( CNN ) -- Doctors in Colorado spotted it first -- a group of 10 children hospitalized with signs of a neurologic illness : limb weakness , cranial nerve dysfunction and abnormalities in their spinal gray matter . Now doctors at Boston Children 's Hospital have identified four patients with the same symptoms . Dr. Mark Gorman said the patients , who ranged in age from 4 to 15 , all suffered from a respiratory illness before being hospitalized with significant weakness in one or more of their limbs . Each case was different , he said ; some had trouble lifting their arms ; others had difficulty walking . One has been discharged . Three are still in the hospital . The Centers for Disease Control and Prevention is investigating the cause of this unknown neurologic illness . Health officials are asking hospitals across the country to be on the lookout for similar cases . `` We do n't know , at this point , if there is any association between the enterovirus EV-D68 that 's circulating and the paralytic conditions some of the children in Colorado are experiencing , '' CDC spokesman Tom Skinner said Monday . Tests of the Colorado children 's cerebrospinal fluid came back negative for enteroviruses and West Nile virus . But a test of their nasal passages found enterovirus in six out of eight patients who were tested . Of those six , four tested positive for enterovirus D68 , which has been sending children across North America to the hospital with severe respiratory illnesses . The other two test results are pending . Boston Children 's Hospital is still awaiting test results for three of the children , Gorman said . One has tested negative for enterovirus D68 . `` We are approaching each patient on a case-by-case basis to work them up thoroughly for causes of their weakness and to determine the appropriate treatment , '' he said . What parents should know about Enterovirus . Enterovirus D68 is part of the Picornaviridae family , which also includes the poliovirus , other enteroviruses and rhinoviruses . Enteroviruses are very common , especially in late summer and early fall . The CDC estimates that 10 million to 15 million infections occur in the United States each year . These viruses usually appear like the common cold ; symptoms include sneezing , a runny nose and a cough . Most people recover without any treatment . But some types of enterovirus are more serious . These can cause hand , foot and mouth disease ; viral meningitis ; encephalitis ( inflammation of the brain ) ; an infection of the heart ; and paralysis in some patients . Enterovirus : My son was 'like a goldfish gasping for breath ' This year , enterovirus D68 seems to be exacerbating breathing problems in children who have asthma . The virus has infected at least 443 people in 40 states , according to the CDC . Cases have also been reported in Canada . CNN 's Georgiann Caruso contributed to this story .
Doctors at Boston Children 's Hospital identify four patients with limb weakness . Patients ' symptoms are similar to those seen in 10 children in Colorado . CDC is investigating the cause .