Intraoperative Ultrasonography During Foramen Magnum Decompression for Syringomyelia Associated with Chiari I Malformation

Foramen magnum decompression (FMD) for syringomyelia associated with Chiari I malformation comprises a midline suboccipital expansive cranioplasty, C1 laminectomy, opening of the dura (with the arachnoid intact), and expansive duraplasty using Gore-Tex membrane. In this study we assess the utility of intraoperative ultrasound study for FMD. A Sonolayer alpha SSA-260A was used to perform an ultrasound study during FMD of 12 patients (7 women and 5 men), ranging in age from 13 to 60 years (average, 25.8 years). After craniotomy and laminectomy, the ultrasound scan was performed with the aid of a 7-MHz sector transducer both before and after the dura was opened. Before opening the dura, the ultrasound study demonstrated that the cerebellar tonsils abruptly descended during cardiac systole and ascended during diastole in all patients and that the syrinx contracted in synchrony with tonsillar descent. Opening of the dura reduced the amplitude of the pulsatile movement of the cerebellar tonsils and eliminated contraction of the syrinx. Postoperatively, all the patients were followed up (3 months-13 years; mean, 4.8 years) with neurological and radiological evaluations. MR imaging was used for all patients until the syrinx had disappeared. Increase in syrinx size was not observed in any patient. Ultrasound could detect craniocervical lesions very clearly in all cases. Pulsation of cerebrospinal fluid across the foramen magnum after surgery was confirmed intraoperatively. Ultrasound is quick, convenient, noninvasive, is performed in real time, and can be easily repeated to determine whether sufficient decompression has been achieved.