What Causes Unexplained Fever And Abnormal Breathing Pattern In A Child?

5 year female child has ataxia,nystagmus,difficulty swallowing and chocking,dysartheria and abnormal breathing pattern.All these saymptoms heve been developed gradually over the lastyear.Apart from delayed motor mile stones where she started walking in her 3rd birthday ,she has otherwise normal developmental history.Nowadays patient is in PICU on mechanical ventilation because of the desaturation that develops as aresult of her abnormal breathing pattern and chocking with her own saliva.other significant findings is her unexplained fever,hepatomegally that increase with days .what could be the case and iam wondering if you could give me a plan to work on ,in order to reach to the diagnosis. (if you want any additional information ,you can ask me)