Swelling face and leg issues have been going on a while. Two threads below.
Were corrections made as suggested - give them plenty of more space, provide feed stations keeping them clean, giving supplemental heat and keeping water stations raised up and clean?
Not sure why there's swelling of the head unless the chick has an ear infection.
The chicks look to be in poor condition.
I'd tend to treat for Coccidiosis. You'll want Corid.
Liquid Corid dose is 2tsp or Powdered Corid Dose is 1 1/2tsp per gallon of water given for 5-7 days as the only source of drinking water.
Do not add any extra vitamins/electrolytes that contain B1(Thiamine) to food or water during the course of treatment.
Work on providing them with plenty of space and some supplemental heat - the brooder needs to be large enough for the chicks to be able to comfortably get warm without crowding and they also all need to be able to get away from the heat without crowding. Chicks poop a lot, but meat birds will poop even more. Keep your brooder cleaned out a couple of times a day.
Make feed available to them free choice until they get better - you may need more than one tiny feed station - even if you split up those 50 chicks into 2 groups, they need more than one chick feeder. Same goes for water - they need plenty of fluids. Provide at least 2 water stations for each group.
https://www.backyardchickens.com/threads/joint-soreness-and-swolleness.1575491/page-3
https://www.backyardchickens.com/threads/chicks-feet-swollen-help.1575378/