All professional attendances regarded as a single service in a single course of attention involving the prescription and fitting of contact lenses where the patient meets the requirements of an item in the range 10921-10929 and requires a change in contact lens material or basic lens parameters, other than a simple power change, because of a structural or functional change in the eye or an allergic response within 36 months of the fitting of a contact lens covered by item 10921 to 10929