According to the AKC:
Disabled Handlers (March 1998 Board meeting)
The AKC strives to provide an opportunity to participate in its events. In the case of conformation judging, where decisions are based solely on the quality of the dogs, a judge may modify the regular judging procedure to accommodate a disabled handler.
If a modification of normal judging procedures is required, the judge needs only to determine that it would not inhibit the ability of the other handlers in the ring to show their dogs.
A disabled handler may compete using a cane, crutch, or electric-powered wheelchair. However, when gaiting the dogs as a group, the judge should ensure that a handler who cannot maintain a normal gait for the breed being exhibited, be placed in line where it will not interfere with the ability of the other dogs to gait properly.
Blind handlers may have a second person assist them in individual and group gaiting of the dogs. If the ring surface or terrain makes it difficult for the assisted handler to gait the dog at a normal speed, the judge may adjust where the dog is placed in line to give all handlers an equal opportunity to show their dogs. Blind handlers may have the second person assist in placing table dogs onto tables and verify weight and measurements that may be required under the breed standard. The primary function of exhibiting the dog is the responsibility of the handler and not the second person in the ring.
Taken from:
American Kennel Club - AKC Board Policy Manual