Elsevier

Clinical Imaging

Volume 67, November 2020, Pages 62-67
Clinical Imaging

Neuroradiology
The role of imaging in skull base osteomyelitis — Reviewed

https://doi.org/10.1016/j.clinimag.2020.05.019Get rights and content

Highlights

  • Skull base osteomyelitis is a fairly common entity, sometimes masquerading as tumor and is often misdiagnosed.

  • Imaging is crucial in understanding the pattern of spread of infection and identifying drainable abscesses, if any.

  • Proper sampling is essential to provide early diagnosis and treatment.

Abstract

Purpose

Skull base osteomyelitis (SBO) is difficult to diagnose due to a wide array of clinical presentations. It can be life threatening if not treated promptly.

The objective of this study is to identify the various neck spaces involved in skull base osteomyelitis, correlate them with the possible source of infection and identify the related complications.

Methods

Eighty nine consecutive either culture proven cases of skull base osteomyelitis, or culture negative cases with inflammation on histopathology responding to antibiotic therapy, presenting at a single non-government hospital in south India between January 2016 and December 2018 were included in this study. Images were reviewed by two radiologists and imaging findings were documented by consensus.

Results

Involvement of the parotid space, retromastoid space and (temporomandibular) TM joint was associated with otogenic source of infection (p value < 0.05); while, retropharyngeal/prevertebral involvement was associated with sphenoid and nasopharyngeal sources (p value < 0.05). Complications such as cavernous sinus thrombosis (p value = 0.023) and ICA involvement (p value = 0.014) were more commonly seen with central skull base osteomyelitis. Abscess formation was seen in all three groups of patients without a significant difference between the groups.

Conclusion

Imaging plays an important role in determining the possible source of infection by identifying the involved neck spaces and this in turn can guide the clinician to a site for biopsy. Complications related to SBO can also be identified on imaging and can guide appropriate management.

Introduction

Skull base osteomyelitis (SBO) has colloquially been described as malignant otitis externa (MOE), since it is often seen as a complication of otitis externa. However, the term encompasses a broader area of involvement and is now referred to as lateral or central SBO depending on the source of infection. Lateral SBO usually originates from otogenic and odontogenic sources and involves the temporal bones, sphenoid wings and pterygoid plates. Central SBO, which is epicentered on the clivus, sphenoid and occipital bones mostly arises from para-nasal sinus inflammatory disease but can also occur secondary to dental infections, pharyngeal abscesses and by haematogenous spread from a distant source [1,2].

Severe otalgia which is disproportionate to clinical signs, persistent purulent discharge from the external ear with preserved tympanic membrane and intact hearing are some of the initial clinically obvious presentations of patients with otitis externa, many of whom respond to prompt intravenous antibiotics. The presentation of SBO is however more insidious and delayed, occurring up to 4–7 weeks after apparent resolution of an initial otogenic infection and can be suspected in a subset of patients with symptoms refractory to medical treatment, new neurological deficits, persistent headache or other symptoms which are not explained clinically [2,3]. Central SBO again may present initially with non-specific features of headache, nasal discharge and fever and is often diagnosed when there is disease progression and the patient develops neurological complications. Hence, by the time imaging is done, the disease has progressed and complications have already developed.

The evaluation of CT and MRI findings in such a clinical picture is also not straightforward. Trabecular bone erosion of between 30 and 50% is necessary to give a positive CT scan [4]. Features of osteomyelitis on CT include medullary expansion, sclerosis, cortical destruction and periosteal reaction [1]. An early feature is effacement of the fat planes in the subtemporal triangle where the facial nerve exits the stylomastoid foramen [5], but is limited by the inherent lack of good soft tissue resolution of CT. MR findings include T1 hypointensity of involved bone marrow, abnormal marrow signal intensity around the skull base foramina and abnormal soft tissue infiltrating adjacent fat planes and adjacent structures such as the cavernous sinus [6]. The areas of suspected involvement on T1-W images appear hypointense on T2-W images in most cases in contrast to other infectious processes where T2-W images demonstrate a hyperintense signal because of hyperaemia and edema [7]. This is probably secondary to a fibrotic, necrotising pathologic process, thus making diagnosis on imaging a challenge.

