A 2-year-old girl was recently diagnosed with a Chiari I malformation. The patient has a history of poor feeding and swallowing, as well as decreased sensation in her upper and lower extremities. She presents for posterior fossa decompression.
MRI shows Chiari I malformation without evidence of hydrocephalus.
Chiari malformations are structural cerebellar defects in which part of the cerebellum is displaced below the foramen magnum. They are classified by degree of severity and by the parts of the brain that protrude into the spinal canal. In Chiari type I malformations, only the cerebellar tonsils extend into the foramen magnum, without involvement of the brain stem. This is the most common form of Chiari malformation, and it may be asymptomatic until adolescence or even adulthood. Chiari type II malformations (also called Arnold-Chiari malformations) involve extension of both cerebellar and brainstem tissue into the foramen magnum, and are associated with the presence of a myelomeningocele. The rarer Chiari type III and type IV malformations are more serious; type III involves herniation of the cerebellum, brain stem, and possibly the fourth ventricle through the foramen magnum into the spinal canal, while type IV is characterized by cerebellar hypoplasia.
Preoperative evaluation should include a detailed history of any neurologic deficits, as well as any comorbid conditions. There should be extensive discussion with the neurosurgeon prior to the procedure. It is important to know whether or not dural opening is planned, the need for intraoperative neurologic monitoring, the requirement for steroids, and whether or not nonsteroidal anti-inflammatory drugs such as ketorolac will be permissible postoperatively. Parents should be informed that two IV lines and an arterial line will be placed, and that the patient will recover in the pediatric intensive care unit postoperatively with patient-controlled analgesia (PCA) for pain management. A Foley catheter generally is placed only in older patients. A type and crossmatch should be available, although it is rarely necessary to transfuse in these cases. Parents should always be advised that in this or any other neurosurgical case, there is always a chance that the child will remain intubated postoperatively, although the plan would be to extubate in the operating room (OR). The patient’s head will be placed in pins for this procedure, so there is no need for special facial padding when turning the child prone. A nasotracheal intubation may be considered to better ensure stability of the endotracheal tube in the prone position.
Premedication with oral midazolam would not be expected to interfere with somatosensory evoked potential (SSEP) monitoring, and should be administered if appropriate. If there are no contraindications, lines can be placed after the patient has been anesthetized with an inhalation induction.
Neurophysiologic monitoring, although controversial, is frequently used in Chiari malformation surgery; it consists of monitoring of SSEPs, and possibly brainstem auditory evoked responses (BAERs). This is particularly ...