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

INTRODUCTION

Chiari malformation type I is a hindbrain anomaly, characterized by herniation of the cerebellar tonsils ≥5 mm below the foramen magnum, without participation of brainstem or supratentorial structures. A previous study reported the prevalence of Chiari malformation type I in the range of 0.1% and 0.5%.^[1]

The most common clinical manifestations include occipital headache, weakness, or paresthesia of the extremities, ataxia, and ocular disturbances, such as decreased visual acuity, diplopia, oscillopsia, and nystagmus. The classical form of nystagmus typically observed in Chiari malformation type I is downbeat and rotary nystagmus during vertical movements and horizontal gaze, respectively.^[2]

CASE REPORT

A 32-year-old woman presented with oscillopsia when gazing in all directions since 2010, which had progressively worsened over the past 3 years. The patient reported gait imbalance with a tendency to fall toward the left side. Walking also required a widened stance to maintain stability. The recurrent episodes of vertigo lasted approximately 3–4 h, which were not aggravated by changes in position. Intermittent episodes of tinnitus and vertigo were also reported.

Neurological examination showed a Glasgow Coma Scale score of E4V5M6. The pupils were round, isochoric, 3 mm in diameter, and reactive to light bilaterally. Ocular motor examination showed oscillopsia, Bruns–Cushing nystagmus, downbeat nystagmus, and rotatory nystagmus on horizontal, downward lateral, and upward lateral gaze, respectively, all without latency or habituation. A characteristic form of Bruns–Cushing nystagmus was observed during the examination, suggesting significant participation of the cerebellar flocculus. When the gaze was directed to the right side, which corresponded to the size of the larger floccular herniation shown in Figure 1, a high-amplitude, low-frequency nystagmus appeared. However, leftward gaze induced a low-amplitude, high-frequency nystagmus. This combination of gaze-evoked nystagmus on the one side and large-amplitude compensatory nystagmus on the opposite side was consistent with Bruns–Cushing nystagmus. The cerebellar examination showed truncal ataxia and a wide-based gait, while autonomic function was within normal limits.

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Figure 1:

T2-weighted magnetic resonance imaging of the head showed asymmetric herniation of the cerebellar flocculus (arrow), more pronounced on the right side.

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Magnetic resonance imaging (MRI) of the brain with contrast showed herniation of the cerebellar tonsils extending 33.02 mm below the McRae line [Figure 2]. MRI also showed asymmetric herniation of the cerebellar flocculus, more significant on the right side [Figure 1]. A communicating hydrocephalus was identified, with no evidence of syringomyelia. The diagnosis of Chiari malformation type I was established through clinical manifestations, neurological examination, and MRI results. In this study, posterior fossa decompression was discussed as the definitive treatment. The patient declined surgical intervention after counselling and was currently being managed conservatively, with the nystagmus persisting without significant improvement.

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Figure 2:

T2-weighted magnetic resonance imaging of the head showed the cerebellar tonsillar herniation extending 33.02 mm below the McRae line. The McRae line used as the anatomical reference for measuring cerebellar tonsillar descent (blue line). The measured extent of cerebellar tonsillar herniation below the McRae line (red line).

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DISCUSSION

Chiari malformation is a congenital disorder with diverse neurological symptoms. The occurrence of Bruns–Cushing nystagmus in Chiari malformation type I is highly unusual and prevalent in patients with cerebellopontine angle tumors. In this case, dysfunction occurs in the vestibulocerebellar areas, namely the uvula, flocculus, and paraflocculus of the cerebellum, along with symptoms caused by brainstem compression.^[3] Compression of the ipsilateral pons causes dysfunction in neuronal integration (including the cerebellar flocculus), leading to an inability to maintain eccentric gaze toward the ipsilateral side of the lesion. This condition led to nystagmus with high amplitude and low frequency. Meanwhile, impairment of the peripheral vestibular components causes a reduction in tonic neuronal burst activity, producing slow-phase eye movements toward the side of the lesion with compensatory quick phases directed contralaterally.^[4]

The recommended radiological examination for establishing the diagnosis of Chiari malformation is MRI. However, some cases are identified using computed tomography scans. Tonsillar herniation is considered pathological when it exceeds 5 mm, with 3–5 mm regarded as borderline. Other radiologic abnormalities that are occasionally observed include atlanto-occipital assimilation, basilar invagination, and cervical vertebral fusion. These abnormalities may be associated with symptoms of cervical instability. The configuration of the cerebellar tonsils is also an important factor related to the severity of clinical symptoms.^[1]

The mass effect caused by the cerebellar tonsillar herniation measuring 33.02 mm [Figure 2] led to the development of Bruns–Cushing nystagmus. This development represents a combination of peripheral and central vestibular nystagmus, including the cerebellar flocculus [Figure 1], the peripheral vestibular components of the cerebellum, and brainstem compression similar to the cases of cerebellopontine tumors.

CONCLUSION

Chiari malformation type I may present with atypical ocular motor abnormalities, such as Bruns–Cushing nystagmus, which is classically associated with cerebellopontine angle tumors. Based on the results, clinicians are recommended to include congenital anomalies, particularly Chiari malformation, in the differential diagnosis of patients presenting with this type of nystagmus. MRI of the brain remains the gold standard for confirming the diagnosis and evaluating associated structural abnormalities.