In February 2014 I wrote about children in California who developed a poliomyelitis-like paralysis, also called acute flaccid paralysis or AFP. However, the cause of this paralysis was not known. The CDC has released its study of these cases and concludes “The etiology of AFP with anterior myelitis in the cases described in this report remains undetermined.”
A total of 23 cases of AFP* in California were reported to CDC during the period June 2012 through June 2014. These cases were from diverse geographic regions of the state. Specimens from 19 of the patients were available and tested for poliovirus, aroboviruses, herpesviruses, parechoviruses, adenoviruses, rabies virus, influenza virus, metapneumovirus, respiratory syncytial virus, parainfluenza viruses, Mycoplasma pneumoniae, Rickettsia, and amoebas. Rhinovirus was detected in one patient, and enterovirus D68 in two patients; all others were negative for potential etiologic agents.
All 23 patients with AFP also had anterior myelitis, inflammation of the grey matter of the spinal cord, which is characteristic of poliomyelitis. While the rate of AFP in California betweeen 1992-1998 was 1.4 cases per 100,000 children per year, anterior myelitis was not described in any of 245 cases reviewed by CDC. However, poliovirus was ruled out as a cause in the 19 individuals who could be tested.
The cause of AFP is often difficult to determine because there infectious and non-infectious etiologies. Only 2 of the 19 clinical specimens met CDC guidelines for poliovirus detection (two stool specimens collected ≥24 hours apart and <14 days after symptom onset) and the others were likely taken too late to detect the presence of virus. The finding of enterovirus D68 in two of the samples is difficult to interpret, as the virus was detected in respiratory specimens and could have been a coincidental infection.
This investigation began with a request from a San Francisco area physician to the California State Department of Public Health to determine whether poliovirus was present in a 29 year old male with AFP and anterior myelitis. Subsequently this department posted alerts for AFP with anterior myelitis to local health departments, and it is from the cases submitted that the 23 were drawn. Therefore the number of cases of AFP with anterior myelitis might be a consequence of this surveillance.
We are left with the unsatisfying conclusion that these 23 cases of AFP with anterior myelitis were either caused by an undetected infectious agent, or by something else.
*Defined by CDC as “at least one limb consistent with anterior myelitis, as indicated by neuroimaging of the spine or electrodiagnostic studies (e.g., nerve conduction studies and electromyography), and with no known alternative etiology”.