Polymicrogyria

Harvey B Sarnat MD FRCPC MS (

Dr. Sarnat of the University of Calgary has no relevant financial relationships to disclose.

)
Originally released January 19, 1998; last updated July 11, 2018; expires July 11, 2021

This article includes discussion of polymicrogyria, microgyria, micropolygyria, and status verrucous deformans. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.

Overview

Polymicrogyria is a complex malformation of cortical development due to disorder of neuronal organization, in which the process of normal cerebral cortical development is disturbed in an early stage of cortical organization. Macroscopically or grossly, including by MRI, it is defined by clusters of gyri that are too small and too numerous for the area. Microscopically, not only are there numerous gyri in clusters but also many of the gyri are fused to adjacent gyri with continuity of their molecular zones and synaptic activity without the normal intervening subarachnoid space with leptomeninges and small meningeal blood vessels. Lamination of the cortex of these gyri also is abnormal with many displaced and disoriented neurons of all types, but there are no dysplastic megalocytic neurons as seen in focal cortical dysplasia type II, hemimegalencephaly, or tuberous sclerosis. Discontinuities or gaps in the pial membrane and lamina limitans, including the fetal subpial granular glial layer of Brun, are morphological features that lead to gyral fusion.

Key points

 

• Polymicrogyria is a cerebral malformation of cortical development with excessive folding of small, numerous fused gyri and normal or abnormal cortical lamination.

 

• Fusion of gyri microscopically results in continuity of the molecular zone of adjacent small gyri without intervening pial membranes or leptomeninges; pathogenesis involves discontinuities in the pia of the cortex during gyration or sulcation after mid-gestation.

 

• The underlying etiologies are multiple but can be summarized into 3 categories: (1) genetic disorders; (2) association with other malformations of the brain, such as schizencephaly; and (3) vascular infarcts or ischemic zones, resulting in a wide variety of neurologic signs and symptoms.

 

• The distribution of polymicrogyria most frequently is perisylvian; it always is associated with schizencephaly, but in other cases it may be focal in any lobe or can be generalized throughout the cortex of both hemispheres.

 

• High-resolution MRI is the most reliable imaging modality for diagnosis and evaluating the distribution and extent of the dysgenesis and association with other brain malformations; neuropathology provides microscopic details not revealed by imaging.

 

• Surgical resection of epileptogenic foci within polymicrogyria may provide good seizure control with better developmental outcome in children, but polymicrogyria is not always epileptogenic.

Historical note and terminology

Originally coined in the early 1900s by Bielschowsky (Pascual-Castroviejo et al 2001), the term, “polymicrogyria” was largely replaced by the term “microgyria” by the middle of the 20th century. Contemporary preference has seen a resurgence of the term “polymicrogyria,” which more accurately describes the findings in this disorder (Pascual Castroviejo et al 2003).

Classification: Several types with various manifestations and etiologies have been noted. Under the current classification of malformations of cortical development, the following types of polymicrogyria are defined by MRI (Barkovich et al 2005; Barkovich 2013). They also can be so classified by gross neuropathology.

 

(1) Bilateral frontal polymicrogyria (BFP)
(2) Bilateral frontoparietal polymicrogyria (BFPP)
(3) Bilateral perisylvian polymicrogyria (BPP)
(4) Bilateral parasagittal parieto-occipital polymicrogyria (BPPOP)
(5) Bilateral generalized polymicrogyria (BGP)
(6) Unilateral perisylvian polymicrogyria (UPP) (Barkovich et al 2005)

Outdated names used for polymicrogyria include microgyria, micropolygyria, and status verrucosus deformis (Crome 1952).

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