Malignant otitis externa, although it is not a malignancy, it behaves and spreads like one, hence the name.
The first case of MOE was reported in 1938, and the term ‘malignant otitis externa’ was described later by Chandler in 1968, due to its high fatality rate in that time.
Malignant otitis externa is a life-threatening infection that mainly affects the external auditory canal and skull base, and the infection can also invade the stylomastoid and jugular foramina. Infection and inflammation can take different anatomical routes through the osteocartilaginous junction or osseous canal toward the mastoid process posteriorly, or toward the temporomandibular joint, parotid gland and cervicofacial spaces anteriorly, or medially into skull base.
The disease ends up with osteomyelitis of the temporal bone after starting as simple otitis externa. Malignant otitis externa has an association with many serious complications, including cranial nerve involvement and increased rates of morbidity and mortality. Therefore, malignant otitis externa should be identified and treated urgently.
The most common causative organism is Pseudomonas aeruginosa.
Fungi can also cause the disease by 5% to 20% of the general population, making it the second most common causative organism, with Aspergillus fumigatus being the most common cause of fungal MOE.
Other organisms, such as Proteus mirabilis, Proteus sp., Klebsiella sp., and Staphylococci, have been isolated.
Most of the patients diagnosed with MOE share varying degrees of immunosuppression, including patients with diabetes and immunosuppressed patients:
Therefore, do not exclude the diagnosis in young, non-diabetic, or immunocompetent patients.
The prevalence of malignant otitis externa has experienced a drop in recent years due to the development of the modern antimicrobials; however, the incidence of it did not reach the limit to be called as a rare disease.
Malignant otitis externa has been reported in all age groups but is most common in patients who are older than 60 years.
Males are more commonly affected than females.
The infection had a higher incidence rate in areas with high humidity.
Malignant otitis externa rarely occurs in pediatric patients.
The infection can spread, causing bony erosions and invasion of distant tissue, using the fascial planes and venous sinuses, with the involvement of the skull base and the surrounding tissue leading to cranial nerve and intracranial structures invasion.
When the infection reaches the temporal bone through the fissure of Santorini, it invades the stylomastoid and the jugular foramina, containing the facial, glossopharyngeal, vagal, and accessory nerves. On the other hand, the spread through the osteocartilaginous junction to the subtemporal area causes invasion of the retrocondylar fat, the parapharyngeal fat, temporomandibular joint, and the masticator.
The spread can be summarized as follows:
Intravascular involvement can be seen as well, which is more common in fungal MOE; however, fungal MOE usually does not affect the temporal bone.
Normally the external auditory canal (EAC) contains a layer of stratified squamous epithelium covering the underlying connective tissue along the osseous, while the osteocartilaginous junction and regions of the osseous canal are made of a thick subepithelial fibrous tissue. In patients with MOE, Biopsy of the external auditory canal (EAC) might show ulceration and loss of epithelium, with bacteria and inflammation reaching to the dense fibrous tissue. In the areas where the epithelium is not broken, reactive changes vary from mild hyperplasia to prominent pseudoepitheliomatous hyperplasia. Acute and chronic inflammation, including abscess formation, is common. In biopsy samples overlying the cartilaginous canal, the inflammation extends into the apopilosebaceous apparatus. Reactive capillary proliferation is evident within the granulation tissue. However, chronic granulomatous inflammation with granuloma formation is not associated with MOE. The identification of microorganisms is usually made by tissue staining. In case no organism has been identified, polymerase chain reaction (PCR) assay might help in detecting the causative organism.
It can be difficult to differentiate very well-differentiated squamous carcinoma form MOE in the presence of pseudoepitheliomatous hyperplasia; therefore, the presence of MOE should not exclude malignancy, as these can occur concurrently.
The best way to approach malignant otitis externa is to follow the diagnostic criteria which have been described by Cohen and Friedman, and it included major (obligatory) and minor (occasional) criteria as follows:
Cranial nerves examination should be done, as these are commonly affected.
Mental state examination should be performed, and if the mental state is affected, this indicates intracranial complications.
Nuclear studies are not sufficient to show the anatomical extent of the disease. CT and MRI should be accompanied by SPECT bone imaging for the initial diagnosis.
SPECT, in addition to gallium 67 scan, is the investigation of choice to assess the progress of the disease.
Hyperbaric Oxygen Therapy
Malignant otitis externa can be mistaken by many other conditions, these include:
The duration of diabetes, the presence of ESR and CRP, and imaging studies results have all affected the prognosis of MOE.
The poorer prognosis was seen in patients who experienced any of the following:
Elderly patients had a higher chance of developing complications and had a higher mortality rate compared to patients from younger age groups.
The recurrence rate of MOE is high, reaching 15% to 20% when ESR starts to rise again.
MOE can recur up to one year after treatment; therefore, the patient should be followed up regularly for one year before being considered cured.
With the advanced treatment modalities, the mortality rate dropped from a high of 50% in the past to 10% to 20%.
Complications of MOE can occur due to the invasion of the surrounding structures and cranial nerve involvement. Most commonly, the facial nerve is affected; however, glossopharyngeal, vagus, accessory, or hypoglossal nerve can also be involved.
Skull base osteomyelitis arises when the disease spreads to reach the sphenoid, occiput, and clivus, rather than being confined to the temporal bone.
Intracranial involvement might also happen to range from confusional state to severe complications such as meningitis, thrombosis, or death.
Patients should be aware of the consequences of the disease and how to control the factors that might increase the risk of having malignant otitis externa. There are modifiable and non-modifiable risk factors for the disease, studies have shown that diabetes, as a modifiable risk factor, can affect the progress of the disease. Therefore, patients with diabetes should have tight glycemic control to avoid getting the infection.
Moreover, patients should avoid the causes of otitis externa, like swimmer’s ear, which might lead to malignant otitis externa.
Malignant otitis externa is a serious infection and can lead to many severe complications, therefore, a team approach must be carried while taking care of these patients in order to improve the outcome for the patients and decrease the rate of complications. A team of an otolaryngologist, radiologist, infectious disease specialist, microbiologist, primary care, and an endocrinologist should be involved looking after patients with MOE. All working together can enhance better outcomes and work on the comorbidities that might bring up the disease.
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