CLASSIFICATION OF CNS MALFORMATIONS

Objective: to find a clinically useful etiological classification of malformations of the CNS that combine traditional morphology with molecular genetics.

Introduction: Classification is primordial in organizing thoughts for comparing information and concepts; the process begins soon after birth. The classification of malformations of the brain has undergone many revisions over the years as new data and concepts were discovered, but the most important principles are 1) recognition that all malformations must be viewed in the context of disorders of embryological development, and 2) traditional descriptive morphogenesis of the past centuries must be integrated with recent molecular genetic data about developmental programming. Neither pure morphological schemes nor pure genetic schemes provide full insight into normal and abnormal ontogenesis and its application to clinical care of children with congenital malformations. We propose that malformations be classified into categories as disorders of 1) genetic gradients of expression in the three axes of the neural tube; 2) segmentation of the neural tube; 3) neuroblast migration; 4) cellular lineage; 5) neural tube induction of non-neural tissues: neurocristopathies. Examples:

Axes of the Neural Tube:

Vertical axis: Overexpression of a dorsalizing or ventralizing gene leads to duplication or hypertrophy of structures; Underexpression results in midline noncleavage or hypoplasia.

Longitudinal axis: rostrocaudal gradient: holoprosencephaly with noncleavage of diencephalon and mesencephalon, as well as telencephalon; caudorostral gradient: sacral agenesis.

Horizontal axis: mediolateral gradient: holoprosencephaly affecting medial cortex more than lateral cortex; lateromedial gradient: pontocerebellar hypoplasia.

Segmentation of the Neural Tube:

Agenesis of specific neuromeres: absent midbrain and pons; absent basal ganglia.

Ectopic expression: Chiari malformation.

Neuroblast migratory disorders:

Lissencephalies; pachygyrias; schizencephaly.

Disorders of cellular lineage:

tuberous sclerosis; hemimegalencephaly.

Disorders of neural crest:

neural induction of craniofacial structures; neurocutaneous syndromes.

Conclusion: The New Neuroembryology is an integration of anatomical morphogenesis and molecular genetic programming that provides insight as an etiological, rather than a purely descriptive, classification of CNS malformations.