Ocular Motor Abnormalities from Right Superior Cerebellar Peduncle Lesions
A right superior cerebellar peduncle lesion primarily causes ipsilateral cerebellar ataxia and may produce ipsilateral flapping hand tremor (rubral tremor) and ataxia when the red nucleus is involved, but typically does NOT cause isolated ocular motor abnormalities unless the lesion extends to involve adjacent structures. 1
Primary Clinical Features
Neurological Signs (Not Ocular)
- Ipsilateral cerebellar ataxia is the hallmark finding of superior cerebellar peduncle lesions 1
- Ipsilateral flapping hand tremor and ataxia occur when the lesion involves the adjacent red nucleus 1
- Ipsilateral hemiplegia or hemiparesis may develop if the cerebral peduncle is affected 1
Why Ocular Motor Signs Are Typically Absent
The superior cerebellar peduncle primarily carries efferent fibers from the cerebellum (particularly from the dentate nucleus) to the brainstem, rather than the specific pathways responsible for immediate ocular motor control 2, 3. Research has demonstrated that isolated superior cerebellar peduncle lesions can occur without saccadic abnormalities, as saccade signals project through the hook bundle rather than the main superior cerebellar peduncle 2.
Potential Ocular Motor Findings (When Lesion Extends Beyond the Peduncle)
If the lesion is not purely confined to the superior cerebellar peduncle and involves adjacent cerebellar structures, the following may occur:
Saccadic Abnormalities
- Saccadic dysmetria (overshooting or undershooting of eye movements) can occur if the lesion extends to involve the dorsal vermis or fastigial nuclei 3
- Impaired saccadic accuracy may result from disruption of cerebellar control over saccadic pulse generation 3
Smooth Pursuit Deficits
- Impaired smooth pursuit eye movements have been documented in cerebellar peduncle lesions, even when saccades remain intact 2
- This reflects disruption of cortico-ponto-cerebellar pathways that traverse the peduncles 4
Gaze-Holding Abnormalities
- Gaze-evoked nystagmus may occur if the lesion affects cerebellar structures involved in maintaining eccentric gaze 4
Critical Distinction: Middle vs. Superior Cerebellar Peduncle
Middle cerebellar peduncle lesions produce much more prominent ocular motor findings than superior cerebellar peduncle lesions, including 4:
- Spontaneous horizontal/torsional nystagmus
- Ocular tilt reaction
- Bilaterally impaired horizontal smooth pursuit
- Abnormal head impulse responses
This distinction is crucial because the middle cerebellar peduncle carries afferent information related to eye movements, while the superior cerebellar peduncle primarily carries efferent motor output 4.
Clinical Pitfalls to Avoid
- Do not assume that all cerebellar peduncle lesions produce the same ocular motor findings—the specific peduncle involved determines the clinical presentation 2, 4
- Do not overlook the possibility of third nerve palsy if the lesion is in the midbrain near where the third nerve exits, as this would produce ptosis, ophthalmoplegia, and potentially pupillary involvement 1
- Consider neuroimaging (MRI with contrast) if ocular motor abnormalities are present, as this may indicate extension beyond the superior cerebellar peduncle to involve other brainstem or cerebellar structures 1