFM case definition to their local health department. Metagenomic analyses identified EV-D68 in NP swabs from the three patients for whom specimens were available, along with a specimen from a patient who did not meet the AFM case definition; therefore, no single etiology or risk factor was associated with only confirmed cases.

Patient and family history of asthma was the most common comorbidity reported among confirmed AFM cases and should be considered in future case investigations. Expanded analysis of infectious, postinfectious, and noninfectious etiologies might provide further insight into the mechanism of AFM.

Kathryn Putman; Jennifer Adair, Bernny Apodaca, Phoenix Children's Hospital, Neurology Department; Phoenix Children's Hospital Infection, Department of Infection Prevention and Control; Arizona State Public Health Laboratory, Virology Section.

**Conflict of Interest:** David Engelthaler and Jolene Bowers have a provisional patent application in progress for a real-time polymerase chain reaction assay for the detection of Enterovirus D68 in complex specimens. No other conflicts of interest were reported.
