Introduction:
Arnold–Chiari type I malformation (ACH I) is classically associated with neuro-otologic symptoms, including vertigo, imbalance, and characteristic oculomotor disturbances—particularly vertical oscillopsia and downbeat nystagmus. However, variability in oculomotor involvement is increasingly recognized. Among the clinical manifestations of Arnold–Chiari type I syndrome, oculomotor disturbances—especially in the sagittal plane—are commonly reported as characteristic findings.

Aim:
This study aimed to analyze the presence and nature of oculomotor dysfunction in two patients diagnosed with Arnold–Chiari type I syndrome.

Case Study:
Two adult patients with MRI-confirmed ACH I underwent comprehensive neuro-otologic evaluation, including videonystagmography (VNG), optokinetic testing, saccadic and smooth-pursuit assessment, caloric stimulation, vestibular evoked myogenic potentials (VEMP), brainstem auditory evoked responses (BERA), audiometry, and complementary neurological and vascular studies. Oculomotor performance was analyzed in the sagittal, frontal, and horizontal planes.

Results and Discussion:
Neither patient demonstrated oculomotor abnormalities in the sagittal (vertical) or frontal planes. Expected features such as vertical gaze palsy, downbeat nystagmus or torsional deviations were absent. In contrast, both individuals exhibited isolated horizontal-plane dysfunction, including impaired smooth pursuit, asymmetrical optokinesis, horizontal nystagmus, and, in one case, canal paresis. The selective involvement of horizontal oculomotor pathways, with preservation of vertical and torsional mechanisms, suggests heterogeneous patterns of brainstem and cerebellar involvement in ACH I.

Conclusions:
Arnold–Chiari type I malformation may present with isolated horizontal oculomotor abnormalities rather than the traditionally described vertical disturbances. Recognition of such atypical patterns is important for accurate diagnostic interpretation and underscores the need for larger studies to elucidate the full spectrum of oculomotor involvement in ACH I.