Acute flaccid myelitis appears to present most commonly as asymmetric weakness after respiratory viral infection and has distinctive MRI features that could help with early diagnosis.
In a paper published in JAMA Pediatrics, researchers presented the results of a retrospective case series of 45 children who were diagnosed between 2012 and 2016 with acute flaccid myelitis, or “pseudo polio,” using the Centers for Disease Control’s case definition.
Matthew J. Elrick, MD, PhD, of Johns Hopkins University, Baltimore, and his coauthors came up with a set of reproducible and distinctive features of acute flaccid myelitis. These were the presence of a prodromal fever or viral syndrome; weakness in a lower motor neuron pattern involving one or more limbs, neck, face, and/or bulbar muscles; supportive evidence either from MRI, nerve conduction studies, or cerebrospinal fluid; and the absence of objective sensory deficits, supratentorial white matter, cortical lesions greater than 1 cm in size, encephalopathy, elevated cerebrospinal fluid without pleocytosis, or any other alternative diagnosis.
The researchers commented that, while the CDC case definition has helped with epidemiologic surveillance of acute flaccid myelitis, it may also pick up children with acute weakness caused by other conditions such as transverse myelitis, Guillain-Barré syndrome, ischemic myelopathy, and other myelopathies.
To identify clinical features that might help differentiate patients with acute flaccid myelitis, the researchers attempted to see how many alternative diagnoses were captured in the CDC case definition.
The patients in their study all presented with acute flaccid paralysis in at least one limb and with either an MRI showing a spinal cord lesion spanning one or more spinal segments but largely restricted to gray matter or pleocytosis of the cerebrospinal fluid. The researchers divided the cases into those who also met a well-defined alternative diagnosis – who they categorized as “acute flaccid myelitis with possible alternative diagnosis” (AFM-ad) – and those who were categorized as “restrictively defined AFM” (rAFM). Overall, 34 patients were classified as rAFM and 11 as AFM-ad.
Those in the rAFD group nearly all had asymmetric onset of symptoms, while those in the AFM-ad group were more likely to experience bilateral onset in their lower extremities, “reflecting the pattern of symptoms often seen in other causes of myelopathy such as transverse myelitis and ischemic injury,” the authors noted.
While both groups often presented with decreased muscle tone and reflexes, this was more likely to evolve to increased tone or hyperreflexia in the AFM-ad group. Patients with AFM-ad were also more likely to experience impaired bowel or bladder function.
On MRI, lesions were mostly or completely restricted to the spinal cord gray matter in patients with rAFM or to involve the dorsal pons. These patients did not have any supratentorial brain lesions.
Patients in the rAFM category also had lower cerebrospinal fluid protein values than those in the AFM-ad category, but this was the only cerebrospinal fluid difference between the two groups.
All patients categorized as having rAFM had an infectious prodrome – such as viral syndrome, fever, congestion, and cough – compared with 63.6% of the patients categorized as AFM-ad. The pathogen was identified in only 13 of the rAFM patients, and included 5 patients with enterovirus D68, 2 with unspecified enterovirus, 2 with rhinovirus, 2 with adenovirus, and 2 with mycoplasma. Of the three patients in the AFM-ad group whose pathogen was identified, one had an untyped rhinovirus/enterovirus and mycoplasma, one had a rhinovirus B, and one had enterovirus D68.
“These results highlight that the CDC case definition, while appropriately sensitive for epidemiologic ascertainment of possible AFM cases, also encompasses other neurologic diseases that can cause acute weakness,” the authors wrote. However, they acknowledged that acute flaccid myelitis was still poorly understood and their own definition of the disease may change as more children are diagnosed.
“We propose that the definition of rAFM presented here be used as a starting point for developing inclusion and exclusion criteria for future research studies of AFM,” they wrote.
The study was supported by Johns Hopkins University, the Bart McLean Fund for Neuroimmunology Research, and Project Restore. Two authors reported funding from private industry outside the submitted work and five reported support from or involvement with research and funding bodies.
SOURCE: Elrick MJ et al. JAMA Pediatr. 2018 Nov 30. doi: 10.1001/jamapediatrics.2018.4890.
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