Porencephaly is a rare condition in which fluid-filled hollows or cavities develop on the surface of the brain. These cavities usually form at sites where damage has been caused by infection, loss of blood flow, or stroke during brain development, but may also be genetic in origin. Equivalent terms are cerebral porosis, perencephaly, porencephalia, and (no longer in favor) polyporencephaly. The prefix "por" comes from the Latin porus, for hole or cavity.
In porencephaly, large dimples, craters, or clefts develop on the surface of the brain. These cavities or cysts are filled with fluid and lined with smooth tissue. They are usually caused by injuries to the fetal or newborn brain before full development of the convolutions or gyri (singular gyrus) on the surface of the cerebrum, especially by infection, ischemia (reduction of blood flow through a vessel), infarction (blockage of blood flow through a vessel), or stroke (bleeding in the brain). The cerebral gyri develop abnormally around a porencephaly cavity, both anatomically and microscopically, and may take on a radiating pattern. Areas of abnormally small gyri (polymicrogyria) may develop on areas of the cerebrum not directly adjacent to a porencephalic cavity.
Porencephaly cavities sometimes develop symmetrically, that is, with a cavity on one side of the brain being matched by a similar cavity on the other side. When a pair of symmetric cavities are very large, they may leave only a thin arch of cerebral cortex running front to back over the top of the brain like a basket handle, a condition termed basket brain. In the most extreme cases, virtually the entire cerebrum may be replaced by fluid, a condition termed hydranencephaly.
Porencephaly is rare, and its exact incidence is unknown. A 1984 study from the University of Colorado found in a study of 18,000 patients with seizure disorder
or retarded neural development that 11 had porencephaly, a rate of 1:1650 in that abnormal population.
Causes and symptoms
Any agent or event that causes localized tissue death in the brain during development can cause porencephaly. The body walls off the injured area with a barrier of smooth tissue (encysts it), and eventually the dead tissue is cleared away and replaced with cerebrospinal fluid. One infectious agent that can cause porencephaly is cytomegalovirus, which can also cause microcephaly (small brain). Ischemic brain necrosis, the death of a portion of the brain due to restriction of blood flow through a specific vessel, most often the middle cerebral artery, can also cause porencephaly. Rarely, porencephaly can be caused by a mechanical injury such as accidental penetration of the skull by an amniocentesis needle.
Because porencephaly usually follows from a disruption during development rather than from a genetic defect, it falls into a class of cerebral defects in between primary malformations (those occurring without any specific injury or trigger, and usually genetic in origin) and secondary malformations (those resulting from injury, infection, or some other external cause). The question of whether a given case of porencephaly is primary (genetic) or secondary is important because geneticists wish to provide accurate counseling to prospective parents with family histories of porencephaly. If a familial case of porencephaly is due to infection or injury, there is probably no increased genetic risk for future generations. If, on the other hand, a familial case of porencephaly is due to heritable genetic abnormalities affecting clotting factors, for instance, there may be increased risk for a fetus affected in future pregnancies. Research by the National Institute of Neurological Disorders and Stroke, an arm of the National Institutes of Health, commenced in 2000 to determine if acquired and/or genetic abnormal coagulation factors in the blood are associated with porencephaly, stroke, and cerebral palsy.
The symptoms of porencephaly are varied, and depend on the severity of the defects in each individual case. Persons with porencephaly may suffer early death, epilepsy, moderate or severe mental retardation, blindness, epilepsy, rigidity, and paralysis.
Imaging technologies such as ultrasound, x-ray computerized tomography, and magnetic resonance imaging (MRI) can diagnose porencephaly before or after birth. Ultrasound is preferred for fetal imaging, both because it is cheaper than MRI or computed tomography (CT) scan and, in most cases, just as informative; and because of lingering concerns that magnetic resonance imaging might, by some unknown mechanism, be capable of disrupting the normal formation of organs. (X rays are not used because fetuses are known to be extremely vulnerable to ionizing radiation.) An initial diagnosis can sometimes be made by shining a light through the newborn's skull.
As with other severe congenital defects of the brain, the membership of a porencephaly patient's treatment team will depend on the severity and exact nature of the damage. A pediatric neurologist and physical therapist will probably be involved, at minimum.
Treatment is addressed to alleviating symptoms, not to curing the underlying problem, as there is no treatment to induce the brain to grow missing sections of the cerebrum. Treatment includes physical therapy for rigidity, spasticity, or movement difficulties; medication to prevent seizures; and, if necessary, the installation of a shunt or drain to remove excess cerebrospinal fluid from the inside of the skull.
As of early 2004, the National Institute of Neurological Disorders and Stroke was sponsoring research entitled "Study of Abnormal Blood Clotting in Children with Stroke." More information can be found by contacting the National Institute of Neurological Disorders and Stroke (NINDS), 9000 Rockville Pike, Bethesda, Maryland, 20892, Patient Recruitment and Public Liaison Office, telephone: (800) 411–1222, email: email@example.com.
Most persons with porencephaly die before reaching adulthood. Each individual's prognosis will depend on the location and severity of the lesions on their cerebrum.
Graham, David I. and Peter L. Lantos. Greenfield's Neuropathology, 6th edition. Bath, UK: Arnold, 1997.
National Institute of Neurological Disorders and Stroke. NINDS Porencephaly Information Page. <http://www.ninds.nih.gov/health_and_medical/disorders/porencephaly.htm> (April 7, 2004).
National Organization for Rare Disorders. 55 Kenosia Avenue, Danbury, CT 06813-1968. (203) 744-0100 or (800) 999-6673; Fax: (203) 798-2291. firstname.lastname@example.org. <http://www.rarediseases.org>.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Avenue, White Plains, NY 10605, USA. (914) 428-7100 or 888-MODIMES (663-4637); Fax: (914) 428-8203. email@example.com. <http://www.marchofdimes.com>.
Larry Gilman, PhD