Hi My 4 month old hen had this condition. Started off falling over and not being able to right herself. When put back on her feet she toddled off, very wobbly. Then if unbalanced , fallon her side and not able to right herself again. She was eating and drinking well and looked alert. Vet charged me $70 to tell me she had hurt her leg!!!? After several days my hen became worse and could not stand. I rang an expert (President of local poultry Club) and was told it was Mareks Disease. Mareks affects the nerve endings I was told. No cure , sadly.
Posted this as I could not find any answers anywhere.