Which imaging techniques are relevant in the primary diagnosis of necrotizing otitis externa?

Balance between desired and undesired effects

Different modalities

A systematic search was performed to determine the diagnostic accuracy in NEO of FDG- PET/CT or MRI, versus  CT. Only one study by Kulkarni (2020) was included, which used clinical outcome measures as a referral. It used FDG-PET/CT as index test, with a comparison to MRI. Although the diagnostic accuracy for FDG-PET/CT can be considered high, the evidence shows a high level of uncertainty comparing with MRI. In addition, no comparison with CT was made. The level of evidence was graded as very low.

A large meta-analysis was done by Kim (2023), in which the diagnostic accuracy for diagnosing necrotizing otitis externa of [⁶⁷Ga]-citrate, [^99mTc}-labelled diphosphonates for bone scintigraphy, MRI and CT was measured. The sensitivities were 0.94 (0.77–0.99), 0.97 (0.88–0.99), 0.94 (0.70–0.99) and 0.96 (0.90–0.98), respectively. Thus, showing comparative results in sensitivity when using only CT or MRI.

Quality of the evidence

The overall quality of evidence is very low. This means we are very uncertain about the estimated effect of the crucial outcomes found.

There is downgrading due to very serious:

• Risk of bias: patient selection; index test.
• Imprecision: inaccuracy, due to a very small number of events in a small sample size.

Clinical application and specific characteristics per modality

Based on the literature, there is no available evidence that shows that FDG-PET/CT or MRI leads to significantly higher accuracy compared to CT in diagnosing NOE. Diagnosing NOE can be challenging due to nonspecific symptoms, the potentially prolonged clinical course, and its clinical presentation. The diagnosis is typically made through a combination of clinical symptoms, laboratory findings (microbiology), and imaging results (Chawdhary, 2017). Various imaging modalities can be used not only to diagnose NOE but also to assess the extent of the disease and evaluate potential complications.

The imaging techniques used to diagnose NOE include non-contrast (most used) and contrast-enhanced CT, contrast-enhanced MRI, and nuclear medicine imaging (FDG-PET/CT) (Curtin, 1982; Tsuno, 2022; Shavit, 2018). In patients where limited disease is expected (external ear with surrounding soft tissues), an unenhanced CT is the best option for evaluating bone erosion and demineralization. With CT, opacification of the middle and/or mastoid cells can also be assessed. Adding a soft tissue kernel allows for the evaluation of soft tissue abnormalities. Post-contrast CT helps in evaluating vascular complications (such as venous sinus thrombosis or pseudoaneurysm formation) or abscess formation. CT scan is the initial imaging modality of choice for a suspected case of NOE (Hodgson, 2023).

Contrast-enhanced MRI is useful for evaluating the anatomic location of the NOE and can also be used to assess the extent of the infection in the adjacent soft tissues or bone marrow. Involvement of the skull base foramina and temporomandibular joint can also be visualized. MRI is superior in detecting abscess formation, intracranial extension of the infection (such as dural enhancement or empyema), or ischemic infarcts.

Both CT and MRI provide anatomical information and details on potential complications of NEO; however, they lack functional information. Various nuclear medicine techniques with different radiopharmaceuticals have been used, but fluorine-18 fluorodeoxyglucose ([18F]-FDG) is widely available and is the most commonly used tracer nowadays. This scan provides metabolic information and is taken up in areas of malignancy, infection, and inflammation. As a consequence, this test is highly sensitive but lacks specificity, as it cannot differentiate between infection/inflammation and malignancy. Another limitation is the physiological uptake of FDG in the brain, which makes it difficult to recognize intracranial extension of the infection.

Several neoplastic and inflammatory diseases may mimic the imaging findings of NEO. The most common neoplastic mimickers include nasopharyngeal carcinoma, carcinoma of the external auditory canal, metastases, and lymphoma. The most common inflammatory disease mimickers are IgG4-related disease and granulomatous diseases. Also osteoradionecrosis and medication-related ear canal osteonecrosis (MRECO) may mimic NEO. The combination of imaging modalities may help in narrowing the differential diagnosis.

Values and preferences of patients (and possibly their relatives/caregivers)

Imaging techniques are used to provide an accurate diagnosis without delaying the diagnostic process. In general, for younger children, an MRI scan can be challenging due to the long acquisition time, noise, or lack of cooperation. However, clinicians should be cautious about using contrast-enhanced CT scans in the diagnostic process due to radiation exposure. This limitation also applies to FDG-PET/CT, as it also involves radiation. For older patients with NEO, an MRI can also be challenging due to motion artifacts, claustrophobia, or contraindications (such as cardiac devices, for example).

Cost considerations

In patients suspected of NEO, an unenhanced CT scan is most frequently used because it is easily available, it has very limited contraindications and results in a first impression of disease extent. There is no evidence that MRI or FDG-PET/CT has higher accuracy than CT for the initial diagnosis of NEO. Replacing CT with MRI or FDG-PET/CT could result in higher costs. However, MRI or FDG-PET/CT may better depict potential complications of NEO, which could influence treatment decisions.

Acceptability, feasibility, and implementation.

Regarding acceptability, feasibility, and implementation, no significant problems are expected because CT, MRI, and FDG-PET/CT are widely available. In cases where there is limited capacity for one or two of the imaging modalities, the remaining third modality serves as a good alternative. If there is a lack of experience in image interpretation, referral to a specialized center is an option.

Rationale of the recommendation: weighing arguments for and against the interventions

NOE clinical symptoms are non-specific and overlap with symptoms of acute external otitis, chronic mastoiditis, and several malignancies. To date, no uniform diagnostic criteria for NOE are available. Imaging has a high sensitivity for NOE and can be suggestive but is sometimes non-specific. The diagnosis of NOE is based on a combination of clinical symptoms, clinical findings, and imaging findings.

Currently, there is no evidence that one imaging modality is superior to another in diagnosing NOE. Depending on the clinical presentation (or clinical suspicion), patient age, comorbidities, possible contraindications, and the patient’s own preference, a choice between the different imaging modalities can be made.

In daily practice, an unenhanced CT is the first imaging modality used because it is widely available and does not require intravascular contrast. A CT scan may show bony erosions or demineralization in cases of osteomyelitis. A soft-tissue kernel helps evaluate possible infiltration of fat planes, which can be suggestive of NOE.

When the CT scan is inconclusive or more extensive disease is expected, an additional contrast-enhanced MRI can be performed. MRI is superior in evaluating the extent of soft tissue involvement, skull base foramina, intracranial involvement, and ruling out vascular complications. If an MRI is contraindicated, a contrast-enhanced CT can be considered.

In case of complex (/severe) NEO (see module Definition and limiting doctor’s delay in diagnosing necrotizing otitis externa) FDG-PET/CT(or MRI) imaging provides functional imaging which may be helpful for biopsy guiding and/or in therapy evaluation.