Paediatric acute mastoiditis complicated by Bezold abscess and epidural empyema without bone erosion

Clinical History

A 4 year old boy attended with a 4 day history of otalgia, otorrhoea, pyrexia and vomiting. He also complained of headache and dizziness. Admission blood tests showed a raised c-reactive protein of 103mg/L and a leukocytosis of 15.1 x 109/L. Unfortunately, microbiology tests could not identify the causative organism.

Imaging Findings

A post-contrast CT (Fig. 1-3) showed an area of low attenuation with an enhancing rim in the subcutaneous tissues along the left mastoid bone around the sternocleidomastoid muscle consistent with a Bezold abscess. There was also a filling defect in the left sigmoid sinus suggestive of intracranial complication. However, it was unclear on CT whether the filling defect was due to a perisinus epidural fluid, sigmoid sinus thrombosis or both.

MRI was performed with diffusion-weighted sequences, MR venogram and post-contrast T1 fat saturation sequences (Fig. 4-7). The filling defect seen on CT was thought to be a combination of sigmoid sinus thrombosis and an epidural collection. This was based on absence of flow void in the left sigmoid sinus and filling defect on MR venogram, but also a rim of restricted diffusion between the sinus and mastoid explained by an epidural collection.

Discussion

Acute otitis media is a common infection in early childhood with an annual incidence of 21.6% in the UK [1]. However, acute mastoiditis is a rare complication of otitis media affecting only around 1-2 per 100, 000 children per year [2]. Streptococcus and haemophilus influenzae account for 60-80% of cases [3].

The extension of acute mastoiditis through the lateral cortex of the mastoid air cells into the subcutaneous tissues of the neck was described by Bezold and Siebenmann in 1908 [4]. Bezold abscess is less common in the paediatric population, prior to the development of the mastoid air cells. Following pneumatisation, the cortical thickness of the mastoid is reduced allowing easier erosion and Bezold abscess formation in the adult population [4].

Extension of acute mastoiditis through the medial wall leads to intracranial infection including extra-axial empyema, parenchymal abscess and meningitis [3, 5]. The sigmoid venous sinus runs medial to the mastoids and can undergo thrombophlebitis [3, 5]. Retrospective reviews at paediatric tertiary care hospitals revealed intracranial complications in 6.8%-20% of acute mastoiditis [6, 7]. Radiology case series have reported higher rates, which may be due to selection bias, as more mild clinical cases may not have imaging [5].

In our case, there was no obvious bony erosion on the CT. We postulate several alternative reasons for spread. Firstly, extension into the intracranial compartment and subcutaneous tissues could be due to secondary inflammatory reaction rather than microbial infection, in which case the pus would be expected to be sterile. This is a possibility as there was no microbial growth on the intra-operative samples. Alternatively, the spread could be through venous pathways draining into the dural venous sinuses or through small cortical tracts or erosions beyond the resolution of CT.

Advantages of CT over MRI are availability, short imaging time which allows imaging in children without sedation or general anesthesia, and assessment of bony erosion. However, MRI is superior for the assessment of the intracranial complications [8]. Sigmoid sinus thrombosis can be identified on MRI by absence of flow void on spin echo sequences and filling defect on MR venography. Diffusion weighted MRI can help to differentiate between extra-axial effusion and extra-axial empyemas. The macrophages in an empyema lead to relative increase in intracellular fluid causing restricted diffusion [9].

The patient had a cortical mastoidectomy and received a 4-week course of IV antibiotics. Warfarin was started for anticoagulation to treat the sigmoid sinus thrombosis.

Differential Diagnosis List

Final Diagnosis

Acute mastoiditis with Bezold abscess, empyema and sigmoid sinus thrombosis

URL:	https://www.eurorad.org/case/13567
DOI:	10.1594/EURORAD/CASE.13567
ISSN:	1563-4086