Imaging is also imperative to guide appropriate intervention, that is, biopsy to prove diagnosis and aspiration of abscesses if present. Given, the clinical presentation of these patients and predominantly elderly age group, close imaging differentials include neoplasms involving the skull base, such as squamous cell carcinoma, lymphoma, minor salivary gland tumours etc. Diffuse trans-spatial, multi-compartmental involvement is seen in SBO whereas neoplastic processes like squamous cell carcinoma and minor salivary gland tumours usually have associated mass-like destructive soft tissue lesion at the skull base [6,8]. Lower ADC values on diffusion weighted imaging seen in neoplasms like lymphoma and nasopharyngeal carcinoma can be used to differentiate these neoplasms from the inflammatory changes of SBO [9]. Foci of diffusion restriction within areas of inflammation in SBO represent abscess formation however they would show a peripheral rim of enhancement unlike in a neoplastic process which would demonstrate homogenous or heterogeneous enhancement in the area of diffusion restriction.

Treatment of these patients is also a challenge, due to the inaccessibility of the skull base to surgical intervention, given the close proximity of vital structures. Additionally, due to the necrotic changes in the bone, treatment is further hampered due to inadequate delivery of antibiotics to these regions lacking blood supply [2]. This calls for the need for early diagnosis, and proper imaging and interpretation of the same. The role of imaging in follow up is limited as changes on both CT and MR frequently lag behind clinical response [3].

The causative organisms for SBO can be broadly classified into bacterial and fungal. The most common bacterial organism responsible for SBO is Pseudomonas aeruginosa and fungal organism is Aspergillus. Fungal organisms can occur in diabetic patients, primary or acquired immunodeficiency however the absence of a risk factor does not always preclude fungal etiology [10]. Identifying the responsible fungal or bacterial organism is important to determine appropriate therapy as antibiotic resistant strains of Pseudomonas and fungal etiologies have higher morbidity and mortality [11].

Diabetes mellitus and other immunocompromised states are major risk factors for the development of skull base osteomyelitis. Given the rising incidence of diabetes mellitus worldwide, clinicians and radiologists need to be aware of the imaging findings and complications related to skull base osteomyelitis, to initiate treatment at the earliest, as the mortality and morbidity rates, despite advances in medical and surgical therapy, is high.

The main objective of our study is to review the imaging appearances of SBO on CT and MRI, specifically in order to identify the source of infection and define patterns of trans-spatial spread and potential complications, in order to help guide appropriate management.

Section snippets

Study population & recruitment

This is a retrospective study conducted in a single non-government teaching hospital in South India. The study was approved by the Institutional Review board and informed consent was waived due to the retrospective nature of the study. Culture proven cases of skull base osteomyelitis, or culture negative cases with inflammation on histopathology responding to antibiotic therapy were included between January 2016 and December 2018. Imaging (both CT and MRI) were reviewed by two radiologists and

Results

A total of 89 patients were included in the study, of which 75 were male and 14 were female. Patient demographics are depicted in Table 1. Majority of the patients were above the age of 50, of which 49 (55%) were >60 years old. Two patients were <20 years of age and both were not immunocompromised. Majority of the patients (85%) were diabetic and one patient was post renal transplant. Among the 50 patients for whom ESR was done, 43 (86%) had elevated levels and among the 30 for whom C-reactive

Discussion

Skull base osteomyelitis (SBO) often has an insidious clinical presentation and the symptomology is mostly non-specific for clinical diagnosis. Immunocompromised state predisposes to this disease entity and at the same time, masks the usual signs of infection. Our study is so far the largest collection of biopsy proven SBO to our knowledge. We have elaborated upon the imaging manifestations of the primary disease and its complications, as identified on imaging.

As expected, significant majority

Conclusion

In this large collection of cases of SBO, we have found that imaging plays an important role, not only in the diagnosis but also determines the possible source of infection by identifying the involved neck spaces. Imaging also excels at identifying imminent complications and guides the clinician on sites suitable for biopsy which would in turn guide appropriate management.

Declaration of competing interest

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

References (14)

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1

Present address: Department of Radiology, Alberta Children's Hospital, Oki Drive NW, Calgary, Alberta Canada T3B 6A8.

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