Case of the Week

Background:

• Posterior quadrantic dysplasia (PQD), also sometimes called hemi-hemimegalencephaly, is a rare disorder of cortical development that comprises 3–15% of cases of multilobar cortical dysplasia.
• PQD typically involves the temporal, parietal, and occipital lobes of one cerebral hemisphere (the posterior quadrant of the cerebrum) with sparing of the frontal lobe.   
• Like hemimegalencephaly, PQD results from hamartomatous overgrowth of the involved parenchyma.

Clinical Presentation:

• Seizures are the hallmark clinical presentation of PQD, and patients most commonly present with infantile spasms prior to 1 year of age.
• As patients grow older, seizures often evolve to simple or complex partial seizures.
• Additional symptoms common to PQD include intellectual disability, developmental delay, hemiparesis, and visual deficits.

Key Diagnostic Features:

• PQD results in overgrowth of the posterior half of one cerebral hemisphere with asymmetric cortical thickening with decreased gray-white matter differentiation, increased white matter volume, and enlargement and abnormal morphology of the trigone and occipital horn of the lateral ventricle.
• Accompanying abnormalities of cortical migration and development may be seen, including gray matter heterotopia, polymicrogyria, pachygyria, and lissencephaly.
• In some cases, coarse parenchymal calcification can be seen.

Differential Diagnoses:

• Hemimegalencephaly: This disorder is identical to PQD except that the entire cerebral hemisphere is involved in hemimegalencephaly and only the posterior quadrant of a cerebral hemisphere is involved in PQD, with sparing of the frontal lobe. In total hemimegalencephaly, there is additional involvement of the cerebellum and brainstem.
• Multilobar cortical dysplasia: Cortical dysplasias are often focal but rarely occur at multiple locations in multiple lobes. In this disorder, there is less uniform cortical involvement compared with hemimegalencephaly and PQD. Also, in contrast to PQD, multilobar cortical dysplasia does not demonstrate frontal sparing.
• Petalia: This common anatomic variant depicts developmental asymmetries of the frontal and occipital lobes with deviation of the falx cerebri from midline. In right-handed individuals, the most common pattern seen is that the left occipital lobe bulges to the right (also referred to as occipital bending) and the right frontal lobe bulges to the left. Unlike PQD and hemimegalencephaly, petalia is not accompanied by asymmetric cortical thickening, increased white matter volume, or abnormalities of cortical migration or formation.

Treatment:

• Treatment of PQD centers around management of refractory epilepsy.
• If surgical management of epilepsy is required, less extensive and therefore less debilitating functional disconnective surgery is often possible with PQD in comparison with hemimegalencephaly and multilobar cortical dysplasias, given more localized cerebral involvement. 
• Early intervention boasts high success rates, with seizure freedom reported in as many as 92% of pediatric cases.