What Causes Immobility In The Legs And Body Tremors After Falling Down The Stairs?

Hi Dr, A few weeks ago my little cousin fell down the stairs. He was unable to move after he fell down, As a result, my uncle picked him and took him to the ER where they said everything was OK there were no broken bones or anything. They did X-rays, EKG, etc. and didn t find anything. However, my cousin was still unable to walk. He was able to move the legs normally but when he tries to walk his whole body starts shaking and drops to his knees after like 3 steps. so he was transferred in an ambulance to a children hospital for further diagnosis and they still were unable to explain why he was unable to walk. They didn t say it was a vertigo case or anything of that matter. my little cousin is 11 years old by the way. 2 weeks later have passed by and he is still unable to walk. This is a very weird case. Any information will be immensely appreciated.