[3] Diagnosis may be supported by medical imaging of the spine, nerve conduction studies, and cerebrospinal fluid testing.

[7] Symptoms include: A summary of the condition by the Centers for Disease Control and Prevention (CDC) in 2014:[9] Common features included acute focal limb weakness and specific findings on magnetic resonance imaging (MRI) of the spinal cord consisting of nonenhancing lesions largely restricted to the grey matter.

In most cases, cerebrospinal fluid (CSF) analyses demonstrated mild-moderate pleocytosis (increased cell count in the CSF) consistent with an inflammatory or infectious process.As of October 2018[update], the U.S. CDC regarded the cause of AFM as having "a variety of possible causes such as viruses, environmental toxins, and genetic disorders", listing poliovirus, nonpolio enteroviruses, West Nile virus, and adenoviruses among the potential causes.

[14] Avindra Nath, clinical director of the National Institute of Neurological Disorders and Stroke and president of the International Society for NeuroVirology, compared the situation to the prolonged investigations that led to confirmation of HIV as the cause of AIDS.

CDC urges healthcare providers to consider EV-D68 as a possible cause of acute, severe respiratory illness and a potential increase in AFM cases.

AFM is diagnosed by examining a person's nervous system in combination with reviewing images of the spinal cord.

[18] A doctor can examine a person's nervous system and the places on the body where he or she has weakness, poor muscle tone, and decreased reflexes.

In addition, a doctor can do magnetic resonance imaging to look at a person's brain and spinal cord, do laboratory tests on the cerebrospinal fluid (CSF, the fluid around the brain and spinal cord), and may check nerve conduction (impulse sent along a nerve fiber) and response.

[20] The CDC recommends, "To prevent infections in general, persons should stay home if they are ill, wash their hands often with soap and water, avoid close contact (such as touching and shaking hands) with those who are ill, and clean and disinfect frequently touched surfaces.

[22][23][24] A seasonal pattern is seen in outbreaks, with a marked increase in cases reported in the late summer and early fall.

"[26] While rates of paralytic symptoms appear to be correlated with the number of respiratory infections, in initial anecdotal reports, the cases are not clustered within a family or school, suggesting that the paralysis per se is not directly contagious, but arises as a very rare complication of the common respiratory infection.

[citation needed] A group in Texas reported having observed a pattern in 2013 of one to four cases per year with similar polio-like characteristics.

[31] Diagnosis included a detailed medical history, MRI imaging, and the elimination of transverse myelitis or Guillain–Barré syndrome as potential causes.

There might be slightly decreased sensation in the other arm, but these are younger kids, so they're not always so cooperative in giving you a good sensory exam.

[31]Children's Mercy Hospital, which had three or four cases in 2014, reported that the MRI images and symptoms closely mimicked polio.

They reported: "The sudden onset of flaccid paralysis in single or multiple limbs with absolutely no sensory findings, the MRIs all showing uniformly a signal increase in the ventral horns of the spinal cord — this is exactly the same region of the spinal cord affected in polio ...

"[31] Of 64 patients meeting the CDC criteria before October 29, 2014, 80% had had a preceding respiratory illness and 75% reported fever in the days leading up to limb weakness, the onset of which was generally abrupt.

[33] The CDC requested that physicians provide information about cases meeting these criteria: patients diagnosed after August 1, 2014, who are no older than 21 years of age, showing acute onset of focal limb weakness, with a spinal-cord lesion largely restricted to grey matter visualized by MRI.