Septic embolic encephalitis

A 14-year-old boy came to the emergency department with sudden onset of a left side weakness, associated with high fever, which was intermittent in nature. He was a known case of a Tetralogy of Fallot. Investigation of his blood revealed leucocytosis with total WBC count of 14, 000 cells/ mm3.

A CECT Brain revealed three well-defined ring enhancing rounded hypodense lesions, two of them were in the cortex-subcortical white matter junction of right parietal lobe and one was in left temporal lobe with a significant surrounding white matter oedema (Fig. 1 and 2; yellow arrows).

The MRI Brain revealed similar findings.

The lesion had peripheral hypointense rim on T2WI with internal heterogeneous intensity content (Fig. 3).

The lesion had peripheral hyperintense rim on T1WI with internal hypointense content (Fig. 4).

The lesion had peripheral hypointense rim on FLAIR with internal hypointense content (Fig. 5).

The lesions demonstrated peripheral rim of blooming, confirming it to be haemorrhagic rim (Fig. 6).

The lesions showed diffusion restriction on DWI with low values on ADC (Fig. 7).

The above findings of peripherally ring enhancing lesions with surrounding white matter oedema in cortex-subcortical white matter junction, in a known case of Tetralogy of Fallot, are consistent with septic emboli with secondary abscesses formation.

Tetralogy of Fallot is the second most common congenital heart disease. It has four features; ventricular septal defect, right ventricular outflow tract obstruction, right ventricular hypertrophy and overriding of aorta. The prevalence of infective endocarditis in patients with congenital heart disease has been increasing [1]. The most common lesion to develop infective endocarditis after surgery is Tetralogy of Fallot or aorto-pulmonary shunt [1]. The most common pathogen encountered in infective endocarditis is staphylococcus aureus followed by coagulase negative staphylococcus [3]. The prevalence of cerebral septic emboli in patients with infective endocarditis in congenital heart disease is 20.1%, among them 10.5% of the patients present with a stroke, which is its most common presentation [1]. The other modes of presentation are transient ischaemic attack, cerebral haemorrhage, mycotic aneurysms, meningitis, cerebral abscess and encephalopathy [2]. Septic cerebral emboli result from dislodgement of cardiac vegetations, which then cause vessel occlusion resulting in ischaemia and infarction [2]. Septic emboli can also result in a haemorrhagic stroke; possible mechanisms are bleeding into infarcts, rupture of vessel wall affected by septic arteritis or rupture of mycotic aneurysm [4]. The most common artery affected in the brain is the middle meningeal artery [2]. The most common area affected is the grey matter - white matter junctions [5]. Septic embolic abscess typically manifests as a peripherally ring enhancing hypodense lesion with surrounding oedema on CT and on MRI, as an expansile lesion with diffusion restriction in its centre, a peripheral white matter oedema which appears hyperintense on FLAIR, and shows ring enhancement on post-contrast examination [5].

Take home message:
Peripherally enhancing lesions showing central restricted diffusion in a patient with congenital heart disease should raise suspicion of septic embolic abscesses due to infective endocarditis. Immediate start of an empirical antimicrobial treatment is a must. Culture reports are vital. If embolic events continue despite the antimicrobial therapy, heart valve replacement is a must.