Neuronal migration disorder refers to a group of disorders that arise from the abnormal migration of nerve cells during embryonic development. If the migration of neuroblasts is disturbed during neurogenesis, neural circuits do not form properly in the correct parts of the brain. This is referred to as cerebral dysgenesis.
The migration commonly takes place in the second month of pregnancy. In cases of migration disorder, only some of the neuroblasts reach the cortical layer or neuroblasts overshoot their path and instead reach the subarachnoid space. In addition, the organization of the neuronal layer in the cortex may be disrupted. Abnormal migration of the neuroblasts leads to abnormal formation of the gyri, the ridges of the cerebral cortex in the brain.
Some examples of neuronal migration disorders include lissencephaly, schizencephaly, porenchephaly, pachygyria, agyria, macrogyria, microgyria, neuronal heterotopias, agenesis of corpus callosum, agenesis of cranial nerves and band heterotopias.
Symptoms of neuronal migration disorder vary according to the type and degree of abnormality. The most common symptoms include:
- Loss of muscle tone
- Decreased motor function
- Developmental delay
- Mental retardation and failure to thrive and grow
- Difficulty feeding
- Smaller than average head size. This is called microcephaly.
- Some infants have the characteristic facial features of neuronal migration disorder.
A diagnosis is made based on clinical investigations and radiological imaging studies. Treatment is symptomatic and includes the use of antiepileptic medications to prevent and correct seizures, along with physical, occupational and speech therapies to support the child with the various problems associated with the disorder.