How to Assess Wounds for Tunneling and Undermining Protection Status
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Tunneling Wound

by Laurie Swezey RN, BSN, CWOCN, CWS, FACCWS

As part of a thorough wound assessment, in addition to noting location and measuring size, the entire wound bed should be probed for the presence of tunneling and/or undermining. If you are unsure what tunneling and undermining are and how to recognize these phenomena, here's an explanation of these terms and how to assess wounds for their presence.

Tunneling Wounds

Tunneling is caused by destruction of the fascial planes which results in a narrow passageway. Tunneling results in dead space that has the potential for abscess formation. To measure tunneling, a probe is gently inserted into the passageway until resistance is felt. The distance from the tip of the probe to the point at which the probe is level with the wound edge represents the depth of the tunnel. Clock terms are often used to describe the position of the tunnel within the wound bed. This is helpful in identifying and remeasuring tunnel depth at a later time in order to assess progress of wound healing. Tunneling can occur in any wound, but it occurs most commonly in surgical wounds and wounds occurring from a neuropathic cause.

Undermining Wounds

Undermining is caused by erosion under the wound edges, resulting in a large wound with a small opening. Much like an iceberg, what you see on the surface is not indicative of what lies below. Undermining is measured directly under the wound edge with a probe held almost parallel to the wound surface, stopping when resistance is felt. The distance from the probe tip to the point at which the probe is level with the wound edge represents the amount of undermining present. Clock terms can also be used to describe the location of undermining. Undermining generally includes a wider area of tissue than tunneling. Tunneling generally occurs in one direction, whereas undermining may occur in one or more directions. Undermining occurs most often in patient with pressure wounds or neuropathic ulcers.

Sinus Tract

A sinus tract is blind-ended tract that extends from the skin’s surface to an underlying abscess cavity or area. Sinus tracts often expel drainage. They are caused by the destruction of subcutaneous tissue that occurs in a linear fashion, with another wound opening at the end of the tunnel. Sinus tracts are most often associated with surgical infections. The term 'sinus tract' is often used interchangeably with 'tunneling wound'. The biggest difference between sinus tracts or tunneling and undermining is that tunneling and sinus tracts are unidirectional, whereas undermining may occur in more than one direction.

Tunneling and undermining are not always easily visualized, therefore all wounds should be thoroughly probed in order to determine the full extent of tissue damage. Obviously, wounds with tunneling and/or undermining will take a longer period of time to heal due to the fact that these defects must be filled in.

Image Credit: Medetec (

Moore, M. Wound Assessment. Wound Care Education Institute.

Meyers, B. (2008). Wound management: Principles and practice. 2nd edition. Pearson Prentice Hall. Upper Saddle River, New Jersey. pg 52-53.

Wound Watch: Assessing Pressure Ulcers. LPN2009; Jan/Feb 2009 5(1), pg 20-23.

About The Author
Laurie Swezey RN, BSN, CWOCN, CWS, FACCWS is a Certified Wound Therapist and enterostomal therapist, founder and president of, and advocate of incorporating digital and computer technology into the field of wound care.

The views and opinions expressed in this blog are solely those of the author, and do not represent the views of WoundSource, Kestrel Health Information, Inc., its affiliates, or subsidiary companies.

WoundSource ENEWS


This is excellent information, and it is so important. Fistulas are another type of below the skin defect that must be carefully managed and documented.

The most common cause of undermining in pressure ulcer-like wounds is shear, which both damages blood vessels by stretching them and disconnects the various levels of tissue from one another. Because inflammation from shear causes intense internal pressure, the defect underneath the skin can be significantly larger than the visible opening.

More details (with references) along with detailed prevention and treatment protocols and procedures can be obtained from Pressure Ulcer Clinical Education, available free (

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