Bruns–Cushing nystagmus in Chiari malformation type I mimicking a cerebellopontine angle tumor

Chiari Malformation Type I is a form of abnormality in the hindbrain, characterized by herniation of the cerebellar tonsils by ≥ 5 mm into the foramen magnum. The most common clinical symptoms included headache in the occipital area, weakness or numbness in the extremities, ataxia, and eye disorders, such as decreased vision, nystagmus, extraocular muscle paresis, diplopia, and visual field disturbances. A 32-year-old woman presented with complaints of vibrating vision when looking in all directions since 2010. Neuroophthalmological examination showed oscillopsia, Bruns–Cushing nystagmus in horizontal movements, downbeat nystagmus, and upward rotatory nystagmus without latency and habituation. However, the head Magnetic Resonance Imaging (MRI) showed herniation of the cerebellar tonsils by 33.02 mm from the McRae line. Communicating hydrocephalus was found without evidence of syringomyelia. Chiari Malformation Type I was diagnosed through clinical symptoms, neurological examination, and MRI results. The patient had not yet received posterior fossa decompression, and the condition was managed conservatively, where the nystagmus persisted without significant improvement. Although Bruns–Cushing nystagmus was classically associated with cerebellopontine angle tumors, it rarely occurred in Chiari Malformation Type I due to participation of the cerebellar flocculus and the peripheral vestibular component, as well as brainstem compression. Recognition of this atypical manifestation was crucial for accurate diagnosis and appropriate management.