July 4, 2019
A 52-year-old man with progressive, left sided weakness and numbness for 2 weeks.
Acute Hemorrhagic Encephalomyelitis (AHEM) Mimicking Abscess
- Background
- AHEM, also known as acute hemorrhagic leukoencephalitis or Hurst disease, is a rare, demyelinating disease of the CNS characterized by an acute and rapidly progressive inflammation of the white matter.
- It is considered the most severe form of acute disseminated encephalomyelitis (ADEM), due to similarities in their suspected post-infectious autoimmune pathogenesis.
- AHEM has a high morbidity and mortality, often causing severe neurological impairment in hours, and death within one week.
- Clinical Presentation
- Acute onset of headache, focal neurological deficits, confusion, somnolence, and seizures. Frequently preceded by a nonspecific viral illness.
- Often poor prognosis with rapid progression to coma and death.
- Key Diagnostic Features
- AHEM presents with lesions in the cerebral white matter, but has been seen in basal ganglia, thalamus, brainstem, cerebellum, and spinal cord.
- Lesions are hyperintense on T2-weighted and FLAIR sequences .
- Gadolinium enhancement of MRI lesions is present in 30% of cases. Ring, focal, and diffuse-patchy patterns have all been reported.
- Because of the hemorrhagic nature of the lesions SWI or GRE T2* hypo-intense rim or foci are usually present.
- Many factors affect how DWI signal appears in the presence of hemorrhage. During the hyperacute stage, oxyhemoglobin is present. During the late sub-acute stage, extracellular methemoglobin is present. These substances are thought to give parenchymal hemorrhage the hyperintense appearance on DWI and low ADC values, which is similar to the strong restricted diffusion seen in an abscess.
- This man’s lesion was most concerning for an abscess given the ring-enhancement, the location, peripheral GRE T2* hypointensity and the pattern of restricted diffusion.
- Pathological findings of a tissue biopsy are often helpful in the diagnosis. This man had a biopsy showing predominantly lymphocytic inflammation with abundant macrophages, microglial, and astrocytic activation, myelin pallor and loss, and hemosiderin; suggesting pre-biopsy microhemorrhages.
- CSF findings have shown pleocytosis in AHEM cases, though it is nonspecific as up to half of patients do not have this finding.
- Differential Diagnosis
- Pyogenic Abscess: Thin, ring-enhancing with strong central restricted diffusion (caused by the high viscosity and cellularity of pus). A dual SWI or GRE T2* hypo-intense rim sign may be present in pyogenic abscess due to the respiratory burst of bacteria converting hemoglobin to methemoglobin. Although a helpful and specific marker, the absence of the dual rim sign does not rule out a pyogenic abscess.
- HSV: May have a hemorrhagic component, not in usual location (typically bilateral corticosubcortical involvement of the limbic system), no ring enhancement.
- Glioblastoma: Thick, nodular, and ring-enhancing. Usually located in white matter with variable, but usually low, DWI signal. A single SWI or GRE T2* hypo-intense ring can help differentiate from abscess.
- Brain Metastasis: Ring-enhancing. Located in grey-white junction, usually no restricted diffusion.
- Deep Cerebral Venous Thrombosis: Thrombosis of the internal cerebral veins usually affects the bilateral thalami, non-contrast CT with hyperdense clot sign, lack of flow void on non-contrast T2WI, and contrast filling defect or lack of opacification on contrast enhanced CTV, MRV or 3D-T1 MPRAGE .
- Treatment
- Data is limited in treatment for adult patients.
- High dose IV corticosteroids.
- If unresponsive, IVIG or plasma exchange are recommended.
