Sensory Substitution in the Diabetic Neuropathic Foot

In recent years, the mechanism of production and maintenance of neuropathic ulcerations have been fairly clearly elucidated. Cumulated small trauma, intense focal trauma, shearing forces, and tissue degradation secondary to the disease process all con-tribute. Loss of protective sense is the element that makes neuropathic ulcerations possible.

Protective senses revisited: Proprioception is probably equally important to the loss of pain in the production of neuropathic ulceration. It is a subcon-scious automatic process that rarely comes to our notice unless we step on something painful. Proprio-ception is normally handled by the subconscious but is absent in the neuropathic foot. In the diabetic patient with neuropathy, we require compliance to nonambulatory prescriptions, assuming that proprio-ception is subject to conscious control. We now know that to heal neuropathic ulcerations, we must allow ambulation while subordinating or substituting for the proprioceptive sense. [1,2,3,4,5,6]

Sensory substitution: Kaczmarek et al [7] and Bach-y-Rita [8] clearly demonstrated that tactile image feedback of visual signals could be used to do indus-trial tasks. However, the system required intense cognitive resources and complex, failure-prone tech-nology. Intense cognitive demand takes away from other higher cognitive functions. I have termed this phenomenon “cognitive steal.” Complex systems have been developed that translate plantar pressures into intricate tactile, visual, and auditory sensory gra-dients. These systems have tended to fail secondary to cognitive steal and to failure-prone technology.

The digital electronic gait trainer [9]: This con-cept grew from a comment by Paul Brand, MD, when he said that if we could only teach a patient to limp we would go a long way toward helping neuropathic ulcerations heal. Past attempts to build such a device had been unwieldy and crude. In reconsidering the issue, I developed a simple, reliable sensory substitu-tion device that answered the call by Brand for a limping machine. Research with this device clearly and significantly demonstrated that patients quickly learned to modify gait patterns based on device feed-back, and durability tests have been very promising with no failures to date. This device uses solid-state technology to detect ground contact, time it, and tell the patient through an auditory signal to simply remove the device from ground contact. It is light, simple, durable, and cheap to manufacture. The feed-back modality seems to require little in the way of cognitive resources and cognitive steal is minimized.

Editor’s Note:

The concept of sensory substitution is a potentially promising one that is often overlooked in the zeal to treat the consequences of lack of sensation: namely ulceration, infection, and Charcot’s arthropathy. To that end, Dr. Steven Walker has provided a summary of his presentation given at the Diabetic Foot Update sponsored by the Department of Orthopaedics, Podiatry Section, University of Texas, San Antonio in 1996.

The complete manuscript, which is the culmina-tion of more than a decade of Dr. Walker’s work, should be published in the near future.

David G. Armstrong, DPM