Consider AFM in patients presenting with rapid-onset weakness (period from onset to nadir: hours to 10 days approximately), particularly when occurring during or shortly after a suspected viral illness or fever.
Be alert between the months of August and November, where most cases have been reported.
Clinical Exam
The clinical syndrome of AFM is defined by acute onset of limb weakness with lower motor neuron findings evident on exam. As such, conducting a complete neurological examination is the most important aspect of the initial clinical assessment.
The motor examination portion of the neurological exam should be emphasized, specially to look for signs of lower motor neuron compromise. Include:
- Strength of individual muscle groups, including specific tests for proximal muscle weakness (example: standing up from a seated position) and axial weakness (neck flexion/ extension).
- Muscle tone, as decreased muscle tone in at least one weak limb is compatible with AFM.
- Reflexes, as affected limbs usually become hyporeflexic or areflexic.
- Cranial nerve examination, as up to 30% of patients also experience motor deficits localizing to the brainstem, and may present with bulbar weakness, and weakness of facial or extraocular muscles.
Neuroradiology
MRI of the spinal cord is the most useful diagnostic test in AFM and should be prioritized. Include cervical, thoracic, and lumbar spine, with T2 and T1 pre- and post-contrast sequences in both axial and sagittal planes.
MRI of the brain with T2 and T1 pre- and post-contrast sequences should also be obtained.
The characteristic MRI abnormality is gray-matter predominant T2 hyperintensity of the spinal cord with associated spinal cord edema; lesion(s) are usually longitudinally-extensive and non-enhancing. Nerve root enhancement may be present.
Repeat MRI can be considered after further clinical evolution in patients with a suggestive clinical presentation but in whom early MRI of the spinal cord is apparently normal.
Where MRI is not possible, rapid completion of available laboratory testing should be prioritized (CSF analysis, microbiological sampling), and EMG/NCS can be incorporated in the initial evaluation where available.
Laboratory Evaluation
- Obtain specimens as soon as possible (i.e. within hours of clinical presentation).
- Respiratory samples (both nasopharyngeal and oropharyngeal): respiratory viral RT-PCR testing (to include enterovirus RT-PCR). Where possible, a positive enterovirus RT-PCR result should be subtyped (to include EV-D68, EV-A71 and other common subtypes).
- Stool samples or rectal swab: enterovirus RT-PCR, viral culture for poliovirus where epidemiologically relevant (with RT-PCR of isolated virus to differentiate between wild-type and vaccine-derived virus).
- Blood sample: microbiologic tests (enterovirus RT-PCR and other epidemiologically appropriate micro-organism tests e.g. West Nile virus serology), and testing for specific alternative myelopathy diagnoses to include MOG-IgG and aquaporin-4-IgG.
- Cerebrospinal fluid sample: cell counts, protein, glucose, oligoclonal bands, enterovirus RT-PCR, and other epidemiologically appropriate micro-organism tests.
- Where RT-PCR is not readily available, samples can still be acquired and frozen for future analysis or transfer to public health authorities.
- Respiratory, stool, serum and CSF samples should also be sent to the relevant public health authorities, according to local protocols. For more information about reporting and sending samples to the CDC, please follow the link: https://www.cdc.gov/acute-flaccid-myelitis/downloads/job-aid-for-clinicians-508.pdf
Neurophysiology
- Electromyography and Nerve Conduction Studies (EMG/NCS) are valuable tools in the investigation of neuromuscular disorders, and every day we learn more about its uses in AFM. Current cases reported show some degree of motor neuropathy, evidenced by diminished compound motor action potential (CMAP) amplitudes, and reduced voluntary motor unit potential (MUP) recruitment, with the absence of sensory nerve conduction abnormalities.
- Although it is not required to make a diagnosis, electrophysiology may be useful in certain patients with atypical features, especially to help to distinguish from alternative diagnoses such as botulism or Guillain-Barre Syndrome.
- In a more chronic setting, it can be used in the pre and post-surgical evaluation for nerve transfer procedures.
Proposed Diagnostic Criteria
Taking into account the previously outlined elements of clinical history (H), examination (E), neuroimaging (NI), and CSF analysis (CSF), we propose the following diagnostic criteria for AFM.
These consensus diagnostic criteria are designed to be applied in the acute phase of the illness to classify the level of certainty of a diagnosis of AFM and to help distinguish AFM from other causes of acute flaccid paralysis.
Our consensus criteria classify AFM cases using typical features, but as a clinician, you may encounter patients with atypical features. They are not meant to replace reasoned clinical judgment on a case to case basis.
Additionally, the diagnostic criteria outlined here are also not intended to replace epidemiologic case definitions for acute flaccid paralysis or AFM that public health organizations (such as the World Health Organization or US Centers for Disease Control and Prevention) use for surveillance purposes.
a. Subjective (H1) or objective (E1) weakness must be present in any of: limb(s), neck, or cranial nerves.
b. Prodromal illness may include respiratory, gastrointestinal, or other symptoms of viral illness.
c. Normal or increased reflexes may be found in other limbs.
d. If MRI obtained very early (within hours of neurological onset) appears normal, repeat MRI after clinical evolution may show diagnostic findings. MRI obtained at late stages (≥4 weeks) may be normal.
e. CSF may be normal at very early (hours) or late (≥4 weeks) stages of AFM.
f. At present, there are a lack of data describing the frequency of these features in patients with AFM