All I know is what I found in IRS Pub 907

Impairment-Related Work Expenses
If you are disabled, you can take a business deduction for expenses that are necessary for you to be able to work. If you take a business deduction for these impairment-related work expenses, they are not subject to the 7.5% limit that applies to medical expenses.
You are disabled if you have:
• A physical or mental disability (for example, blindness or deafness) that functionally limits your being employed; or
• A physical or mental impairment (including, but not limited to, a sight or hearing impairment) that substantially limits one or more of your major life activities, such as performing manual tasks, walking, speaking, breathing, learning, or working.

Impairment-related expenses defined. Impairment-related expenses are those ordinary and necessary business expenses that are:

• Necessary for you to do your work satisfactorily;
• For goods and services not required or used, other than incidentally, in your personal activities; and
• Not specifically covered under other income tax laws.