CASE

11 Intracranial abscess

Ciaran Scott Hill

Expert commentary George Samandouras

Case history

A 20-year-old right-handed man presented to the Emergency department with a 4-week history of right-sided earache associated with a foul smelling purulent discharge. He had suffered from intermittent ear discharge since childhood, but he had been well for the previous year. The current episode had been treated with a 1-week course of antibiotics by the general practitioner without any effect. The patient then developed general malaise, positional headaches, and was now describing intermittent horizontal vertigo, the sensation of movement as if the environment were spinning. There were no meningitis symptoms. He had no headache, neck stiffness, or photophobia. His past medical history was otherwise unremarkable.

On examination, there was an erythematous, boggy swelling over the right mastoid process. The right external auditory meatus was completely occluded by pus and the pinna was pushed anteriorly.

The patient was admitted under the ear, nose, and throat surgeons who requested routine laboratory investigations and a microbiology swab that was sent for microscopy, culture, and sensitivity. A CT scan was performed and the CT images are shown in Figure 11.1.

A diagnosis of mastoiditis was made and the patient was placed on the emergency theatre list for an exploratory mastoidectomy. However, the next day the patient was noted to have developed a mild right-sided hemiparesis and was referred to neurosurgery. Review of the CT scans (Figure 11.2) with brain windows demonstrated a hypodensity of the right cerebellum in association with subtle triventricular hydrocephalus and displacement of the IVth ventricle.

It was felt these images were consistent with cerebritis and a T1, T2 and T2 FLAIR MR scan was requested (Figure 11.3). Additionally, a T1 scan with contrast (Figure 11.4), diffusion-weighted imaging (Figure 11.5) and magnetic resonance venography (MRV) was performed (Figure 11.6).

Expert comment

The role of steroids remains controversial in the literature, with some studies supporting their use, while others advocate against them.

Cerebral oedema is a major cause of morbidity and mortality in patients with brain abscess. When the patient is on a targeted antibiotic treatment, administration of dexamethasone, in the presence of oedema on imaging, is often an essential part of the patient’s management. Long-term use should be discouraged.

Rapidly deteriorating patients referred from district general hospitals requiring urgent treatment can have, prior to transfer, administration of broad spectrum antibiotics and dexamethasone after obtaining blood cultures.

Expert comment

The classic clinical triad of headache, high temperature, and focal neurological deficit occurs in <50% of cases. When no obvious source of infection is identified an extensive septic work up is mandatory.

Clinical tip

Temporal lobe abscesses secondary to middle ear infection are

best managed operatively in conjunction with the ear, nose, and throat (ENT) surgeon. Petrosectomy or mastoidectomy are often necessary and, ideally, when indicated, should be performed at the same operative session as the abscess drainage.

The overall score is calculated by measuring the movement of the mid-portion of the superior aspect of the eyebrow in a superior direction and the movement of the angle of the mouth laterally. The eye is scored from 0 to 4 with one point given for each 0.25cm of cephalic movement. The mouth is also scored out of 4 with 1 point given for each 0.25cm of lateral movement. The maximum score is 8. This does not consider the sensory or parasympathetic innervation of the facial nerve. A graphical

version was produced by Lazarini et al., this is simple to use and offers the advantage of speed over the tabulated scale [5].

Two days later he underwent a mastoidectomy and tympanoplasty. A pneumatized mastoid cavity full of granulation tissue was drilled to healthy bone. The facial nerve was decompressed by opening its bony canal. Within 2 weeks, his cranial neuropathies had improved with only mild weakness of mastication, normal facial sensation, minimal diplopia, and a House–Brackman Grade 3 facial weakness remaining. The intra-operative pus samples were sterile and a peripherally inserted central catheter (PICC) line was inserted so the patient could receive 6 weeks of intravenous antibiotics.

Discussion

The earliest successful series of posterior fossa intracranial operations were those of the Sir William Macewen over 100 years ago. The original technique involved blind drainage of a cerebellar abscess through a trephined opening in the temporal mastoid bone [6,7]. Macewen was also perhaps the first surgeon to champion the use of the electric burr, a tool that in combination with the operating microscope and suction irrigation would allow surgeons to unlock the complexities of the skull base. Mastoidectomies were popularized by the German otologist Hermann Schwartze and modified to achieve their current form under William F. House [8].

Chronic suppurative otitis media is a longstanding infective disease of the middle ear. It is usually easily treated in the early stages with antibiotics, with or without myringotomy. If treatment is delayed or ineffective the complications can be severe. Most intracranial complications develop in patients with a chronically discharging ear. The complications that are the primary concern to neurosurgeons are extraaxial (such as subdural empyema) or intra-axial (such as brain abscess). A brain abscess is a focal suppurative process that involves the brain parenchyma. At least 50% of all adults brain abscesses are thought to be otogenic in origin [9]. The possible causes of brain abscesses are outlined in Table 11.3.

Table 11.3 Aetiology of brain abscess from The Neurosurgeon’s Handbook by G. Samandouras (reproduced with permission).

Case 11 Intracranial abscess

Spread of contiguous infection from an otogenic source to the brain is thought to be a key cause of cerebellar abscesses and has been found in up to 93% of cases [10]. Intracranial entry can occur by a number of pathways.

The management of brain abscesses from an otogenic origin is controversial. Otological infections that spread to the brain lie at the interface of neurosurgery, ENT, and microbiology. Treatments include pure pharmacological management, single or repeat aspirations, capsule excision, and extensive ENT procedures.

