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M. Dey, R. Rajendram, S. Farook, F. Proudlock, Y. Rajabally, N. Messios, I. Gottlob; Basilar Invagination and Upbeatnystagmus . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2743.
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Purpose: Basilar invagination is a congenital craniovertebral anomaly, which results in abnormal protrusion of the odontoid peg into the foramen magnum. Previous cases of basilar invagination described in the literature have been associated with downbeat nystagmus. We report a case displaying upbeat nystagmus. We have used eye movement recordings to characterize this unusual finding for the first time. Methods: A 41 year-old Indian male underwent ophthalmic and neurological examination. Horizontal, vertical eye movements were recorded with a high resolution infrared system at 250 Hz and torsional eye movements at 50 Hz. He underwent a T2 weighted MRI scan. Results: The patient was symptom free. Visual acuity was 0.66 in each eye. He had upbeat nystagmus in the primary position, which became more prominent on upward gaze and convergence. On neurological examination, cranial nerves were normal. There was no hypertonia or reduction of power, but tendon reflexes were brisk in all four limbs. There was no sensory deficit to suggest the presence of a syrinx. His gait was mildly broad based and he was unable to walk in a straight line. There were no other positive cerebellar signs. Eye movement recordings confirmed upbeat nystagmus of 1 degree amplitude in the primary position with an increase in upgaze. The frequency was 2 to 3 Hz and the velocity of the slow phase was linear. There was no torsional nystagmus. He had saccadic horizontal and vertical pursuit and slow vertical saccades. The MRI revealed basilar invagination with the pons and brainstem flexed over the odontoid process. The deformity was maximal at the pontomedullary junction. There was also evidence of compression of the fourth ventricle and cerebellum. There was no evidence of tonsillar herniation or associated hydrocephalus. Conclusion: In contrast to previous literature our patient with basilar invagination presented with prominent upbeat nystagmus, this finding was documented with detailed eye movement recordings. Since he was not experiencing oscillopsia it is likely that the onset of nystagmus was in early childhood. It is possible that the lesion responsible for the upbeat nystagmus is at the pontomedullary junction as this feature was particularly pronounced in this case, however it is not possible to exclude cerebellar compression.
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