Some children with Chiari malformation and achondroplasia require posterior fossa decompression that typically includes expansion of the dural tube with duraplasty. Infants and young children, however, may have a more distensible dura mater than do older patients. Furthermore, the structures that compress the hindbrain of young patients may be the bone and abnormally thickened atlantooccipital membrane, i.e., dural band, rather than the dura mater. We have treated 7 children who had Chiari malformation or achondroplasia with posterior fossa decompression without duraplasty. All children were symptomatic; 3 had Chiari-I malformations, 2 Chiari-II malformations, and 2 achondroplasia. The age range was 3 months to 2.5 years (mean 15.1 months). The exent of tonsillar herniation and other hindbrain anomalies was assessed on preoperative magnetic resonance imaging. The infants with Chiari-II malformations underwent cervical laminectomies, whereas the other young children with Chiari-I malformations or achondroplasia underwent suboccipital craniectomy as well as cervical laminectomy. In Chiari malformation, the dural band was divided; in achondroplasia, there was no identifiable dural band. Following bony decompression and division of the identifiable dural band, immediate expansion of the stenotic region with visible cerebrospinal fluid space posterior to the neural elements could be ascertained by intraoperative ultrasonography. During a follow-up period ranging from 4.5 months to 4 years (mean 22 months), all patients made improvements in their symptoms, 3 having complete resolution of their symptoms. This preliminary experience indicates that in children 2 years of age or younger, posterior fossa bony decompression without duraplasty can be effective treatment for Chiari malformations or achondroplasia.
Although magnetic resonance imaging (MRI) may be more sensitivee than myelography with CT scanning in detecting spinal metastatic disease (SMD), the limitations of these MRI examinations have not been defined. The feasibility of obtaining early postoperative MRI to screen for SMD, and the type and frequency of imaging abnormalities were evaluated in this prospective study. Twenty-seven patients (78% with nondisseminating neoplasms or non-neoplastic diseases) were studied with postoperative spinal MRI without and with contrast enhancement, obtained within 24 h of craniotomy. Procedural problems with immediate spinal MRI were identified. Physician monitoring of the examination did not guarantee an adequate examination, and small abnormalities were missed or incompletely evaluated at the time of the MRI. Surprisingly, abnormal signal from intraspinal gas, blood, or blood breakdown products was not seen. In the absence of metastatic disease, spinal meningeal enhancement was absent. True SMD was detected, but nonneoplastic focal contrast enhancement (false positive) occurred in 11%. An unusual nonneoplastic subdural cerebrospinal fluid enhancement was seen in 3 of 13 (23%) of all patients undergoing posterior fossa craniectomies. This enhancement could hide small metastases or could be confused for SMD. Although immediate postoperative spinal MRI is potentially effective as a screening tool for the detection of SMD, we have identified some significant limitations of this technique.
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