Hydranencephaly and porencephaly are rare, related malformations associated with failure of development (hypoplasia) and destruction (secondary atrophy) of primarily the neopallial part of the telencephalon (the neocortex and the ventricular zone). The pathogenesis of this anomaly is not always certain.
Porencephaly refers to the occurrence of single or multiple cystic cavities in the cerebrum, usually communicating with the lateral ventricles or subarachnoid space. According to Summers and colleagues, porencephaly might occur if infection occurs later in the period of fetal nervous system vulnerability or if it is less destructive.
Hydranencephaly is characterized by virtual absence of the cerebral hemispheres usually with preservation of olfactory and hippocampal components, fornix, and basal nuclei. The cerebral hemispheres are replaced by cerebrospinal fluid-filled sacs lined by leptomeninges, a glial membrane, and ependymal remnants. Microphthalmia has been reported and there may be loss of nerve fibers and reduced myelin staining in optic nerves. Other structures including the brainstem and cerebellum may or may not be affected. Involvement of structures other than the cerebral cortex may depend on the developmental stage of the nervous system at the time of fetal infection.
Clinical signs are usually seen within several weeks after birth and depend upon nervous structures involved. Animals with predominant cerebral cortex involvement may have behavioral changes, including dummy-like characteristics, indifference to their environment, and episodes of rage, are usually blind, and may be unable to suckle. Urinary and fecal incontinence may be present. In animals with cerebellar involvement ataxia, dysmetria, and difficulty standing may be noted. Other animals can appear active but have difficulty in prehending food and drinking water. Unilateral hydranencephaly has been observed in an 8-month old Miniature Poodle whose only clinical sign was a visual defect. Accordingly, prognosis varies from guarded to poor.
Diagnosis is suggested by clinical signs and neuroimaging techniques, such as MRI, and confirmed by histopathology, immunofluorescence, or DNA studies using polymerase chain reaction.
There is no known treatment for this condition.
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