Employees are entitled to paid personal leave for any period of illness or injury which prevents them from attending work. Generally speaking (but not always), a specialist appointment for ‘non-incapacitating’ injuries or illnesses are not considered sufficient basis for taking paid personal leave.
You could put a policy in place that requests any elective appointments be at either end of the day, but the reality is if someone needs to see a specialist they just have to fit it in however they can.
If they can’t give you evidence of their not being fit for work (e.g. a doctor’s note) then the time off can be unpaid leave, or annual leave if the employee prefers.