The pathology of occipital encephalocoele and a discussion of the pathogenesis.

The midline structures of 5 infant brains with occipital encephalocoeles were studied. Though all cases showed a uniform finding of a midline defect of the occipital bone with a herniated mass, the extent of the bony defect, the amount and nature of the herniated tissues and the degree of distortion of the intracranial structures varied markedly. Two of the cases showed multiple associated systemic malformations and another case had an inverse cerebellum. In spite of such a marked variability in presentation, the common findings of herniation and displacement of the mesencephalon and roof of the diencephalon allowed us to postulate that the primary defect of occipital encephalocoeles is an abnormality of the tissues overlying the mesencephalon of the developing brain. Initial herniation of the mesencephalon and subsequent movement of the rest of the brain as a result of growth determines the tissues present in the hernia sac at birth. Other theories of genesis of occipital encephalocoeles are discussed.