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Show PHOTO ESSAY Isolated Sixth Cranial Nerve Aplasia Visualized With Fast Imaging Employing Steady- State Acquisition ( FIESTA) MRI Svetlana A. Pilyugina, MD, Nancy J. Fischbein, MD, Y. Joyce. Liao, MD, PhD, and Timothy J. McCulley, MD FIG. 1. A. Reduced abduction of the left eye in a 12- month- old girl with left sixth cranial nerve aplasia. Note that there are no changes in lid fissure height with gaze. B- D. MRI axial fast imaging employing steady- state acquisition ( FIESTA) sequence. B. The caudal medulla level shows no abnormalities. C. The pontomedullary junction ( PMJ) level demonstrates the cisternal segment of the right sixth cranial nerve { black arrow) but not of the left sixth cranial nerve. The basilar artery { white arrow) is shown. D. The pons level shows no abnormalities. Abstract: An otherwise healthy 12- month- old girl presented for evaluation of reduced abduction of the left eye detected at 6 months of age. The remainder of the examination was unremarkable. A special MRI sequence- fast imaging employing steady- state acquisition ( FIESTA)- visualized the right but not the left sixth nerve cisternal segment. This is the first reported use of the MRI FIESTA sequence to diagnose aplasia of the sixth cranial nerve. (/ Neuro- Ophthalmol 2007; 27: 127- 128) Departments of Ophthalmology ( SAP, YJL), Radiology ( NJF), and Neuro- Science ( YJL), Stanford University School of Medicine, Stanford, California; and Department of Ophthalmology, University of California, San Francisco, San Francisco, California. None of the authors have proprietary interest in the material discussed. Address correspondence to Timothy J. McCulley, MD, Department of Ophthalmology, Stanford University School of Medicine, 300 Pasteur Drive, Room A157, Stanford, CA; E- mail: mythird@ msn. com An otherwise developmentally normal 12- month- old girl presented for evaluation of reduced abduction of the left eye which had been stable since detection at 6 months of age. The child had no similarly affected family members and was a product of a normal pregnancy and uncomplicated spontaneous vaginal delivery. On examination, the only identifiable abnormality was absent abduction of the left eye ( Fig. 1). Motility was otherwise full and she was orthophoric in primary gaze without evidence of amblyopia. Eyelid position was normal and did not fluctuate with gaze change. There was no nystagmus or globe retraction with eye movement. Pupil, slit- lamp, and dilated fundus examinations, as well as the neurologic evaluation, were unremarkable. Neuroimaging was performed using a 1.5- T MRI scanner ( Signa Infinity Excite; GE Healthcare, Waukesha, WI). The axial fast imaging employing steady- state acquisition ( FIESTA) sequence was obtained to evaluate J Neuro- Ophthalmol, Vol. 27, No. 2, 2007 127 J Neuro- Ophthalmol, Vol. 27, No. 2, 2007 Pilyugina et al the cisternal segments of the cranial nerves. Sequential axial images revealed absence of the left sixth cranial nerve. No other brain abnormalities were observed. Sixth cranial nerve aplasia is most commonly encountered as a component of Duane or Mobius syndrome with isolated cases being exceedingly rare ( 1). Before modern imaging techniques, cranial nerve aplasia was a presumptive clinical diagnosis only confirmed postmortem with brain stem sectioning ( 1- 5). By using standard MRI techniques, the absence of the sixth cranial nerve in Duane syndrome has occasionally been demonstrated ( 6- 9). With improved imaging techniques, the diagnosis of sixth cranial nerve aplasia is more readily substantiated. A relatively new technique is high spatial resolution MRI with three- dimensional ( 3- D) FIESTA. FIESTA generates T2/ T1 contrast because of its short repetition time ( TR) and the symmetrical and balanced gradient around the echo time ( TE). Thus, it provides very high signals from tissues with large T2/ T1 ratios, such as fluid, blood, and fat. Flow- compensating gradients, short TR and TE in the FIESTA sequence, allow for thinner slice imaging and significantly reduced scanning time and cerebrospinal fluid ( CSF) flow artifacts. The FIESTA sequence can therefore provide more detailed information than other heavily weighted T2 imaging of the cisternal segments of cranial nerves ( 10). Although cranial nerves can be visualized using thin-section 3- D fast spin echo, the FIESTA sequence further facilitates the assessment of cranial neuropathies by providing an ultra- thin noninvasive myelography effect with very bright CSF and the nerves and vessels of the basal cisterns appearing as " filling defects." The FIESTA technique may allow more widespread and accurate diagnosis of disorders of the sixth and other cranial nerves and may eventually prove to be a helpful asset in elucidating their etiology and in guiding management. REFERENCES 1. Hickey WF, Wagoner MD. Bilateral congenital absence of the abducens nerve. Virchows Arch A Pathol Anat Histopathol 1983; 402: 91- 8. 2. Philips WH, Dirion JK, Graves GO. Congenital bilateral palsy of the abducens. Arch Ophthalmol 1932; 8: 355- 64. 3. Hoyt CS, Billson FA, Taylor H. Isolated unilateral gaze palsy. JPediatr Ophthalmol 1977; 14: 343- 5. 4. Miller NR, Kiel SM, Green WR, et al. Unilateral Duane's retraction syndrome ( type 1). Arch Ophthalmol 1982; 100: 1468- 72. 5. Hotchkiss MG, Miller NR, Clark AW, et al. Bilateral Duane's retraction syndrome: a clinical- pathologic case report. Arch Ophthalmol 1980; 98: 870^ k 6. Parsa CF, Grant E, Dillon WP Jr, et al. 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