Inblunttrauma,theremaybebraininjurybothattheoriginalpointofcontact(coupinjury)andontheoppositesideofthebrain(contrecoupinjury)owingtoimpactswiththeskull.
Rapiddisplacementoftheheadandbraincantearaxons(diffuseaxonalinjury),oftencausingsevere,irreversibleneurologicdeficits.
Traumatictearingofbloodvessels,dependingonthelocation,leadstoepidural,subdural,orintraparenchymalhematomaaswellassubarachnoidhemorrhage.
The incidence of CNS malformations, giving rise to mental retardation, cerebral palsy, or neural tube defects, is estimated at 1% to 2%. Malformations of the brain are more common in the setting of multiple birth defects. Prenatal or perinatal insults may interfere with normal CNS development or cause tissue damage. During gestation, the timing of an injury determines the pattern of malformation, with earlier events typically leading to more severe phenotypes. Mutations affecting genes that regulate the differentiation, maturation, or intercellular communication of neurons or glial cells can cause CNS malformation or dysfunction. Additionally, various chemicals and infectious agents have teratogenic effects.
Some developmental disorders produce profound neuronal dysfunction in the absence of morphologic abnormalities. Genetic underpinnings for various forms of autism have emerged recently; many of the implicated genes contribute to the development or maintenance of synaptic connections. Rett syndrome is an X-linked dominant disorder associated with mutations in the gene encoding a regulator of epigenetic modifications of chromatin. Development in affected girls initially is normal, but neurologic deficits affecting cognition and movement appear by 1 to 2 years of age.
One of the earliest steps in brain development is the formation of the neural tube, which gives rise to the ventricular system, brain, and spinal cord. Partial failure or reversal of neural tube closure may lead to several malformations, each characterized by abnormalities involving neural tissue, meninges, and overlying bone or soft tissues. Collectively, neural tube defects are the most frequent type of CNS malformation. Folate deficiency during the first trimester sharply increases risk through uncertain mechanisms and represents an important opportunity for prevention, as administration of folate to women of child-bearing age reduces the incidence of neural tube defects by up to 70%. The combination of imaging studies and maternal screening for elevated αfetoprotein has increased the early detection of neural tube defects.
Neural tube defects include the following:
The most common defects involve the posterior end of the neural tube, from which the spinal cord forms. These can range from asymptomatic bony defects (spina bifida occulta) to spina bifida, a severe malformation consisting of a flat, disorganized segment of spinal cord associated with an overlying meningeal outpouching.
Myelomeningocele is an extension of CNS tissue through a defect in the vertebral column that occurs most commonly in the lumbosacral region (
Fig. 23.14 ). Patients have motor and sensory deficits in the lower extremities and problems with bowel and bladder control. The clinical problems derive from the abnormal spinal cord segment and often are compounded by infections extending from the thin or ulcerated overlying skin.
Anencephaly is a malformation of the anterior end of the neural tube that leads to the absence of the forebrain and the top of the skull. Varying amounts of posterior fossa structures may be present.
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Fig. 23.14 Myelomeningocele.Bothmeningesandspinalcordparenchymaareincludedinthecystlikestructurevisiblejustabovethebuttocks.
• An encephalocele is a diverticulum of malformed CNS tissue extending through a defect in the cranium. It most often involves the occipital region or the posterior fossa. When it occurs anteriorly, brain tissue may extend into the sinuses.