Introduction
The external auditory canal (EAC) and auricle comprise the external ear. There are two sections to the EAC. The medial two-thirds are bony, whereas the lateral one-third is cartilaginous. The EAC's primary job is transporting vibrational sound waves to the tympanic membrane. Atresia is the most frequent congenital condition affecting the EAC. Malignant otitis externa and osteomyelitis are two examples of inflammatory lesions.
The most frequent neoplastic lesions seen are bone tumors. The EAC may get injured due to trauma. Other disorders, including cholesteatoma and excessive ear wax, also impact the EAC. Most of these lesions can be identified clinically, although imaging is frequently needed to determine the lesion's extent. A high-resolution CT scan (HRCT) helps evaluate EAC anomalies.
What Are the Various EAC Anomalies?
EAC Atresia:
The EAC might be atresia alone or in combination with middle ear and inner ear dysplasia. Surgery can correct isolated EAC atresias. The results of procedures carried out when middle and inner ear abnormalities are present are not favorable. A temporal bone HRCT is recommended for preoperative planning. The radiologist should check for surgical contraindications such as atretic oval or round windows and an unfavorable facial nerve route before surgery.
The middle ear cavity's diminished volume and the temporal bone's inadequate pneumatization are two structures we should also look for. The existence of severe incudomalleolar dysplasia, which, when present, requires resection, and dysplastic stapes, which may require replacement by a prosthesis, are further anomalies that need to be kept an eye out for. The volume of the middle ear cavity and the separation between the middle ear structures and the atretic EAC should both be visible in 3D reconstructed pictures.
Malignant Otitis Externa:
Because of its aggressive clinical behavior and high death rate, otitis externa is referred to be "malignant." Enhanced antibiotics have led to a mortality reduction range from 30 % to 40 % to 20 %. Pseudomonas aeruginosa infection of the EAC leads to malignant otitis externa. Those with diabetes and the elderly are frequently affected.
The infection quickly spreads to nearby tissues such as the skull base, middle ear, mastoid air cells, and temporomandibular (TM) joint. The EAC has an asymmetric soft tissue density visible on a CT scan, with or without extension into the nearby structures. A temporalis abscess develops when the temporalis muscle is involved and is characterized by widespread muscular thickening and hypodensity. The widening of the joint space and irregularity of the articular borders indicate the TM joint is affected. Temporal bone osteomyelitis is characterized by a rise in bone density, linear periosteal response, and dense soft tissue surrounding the EAC and mastoid. Cholestasis and EAC squamous cell carcinoma are differential diagnoses.
Bone Tumors:
In the EAC, osteochondroma (exostosis) is the most typical benign tumor. The cancers osteoma, bone island, and osteoid osteoma are among others. Ewing sarcoma, osteosarcoma, and squamous cell carcinoma are malignant tumors. Osteomas can have one or more instances.
Gardner syndrome is characterized by a constellation of polyps of the colon, numerous osteomas, and several impacted or unerupted teeth, along with skin and soft tissue malignancies. Osteomas appear in the EAC as pedunculated lumps slowly developing and asymptomatic. A bony protrusion that is protruding into the EAC is visible on HRCT.
Ear Wax (Cerumen):
Wax buildup in the EAC is a physiological phenomenon. Only when it causes symptoms or precludes ear canal examination, the audiovestibular system, or both is cerumen impaction deemed pathological. Most diagnoses are made clinically; a CT scan is only necessary if impaction removal fails. A hypodense lesion occupying the EAC is seen on HRCT. The presence of an air rim around the lesion and fat attenuation within the lesion support the diagnosis.
Cholesteatoma:
The stratified squamous epithelium of the EAC invades the middle ear, which results in cholesteatomas. Cholesteatomas seldom occur in the EAC. Primary or secondary cholesteatomas are both possible. Secondary instances are far more frequent than primary ones and are brought on by trauma or inflammatory diseases.
Based on the CT scan results and clinical observations, EAC cholesteatomas (EACC) are divided into four types. The EAC is the only site of stage I cholesteatoma. Tympanic membrane and middle ear involvement are present in stage II cholesteatoma. If the mastoid air cells are also affected by an EAC cholesteatoma, it is classified as stage III. The lesion goes beyond the temporal bone in stage IV cholesteatomas.
It's important to distinguish between EACC and malignant otitis externa. EACC is a well-localized, chronic illness that advances slowly. The soft tissue density erodes one of the EAC's walls. On the other hand, malignant otitis externa is a condition that spreads widely and affects the majority of the EAC.
In contrast to cholesteatomas, malignant otitis externa exhibits soft tissue augmentation on contrast-enhanced investigations. Under challenging circumstances, diffusion MRI imaging is advantageous. Malignant otitis externa does not show quick diffusion limitation, in contrast to EACC.
What Are External Auditory Canal Injuries?
EAC injuries may result from piercing or blunt trauma. The most frequent source of blunt trauma is motor vehicle accidents. Injury to the pinna, with or without TM joint dislocation, is typically linked to trauma. Fractures of the temporal bone may or may not be related to EAC injuries. High-density fluid, fracture fragments, and accompanying TM joint dislocation can all be shown on an HRCT scan of the EAC. The pinna's laceration can be seen on three-dimensional CT volume-rendered images.
Lacerations: Whenever possible, the skin margins around pinna lacerations are stitched. If the cartilage is damaged, it is healed unless there is not enough skin to cover it. A cotton or gauze pressure dressing that has been antibiotic-impregnated is used to externally splint damaged cartilage, whether it has been healed or not. As with a hematoma, oral antibiotics are used.
Human bite wounds are likely to become infected, especially cartilage, a potentially severe problem. Prophylactic antibiotics, antivirals, and thorough debridement of devitalized tissue are all included in the course of treatment (see table Antimicrobials for Bite Wounds). Older wounds should be allowed to heal secondary, with cosmetic deformities corrected afterward. Injuries that are less than 12 hours old can be closed.
Avulsions: An otolaryngologist, facial plastic surgeon, or plastic surgeon can treat full or partial avulsions.
Conclusion
The EAC, a crucial component of the temporal bone, participates in sound conduction. HRCT fundamentally aids the identification and treatment of lesions in the EAC. A better visual representation of these lesions is made possible by 3D reconstruction, which makes it easier to handle them properly.