Evidence base

Staged operative approach

The classical approach to intracranial complications of chronic supperative otitis media is first to treat the intracranial disease (with either aspiration or capsule excision) and then remove the

offending source via mastoidectomy at a later date. This was described by Joe Pennybacker in 1948 who reported eighteen cases of otogenic cerebellar abscesses. Interestingly, he notes that only two of the nine survived in the pre-antibiotic era, whereas after penicillin was introduced eight out of nine survived, underlining the importance of antimicrobial therapy [13] (Class IV evidence). In 1981, Shu-Yuan Yang reported 400 cases of brain abscess (115 cerebellar) treated over 20 years in China without the aid of CT imaging. They found no difference in mortality between simple aspiration or capsule excision (Class IV evidence). In 2011, a review of 973 brain abscesses (38.6% otorhinogenic) over a 20-year period in Durban, South Africa recommended abscess drainage and separate eradication of infection source. This study was limited by its lack of direct comparison with other

strategies [14]. A review of the literature pertaining to aspiration versus capsule excision over a 78-year period by Ratnaike et al. favoured aspiration because of a 6.6% mortality rate versus 12.7% in the capsule excision group [15]. However, the validity of this final conclusion is questionable because abscesses location, aetiology or adjuvant therapy was not addressed. A modern consensus document on treatment of bacterial brain abscesses states that the type of surgical approach does not appear to be critical in determining outcome but that the speed of the therapeutic operation, including surgery, appear to be the more decisive factors for the final outcome [16] (Class V evidence).

Combined approach (neurosurgery and ENT surgeons)

The location of cerebral abscesses that originate in the ear is remarkably constant. In twenty-six cases of otogenic abscesses that were treated over 15 years all were found immediately adjacent to the petrous temporal bone. In the pre-CT era, this consistency of location allowed blind drainage. More recently, it has facilitated a concurrent approach to the abscess and mastoid infection through a single incision [17]. Morwani & Jayashankar propose a single stage, transmastoid approach as a safe treatment modality for otogenic intracranial abscesses [18] (Class IV evidence). They retrospectively reviewed sixty-one patients who had undergone transmastoid abscess drainage and concurrent tympanomastoidectomy (canal wall up or down depending on pathology). Follow-up was for a minimum of 24 months. Their mortality was 3%, there was a 6% complication rate (CSF leak or

meningitis), and a 3% abscess recurrence rate. They conclude that this is a safe and effective treatment strategy. This view is also supported by the work of Singh and Maharaj who found lower mortality (13% versus 36%) when procedures were combined or performed within 12 hours of each other [19] (Class IV evidence). Kurien et al. also adopted concurrent craniotomy and mastoidectomy, and in their report of thirty-six patients found this to be a safe procedure [20] (Class IV evidence). It has been suggested that early surgical intervention is important to achieve a good outcome and transtemporal drainage of the abscess allows eradication of the primary mastoid disease at the same time as treating the intracranial complications [11,21].

Non-surgical management

Wanna et al. have suggested that an initial non-surgical approach to otogenic intracranial abscess with 6 weeks of broad-spectrum antibiotics (vancomycin, ceftriaxone, and metronidazole) and a shorter intravenous steroid course is safe and effective (Class IV evidence) [12]. They reserve surgical

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Learning point Routes of intracranial infection from the middle ear

●Direct spread through a bone defect in the tegmen tympani (the very thin layer of temporal bone that separates the tympanic cavity from the middle cranial fossa) or via Trautmann’s triangle (demarcated by the angle between the sigmoid sinus, the superior petrosal sinus and the osseous labyrinth).

●A retrograde thrombophelbitis of the emissary veins may allow communication through the skull into the venous sinuses and then to the brain parenchyma [11,12].

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Expert comment

In the absence of definitive diagnosis and clinical deterioration, atypical causes of abscess should be considered, including TB, Nocardia, and fungal infections, such as Aspergillus or Mucorales order fungi.

Nocardia responds to sulphonamides with or without trimethoprim. Aspergillus responds to voriconazole and mucormycosis to amphotericin B.

Challenging concepts in neurosurgery

intervention for patients with an abscess that is expanding with mass effect, despite therapy, or shows a poor response to treatment. They also state that neurosurgical intervention is indicated if the abscess looks likely to rupture into the ventricles as this is a recognized poor prognostic marker, with up to 80% mortality. Interval mastoidectomy is performed once the intracranial disease is stable unless neurosurgical intervention is needed, in which case it is performed as a combined procedure wherever possible. It has been suggested that perhaps mastoidectomy can also be avoided in these patients. In separate studies, Kenna et al. and Dagan et al. treated mastoiditis with daily aural toilet and intravenous antibiotics [22,23]. This approach has not been widely adopted and Wanna et al. urge caution, given the severe potential complications of mastoiditis, including sinus thrombosis and further abscess development. The results of Wanna et al. in ten consecutive patients with intracranial

complications from chronic supperative otitis media (four temporal abscesses, one cerebellar abscess, four sagittal sinus thrombosis, and one subdural empyema) showed 0% mortality and no recurrence. Mean hospital stay was 6.4 days. Although firm evidence is lacking, it has been suggested that medical treatment alone may be more successful if it is begun during the cerebritis stage, if the lesion is less than 2.5cm, if GCS is >12, and a specific organism is known (Class V evidence) [16,24]. Other authors have found higher mortality rates with Hsiao et al. reporting an overall case fatality rate of 48% in thirty-one cases of brain abscess that were managed non-operatively. They identified a low GCS as a key poor prognostic marker in these patients (Class IV evidence) [25].

The optimum antibiotic regimen has not been firmly established, but an ‘Infection in Neurosurgery’ working party review in 2000 suggested ampicillin, metronidazole, and ceftazidime (or gentamicin), as the first line empirical therapy (Class V evidence) [26].