"What are the limits and requirements to claim medical expenses?"
In order to get a tax benefit for medical expenses, you need to itemize your deductions (versus taking the standard deduction). In addition to that, only the amount of your medical deductions that exceed 7.5% of your adjusted gross income count towards itemized deductions. So, if your adjusted gross income is $50,000, the 7.5% threshhold is $3,750. If you have $4,000 of unreimbursed medical expenses, you have a deduction of $250. This $250 becomes one of the components of your "itemized deductions" which need to exceed the "standard deduction" for you to reap a tax benefit.