Abstract
Purpose
Skull base osteomyelitis (SBOM) is an inflammatory process which often arises from malignant otitis externa (MOE); the diffuse skull base and adjacent soft tissue involvement may be mistaken at initial imaging for advanced nasopharyngeal carcinoma (NPC), especially if there is no prior knowledge of MOE, direct spread from the sphenoid sinus or in atypical presentations of MOE. This study aims to evaluate imaging features on MR that may differentiate SBOM from NPC.
Materials and Methods
The MR examinations of 26 patients diagnosed with SBOM between January 1996 and January 2013 were retrospectively reviewed. Comparison was also made with the MR images of 22 consecutive patients with newly diagnosed advanced T3 and T4 NPC between July 2011 and August 2012. Imaging features in both conditions were compared, including the presence of a nasopharyngeal bulge, nasopharyngeal mucosal irregularity, lateral extension, architectural distortion (or lack thereof), increased T2 signal and enhancement patterns.
Results
The most prevalent findings in SBOM were lateral extension, increased T2 signal in adjacent soft tissues, lack of architectural distortion and enhancement greater than or equal to mucosa. The combination of these 4 findings was found to best differentiate SBOM from advanced NPC, and found to be statistically significant ( p < 0.001).
Conclusion
We suggest that the combination of lateral extension, increased T2 signal, lack of architectural distortion and enhancement greater than or equal to mucosa is helpful in differentiating SBOM from advanced NPC.
1
Introduction
Skull base osteomyelitis (SBOM) involves the temporal bone and is a known late complication of malignant otitis externa (MOE). Central skull base osteomyelitis may also result from direct spread from sphenoid sinus infection or less commonly, hematogenous spread. Symptoms include otalgia, otorrhoea and headache and, late in the disease course, cranial nerve palsies. With involvement of the deep soft tissues just below the skull base, intracranial extension and cranial nerve palsies, SBOM may be mistaken for advanced malignancy. In East Asia, SBOM can be mistaken for nasopharyngeal carcinoma (NPC), a common malignancy in East Asia which predominantly affects the ethnic Chinese population. Undifferentiated carcinoma is the most common subtype of NPC in this region, secondary to Epstein-Barr virus (EBV) infection, affecting males more commonly (M:F incidence 2.7:1) . The disease is often advanced at diagnosis, as symptoms have a late onset.
Due to the overlap of features, differentiation between these 2 entities can be difficult, especially for less experienced readers who may be unfamiliar with skull base anatomy and pathology. A lack of relevant clinical information and the presence of atypical features (e.g. painless central skull base involvement or a lack of constitutional symptoms) adds to the diagnostic difficulty. This can lead to delayed diagnosis and treatment, with serious consequences.
As MR imaging is used in the setting of both NPC and SBOM, we attempted to identify imaging features that may help differentiate SBOM from advanced NPC at initial presentation.
2
Materials and methods
A retrospective review was made of patients with a diagnosis of SBOM secondary to MOE imaged with MR. The diagnosis of SBOM was made based on bacteriologic culture results, clinical presentation and elevated inflammatory markers. Institutional board approval to perform this review was obtained. A waiver of consent was granted as this was a retrospective review. These patients were imaged between January 1996 and January 2013. The images were reviewed in consensus by 2 head and neck radiologists. The images were assessed for the following features:
- a)
presence or absence of a nasopharyngeal bulge,
- b)
involvement of lateral structures such as the parotid gland or temporomandibular joint.
- c)
presence or absence of abscess formation.
- d)
presence or absence of architectural distortion, specifically disruption of fascial planes and distortion of normal muscle fibres in the masticator space and prevertebral musculature, in the presence of abnormal signal and enhancement.
- e)
presence, extent and distribution of nodal disease. Only the retropharyngeal and level I and II nodes were evaluated as patients imaged for suspected SBOM were routinely imaged only to the level of the upper neck in our patients. Nodal enlargement was deemed to be present if the shortest trans-axial diameter was greater than 0.8 cm and 1.1 cm for the retropharyngeal and level I and II nodes respectively.
- f)
enhancement of the adjacent deep soft tissues relative to mucosa on MR images. Enhancement was considered significant if this was equal to or greater than that of nasopharyngeal or nasal mucosa.
