Diagnosis: Cholesteatoma from Gunshot Wound
The first issue that needed to be clarified was whether this mass was a new development or a possible complication from the patient's old gunshot injury. Gunshots carry a substantial amount of kinetic energy due to the very high speed of the projectile. Generally, the entrance wound of a gunshot can be very small, and measures approximately the diameter of the projectile caliber or smaller, depending on tissue elasticity. However, the tissue area damaged below the skin is substantially larger than the caliber size. The formula of kinetic energy (KE = ½mv2) determines the amount of energy transferred to the tissues by a bullet. Increasing the velocity of the bullet by 100 percent (doubling it) will increase the energy by 300 percent (four times). Therefore, a long rifle will cause substantially more damage than a handgun, even if the caliber of the bullet is the same due to the rifle bullet's higher velocity. A military-type rifle will inflict even greater damage.
Tissue damage can occur a significant distance away from the bullet path when the bullet's kinetic energy produces a shock wave. As the bullet passes through the tissue, it can create a shock wave that measures much larger than its diameter, creating a cavitary effect that damages the tissues around the bullet's path. After the temporary cavity around the bullet path collapses, a permanent cavity wider than the size of the bullet may persist.
Gunshot wounds to the temporal bone and skull (which can eventually reach the temporal bone) are unique in that the otic capsule bone is very hard, causing the bullet to shatter after it makes contact with the temporal bone's hard surfaces. Each shattered piece of the bullet then scatters within the skull and damages the structures within and around its path (Figs. 2-5). A unique feature of gunshot wounds that traverse the temporal bone is the close proximity of the neurovascular structures. Damage to the carotid artery is possible when the gunshot traverses the temporal bone. The initial workup of a patient with a gunshot wound to the face or temporal bone includes a CT of the brain and CT angiography to evaluate the carotid and vertebral systems.
Another unique feature of gunshot wounds to the temporal bone is the confined intracranial space and the effect of edema. Significant edema can occur in response even to a minor injury to the brain. Because of the confined space of the intracranial cavity, the edema will gradually increase the intracranial pressure and reduce the intracranial arterial blood flow. In general, three things are contained within the skull: the brain, cerebrospinal fluid, and intravascular blood. Increased fluid within the brain displaces the cerebrospinal fluid and eventually the arterial blood. This causes anoxia and herniation of the brain. Rapid action is necessary to decompress the skull and control major vascular injury. Decompression is performed with a ventriculostomy, which reduces the cerebrospinal fluid pressure. In addition, a craniectomy (removal of a portion of the skull) may be performed to allow for expansion of the brain and prevent brain herniation. To treat a vascular injury, a neurointerventional radiologist usually isolates the injury using small intravascular balloons or placing coils within the blood vessel.
As important as hearing and facial nerve functions are to human beings in general, they are of secondary importance to patients with a gunshot wound to the head. After addressing the intracranial issues and stabilizing the patient, the physician can then address the facial and hearing issues. When evaluating the facial nerve of a patient who is experiencing some tension, the physician needs to assess whether the paralysis is due to edema of the facial nerve or the bullet fragments transecting the nerve. To best evaluate the condition, a CT scan of the temporal bone and an electrical test can be performed. If the bullet fragments do not appear to be violating the facial nerve, it is most likely damaged by edema from the cavitary forces of the bullet. Evoked and unevoked electromyography (EMG) can allow the clinician to determine if the facial nerve is in continuity. Unevoked EMG, also known as electroneuronography (ENoG), is inaccurate in the first three days after injury, as the distal facial nerve will continue to be stimulable up to 72 hours even after it has been completely transacted. EMG allows the clinician to determine even if a single nerve fiber is potentially intact. If a single nerve fiber action potential is seen in the face, it means that the nerve is intact and only time is needed for recovery. If the fallopian canal is fractured, the segment of the nerve can be explored further. If bullet fragments are found within the facial nerve canal or if the facial nerve (fallopian) canal is comminuted, the physician must prepare for nerve grafting, as the nerve has likely been transacted. Usually, the nerve becomes too damaged for spontaneous regrowth due to the bullet's high velocity and force energy; thermal injury can occur in these situations. The ends of the facial nerve need to be trimmed back to normal healthy nerves to facilitate the regeneration process. If the patient is stable, it is best to perform surgery or explore the nerve early in the course since the development of granulation (healing) tissue will make it challenging to identify the nerve and the normal anatomy.
Very little can be done to restore the hearing of patients with a damaged otic capsule or cochlear nerves after a gunshot injury. Another consideration is cerebrospinal fluid leakage, which needs to be addressed immediately after the injury. This may require mastoid obliteration through the middle ear or intranasal closure of the Eustachian tube. Finally, stenosis of the canal or trapping of epithelium (skin) can lead to cholesteatoma formation.
This patient underwent a retrosigmoid craniectomy with removal of a portion of the skull. The imaging showed the dura sitting very superficially under the skin. During resection, the postauricular mass was found to be partially filled with cholesterol granuloma fluid, squamous epithelium, and keratin (dead skin). The mass involved the dura but not the intracranial cavity. Histopathologic examination showed a squamous epithelium that indicated cholesteatoma. The skin trapped in the mastoid most likely started out at the facial skin surface where the bullet entered. The bullet carried a piece of the skin into the temporal bone, which led to a cholesteatoma that grew for over 20 years and became a mass behind the ear.
BONUS ONLINE VIDEOS: VISUAL DIAGNOSIS
Read this month's Clinical Consultation case, then watch the accompanying videos from Hamid R. Djalilian, MD, to review the patient's imaging for yourself.
- Video 1. Axial (horizontal) bone window CT of the temporal bones showing the mass and bullet fragments scattered in the skull. The intense white in the carotid canal is previous coiling of the carotid artery.
- Video 2. Axial (horizontal) soft tissue window CT of the temporal bone showing the extent of the mass.
- Video 3. Coronal (parallel to face) bone window CT of the temporal bone demonstrating the mass and its relationship to the temporal bone structures.
- Video 4. Sagittal CT of the right temporal bone showing the relationship between the mastoid facial nerve and the mass.
- Video 5. Sagittal CT of the left temporal bone showing the normal temporal bone with scattered bullet fragments.
- Video 6. Axial CT of the brain showing atrophy of the brain in the areas of previous surgery.
Watch the patient videos online at thehearingjournal.com https://journals.lww.com/thehearingjournal/Pages/collectiondetails.aspx?TopicalCollectionId=23.Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.