Definition : Rasmussen encephalitis, or chronic focal encephalitis, is a chronic inflammatory disorder of unknown origin that mainly affects one hemisphere of the brain. While 85% of cases occur in children under 10, more adult cases are being identified through routine MRI for intractable seizures [1].

Mechanism : The exact cause remains unclear; however, various viral infections and inflammatory events have been proposed as triggers, alongside a suggested autoimmune mechanism.

Clinical features :

Rasmussen encephalitis progresses through three clinical phases [2].

1. Prodromal phase

   • Occasional focal seizures.
   • Mild neurological symptoms, such as slight hemiparesis.

2. Acute phase

   • Frequent drug-resistant seizures, including epilepsia partialis continua and focal-to-bilateral tonic-clonic seizures.
   • Worsening neurological deficits, such as hemiparesis, dysphasia (if the dominant hemisphere is involved), or homonymous hemianopia.
   • Possible cognitive decline.

3. Chronic (residual) phase

   • Persistent focal epilepsy that does not respond to treatment.
   • Permanent neurological deficits and cognitive impairment.

MRI findings : Initial atrophy is seen in the ipsilateral caudate nucleus, progressing to more widespread atrophy and signal changes in the affected hemisphere.

The imaging characteristics include [3]:

• T1 : Unilateral cortical atrophy with ex vacuo ventricular dilatation.
• T2 / FLAIR : Hyperintense signal in cortical and subcortical regions.
• DWI/ADC: Restricted diffusion in altered signal areas.
• T1 with contrast : No significant post-contrast enhancement.

Fig 1: Fig 1: Axial T2-weighted showing unilateral cortical atrophy with ex vacuo ventricular dilatation.

Fig 2: Fig 2: Axial FLAIR in the same patient (Fig 1) clearly showing the cortical and subcortical hyperintense signal.

Fig 3: Fig 3: Axial T2-weighted showing unilateral temporo-occipital cortical atrophy with volume loss.

Fig 4: Fig 4: Axial FLAIR showing hyperintense signal areas in cortical and subcortical regions in the same patient (Fig 3), with parietal lobe involvement.

Differential diagnoses :

• Dyke-Davidoff-Masson syndrome : 

  Condition characterized by hemicerebral atrophy/hypoplasia secondary to brain insult usually in fetal or early childhood period and is accompanied by ipsilateral compensatory osseous hypertrophy and contralateral hemiparesis.

  It is characterized by:

  • Thickening of the skull vault (compensatory)

  • Enlargement of the frontal sinus (also ethmoidal and mastoid air-cells)

  • Elevation of the petrous ridge

  • Ipsilateral falcine displacement

  • Capillary malformations (are a novel finding for children with Dyke-Davidoff-Masson syndrome)

• Sturge-Weber syndrome :Also referred to as encephalotrigeminal or encephalofacial angiomatosis, this phakomatosis is marked by a facial port-wine nevus (capillary malformation) and pial angiomas. It falls within the broader spectrum of cerebrofacial arteriovenous metameric syndrome (CAMS) phenotypes.

It is characterized by:

• Subcortical calcification, associated withparenchymal volume loss
• Tram-track sign of cortical and subcortical calcification.
• Possible calvarial and regional sinus enlargement.
• Enlargement of ipsilateral choroid plexus.
• Asymmetric cavernous sinus enlargement

• Unilateral megalencephaly.