- g)
perineural extension and intracranial extension. Perineural extension was deemed present if there was thickened enhancing tissue (with or without nodularity) extending along the cranial nerves.
Comparison was then made with the MR images of 22 consecutive patients with newly diagnosed advanced T3 and T4 NPC between July 2011 and August 2012, with an aim to identify the above findings.
The medical records were reviewed, looking for the presence of underlying medical conditions such as diabetes mellitus; elevated inflammatory markers (specifically ESR and CRP); microbiology; and histology (where available).
Statistical evaluation of the results was performed using the Fisher’s exact test.
2
Materials and methods
A retrospective review was made of patients with a diagnosis of SBOM secondary to MOE imaged with MR. The diagnosis of SBOM was made based on bacteriologic culture results, clinical presentation and elevated inflammatory markers. Institutional board approval to perform this review was obtained. A waiver of consent was granted as this was a retrospective review. These patients were imaged between January 1996 and January 2013. The images were reviewed in consensus by 2 head and neck radiologists. The images were assessed for the following features:
- a)
presence or absence of a nasopharyngeal bulge,
- b)
involvement of lateral structures such as the parotid gland or temporomandibular joint.
- c)
presence or absence of abscess formation.
- d)
presence or absence of architectural distortion, specifically disruption of fascial planes and distortion of normal muscle fibres in the masticator space and prevertebral musculature, in the presence of abnormal signal and enhancement.
- e)
presence, extent and distribution of nodal disease. Only the retropharyngeal and level I and II nodes were evaluated as patients imaged for suspected SBOM were routinely imaged only to the level of the upper neck in our patients. Nodal enlargement was deemed to be present if the shortest trans-axial diameter was greater than 0.8 cm and 1.1 cm for the retropharyngeal and level I and II nodes respectively.
- f)
enhancement of the adjacent deep soft tissues relative to mucosa on MR images. Enhancement was considered significant if this was equal to or greater than that of nasopharyngeal or nasal mucosa.
- g)
perineural extension and intracranial extension. Perineural extension was deemed present if there was thickened enhancing tissue (with or without nodularity) extending along the cranial nerves.
Comparison was then made with the MR images of 22 consecutive patients with newly diagnosed advanced T3 and T4 NPC between July 2011 and August 2012, with an aim to identify the above findings.
The medical records were reviewed, looking for the presence of underlying medical conditions such as diabetes mellitus; elevated inflammatory markers (specifically ESR and CRP); microbiology; and histology (where available).
Statistical evaluation of the results was performed using the Fisher’s exact test.
3
Imaging technique
MR was performed using either a 1.5 T (Signa; GE Healthcare, Milwaukee, Wisconsin) or 3 T magnet (Trio; Siemens Medical Systems, Erlangen, Germany). Precontrast axial and coronal T1W images were acquired, followed by post-contrast axial and coronal T1W fat-saturated images, unless there was impaired renal function. Axial and coronal STIR images were also obtained.
4
Results
A total of 26 cases of SBOM were identified (M:F 15:11, age range 48–82 years). There were 17 Chinese and 9 non-Chinese patients (4 Malay, 4 Indian, 1 Thai). 24 patients were found to be diabetic. 2 patients were immunocompetent. These findings are summarized in Table 1 .
| N = 26 | |
|---|---|
| Male | 15 |
| Female | 11 |
| Ethnicity | Chinese 17; non-Chinese 9 |
| Diabetic | 24 |
| Raised inflammatory markers (CRP, ESR) | 26 |
23 patients with SBOM received intravenous gadolinium. 3 patients did not receive intravenous contrast as they had impaired renal function. Of these 23 patients, 21 showed significant enhancement. In contrast, only 1 NPC patient demonstrated significant tumor enhancement.
A nasopharyngeal bulge was seen in 14 patients with SBOM, and in all 22 patients with NPC.
Involvement of lateral structures was demonstrated in 24 of 26 patients with SBOM ( Fig. 1 a, b and 2 a ). However, only 1 patient with NPC showed involvement of lateral structures; this was an advanced case with extensive locoregional spread.
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