Prenatal diagnosis of central nervous system abnormalities

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Temporomandibular Joint Radiology Update MAGNETIC RESONANCE IMAGING (MRI) with small surface coil receivers appears superior to computed tomography (CT) for visualizing the articular discs in both the normal and displaced configurations of the temporomandibular joint. MRI promises to give sufficient detail of the soft tissues of the joint so that using the more invasive positive contrast arthrography may no longer be necessary. Whether or not perforations and tears of the disc can be diagnosed and the state of the bony articular surfaces can be defined still need investigating. Correlative studies need to be done to document the sensitivity and specificity of MRI. There are still investigative and clinical roles for CT and arthrography. CT may be indicated in communities where MRI is not yet available to screen for disc displacement. In patients with trauma, CT is superior to both conventional radiography and MRI for showing fractures and bleeding. Altered bony joint components and degenerative changes are better seen with CT. Arthrography may be an important test to consider for preoperative evaluation, particularly if CT or MRI findings are equivocal for showing the articular disc. Its ability to show disc movement during function is unsurpassed. Because of the expense of these sophisticated tests, the significant x-ray exposure from tomograms, arthrography and CT and the current lack of one test to satisfy all diagnostic requirements, the use of these tests must be tailored to each patient. For patients with acute trauma, CT is the first choice. When a clinical evaluation points to disc dislocation, unnecessary x-ray exposure and expense can be avoided by going directly to MRI. A patient with long-standing joint problems may be best examined with CT, particularly if a screening radiograph shows joint deformity. JOSEPH R. THOMPSON.

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Christiansen EL. Moore RJ, Thompson JR, et al: Radiation dose in radiography, CT, and arthrography of the temporomandibular joint. AJR 1987 Jan: 148:107-109 Christiansen EL. Thompson JR. Hasso AN. et al: CT evaluation of traumna to the temporomandibularjoint. J Oral Maxillofac Surg. in press Katzberg RW. Bessette RW, Tallents RH. et al: Normal and abnormal temporomandibularjoint: MR imaging with surtace coil. Radiology 1986 Jan: 158:183-189

Prenatal Diagnosis of Central Nervous System Abnormalities As ULTRASOUND image resolution has improved, its role as a low-risk imaging modality for detecting fetal morphologic abnormalities has dramatically increased. Abnormalities of the central nervous system (CNS) have been particularly well studied because the fetal head is routinely imaged in the course of pregnancy dating, CNS abnormalities are relatively common and there is interest in preventing the devastating clinical consequences of many of these abnormalities. The most common congenital CNS abnormalities are the neural tube defects-anencephaly, encephalomeningocele, myelomeningocele-occurring in aggregate in about 0.2 % of births. Anencephaly is readily detectable sonographically from the end of the first trimester onward. Encephalomeningocele and myelomeningocele are detectable by 18 to 20 gestational weeks in most cases. Because 90% of these defects occur sporadically, detecting these abnormalities based on ultrasound screening alone is not practical. Patients with elevated maternal serum or amniotic fluid ce-fetoprotein levels

constitute a high-risk group for neural tube defect and should be evaluated with high-resolution sonography. The state of California has instituted a statewide maternal serum AFP screening program that should allow detection of the vast majority of neural tube defects. Fetal cerebral ventricles can be consistently visualized from early in the second trimester. This allows detection -of hydrocephalus, which may occur as a result of aqueductal stenosis, the Chiari II malformation associated with myelomeningocele or other causes. Anatomic features usually allow distinction from other causes of dilated cerebrospinal fluid spaces, including hydranencephaly and holoprosencephaly. The posterior fossa can also be well imaged, allowing prenatal diagnosis of the Dandy-Walker malformation. When ultrasonography detects a definite or possible CNS abnormality, a follow-up examination is frequently useful to further characterize the anomaly, follow progression or plan treatment. In cases of diagnostic difficulty, a second imaging modality-intrauterine magnetic resonance imaging or computed tomography-can be useful. This is necessary in only a small percentage of cases, however. Because many of these abnormalities are rare, referral to a center specializing in prenatal diagnosis may be indicated. An accurate prenatal diagnosis of CNS abnormalities allows informed patient counseling. This may result in terminating the pregnancy or optimizing obstetric and neonatal care to allow the best possible outcome. RONAL D R. TOWNSEND. MD ROY A. FILLY, MD

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Fiske CE. Filly RA: Ultrasound evaluation of the norimial and abnormilal tetal neural axis, chap 6, 1hi Callen PC (Ed): Ultrasonography in Obstetrics and Gynecology. Philadelphia. WB Saunders, 1983, pp 97-1 12 Harrison MR, Golbus MS, Filly RA: The Unborn Patient: Prenatal Diagnosis and Treatnient. Orlando. Fla, Grune& Stratton, 1984. pp 33-123 Hofiman HJ. Epstein F (Eds): Disorders of the Developing Nervous System: Diagnosis and Treatmnent. Boston. Blackwell Scientific. 1986

Percutaneous Gastrostomy THE ABILITY TO CREATE a gastrostomy by using percutaneous techniques without general anesthesia has simplified patient care. Total parenteral nutrition can frequently be replaced with percutaneous enteral feeding in acutely or chronically ill patients in whom the gastrointestinal tract is intact and functioning. Medical costs can be significantly reduced. Long-term nasogastric feeding is uncomfortable and problems of reflux and esophagitis are common. Percutaneous gastrostomy can largely eliminate the problem of reflux, particularly if the feeding is done in the jejunum. The stomach is ideally suited for placement of a percutaneous tube because it is a "self-sealing" system. Three layers of interlacing muscular fibers contract around the tube, preventing significant leakage. When a surgical gastrostomy is done, the muscle fibers are cut, and it is therefore necessary to suture the stomach to the anterior abdominal wall to prevent leakage. Several techniques are available for placing a percutaneous gastrostomy. Using fluoroscopy, the stomach is initially inflated with air and, under fluoroscopic guidance, is punctured with a needle and a guide wire is inserted. A small catheter is placed over the guide wire and, under fluoroscopic guidance, the guide wire is advanced beyond the ligament of Treitz. The tract is then serially dilated and a sheath is placed. A 12 French catheter is advanced over the guide wire and

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