Midbrain cleft as a cause of chronic internuclear ophthalmoplegia, progressive ataxia, and facial weakness.

A 44-year-old man with progressive ataxia, facial weakness, bilateral adduction deficits, and abducting nystagmus was initially misdiagnosed and treated for multiple sclerosis because a midbrain anatomic cleft had been overlooked on brain MRI. Six cases of "midbrain (or mesencephalic) cleft" or "keyhole aqueduct syndrome" have been previously reported. This developmental anatomic abnormality always manifests bilateral internuclear ophthalmoplegia (INO), often together with ataxia, which may be progressive and debilitating. Because the INO is chronic, patients may have no visual symptoms. The cause of a midbrain cleft is uncertain, but it may be the midbrain version of a syrinx. There is no known effective treatment.