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في هذا الموضوع تجدي كيفية اكتشافه و كيف نمنعة و يا ليت اطبائنا يقراون ليعرفوا ان الكثير من امراض الوراثة يمكن تفاديها . كما قلت لكي اقرائي جيدا و اي شيئ غير واضح انا موجود و اعذريني لعدم اتساع وقتي للترجمة فانا استعد لامتحان الدكتوارة في المانيا بعد 3 اسابيع . دعواتكم


Prenatal screening and diagnosis of neural tube defects

Lauri Hochberg, MD
Joanne Stone, MD


UpToDate performs a continuous review of over 350 journals and other resources. Updates are added as important new information is published. The literature review for version 14.2 is current through April 2006; this topic was last changed on April 28, 2006. The next version of UpToDate (14.3) will be released in October 2006.

INTRODUCTION — Neural tube defects (NTD) are the second most prevalent congenital anomaly in the United States, second only to cardiac malformations. Three factors have played a significant role in the assessment and prevention of this disorder in developed countries:

The widespread use of maternal screening programs to identify pregnancies at high risk [1]
Sonographic imaging combined with amniocentesis for diagnosis of affected fetuses [2,3]
Administration of folic acid supplements for prevention of the disorder [4,5].
Prenatal screening and diagnosis of NTDs will be reviewed here. Prevention of NTDs is discussed separately. (See "Prevention of neural tube defects").

INCIDENCE AND EPIDEMIOLOGY — The incidence of NTDs is highly variable and depends upon ethnic and geographic factors. Studies performed before the availability of prenatal screening and prophylactic vitamin supplementation reported the birth incidence of both spina bifida and anencephaly were higher in Caucasians than in blacks. In the United States, higher rates of NTDs were observed in the East and South than in the West [6].

Prenatal maternal serum screening programs for alpha fetoprotein (instituted in the 1970s and 1980s) combined with periconceptional folic acid supplementation and food fortification (instituted in the 1990s) have led to a decrease in the prevalence of NTDs where these interventions are practiced (see "Prevention of neural tube defects") [4,5,7-11]. As an example, the prevalence of NTDs in England and Wales declined by 96 percent between 1970 and 1997: from approximately 3.2 per 1000 births to 0.1 per 1000 births [7]. Forty percent of the decline was attributed to antenatal screening with termination of affected pregnancies and 56 percent was attributed to a decline in incidence, due at least in part to an increase in dietary folate.

EMBRYOLOGY — The central nervous system appears as a plate of thickened ectoderm called the neural plate at the beginning of the third week of embryonic life. The lateral ees of the neural plate become elevated to form the neural folds. These folds subsequently become further elevated, approach each other, and fuse to form the neural tube; the fusion begins in the cervical region and proceeds in both the cephalad and caudal directions. Since fusion is delayed at the cranial and caudal ends of the embryo, the cranial and caudal neuropores form temporary open communications between the lumen of the neural tube and the amniotic cavity until closure of the cranial neuropore on the 25th day after conception and closure of the caudal neuropore about two days later [12]. NTDs result from failure of the neural tube to close normally between the third and fourth weeks after conception (the fifth and sixth weeks of gestation).

TYPES OF NTDS — There are several anatomic types of NTDs, which affect either the spine or cranium. They can be classified as open (neural tissue exposed) or closed (neural tissue not exposed). Open NTDs often involve both the spine and cranium, while closed NTDs are usually localized and confined to the spine.

Spinal defects

Spina bifida — Spina bifida refers to a cleft in the spinal column. Spina bifida may be closed (the skin covering the defect is intact) or open (not covered by skin) (show ultrasound 1A-D).
Spina bifida occulta is the simplest form in which there is a failure of the dorsal portions of the vertebrae to fuse with one another. This abnormality, usually localized to the sacrolumbar region, is covered by skin and is not noticeable on the surface except for the presence of a small tuft of hair or other dermal lesion over the affected area. It is diagnosed, usually incidentally, by radiographs of the spinal vertebrae.

A meningocele develops if more than one or two vertebrae are involved in the defect and only the meninges of the spinal cord herniates through the opening. It is not associated with hydrocephalus or neurologic defects, but may be contiguous with a subcutaneous lipoma (ie, lipomeningocele).

A meningomyelocele or myelomeningocele occurs when both the meninges and the spinal cord herniate through the vertebral defect. It is often associated with hydrocephalus. (See "Myelomeningocele").

Cranial defects

Anencephaly — Anencephaly is the congenital absence of a major portion of the brain, skull, and scalp due to failure of the cephalic part of the neural tube to close. It is the most common NTD and is readily detected antenatally. (See "Anencephaly and encephalocele").
Exencephaly — In exencephaly the skull and scalp are absent, with exteriorization of the abnormally formed brain.
Encephalocele — Encephalocele refers to the herniation of cranial contents through a defect in the skull. (See "Anencephaly and encephalocele").
Iniencephaly — Iniencephaly is a rare malformation characterized by the triad of an occipital bone defect, cervical dysraphism (ie, defective fusion), and fixed retroflexion of the fetal head.