Paramedian pontine reticular formation

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The paramedian pontine reticular formation (PPRF) is a group of neurons in the brainstem that acts as the horizontal gaze center. It sits in the pons near the abducens nucleus and is just in front and to the side of the medial longitudinal fasciculus.

The PPRF controls horizontal eye movements. It sends bursts of signals to the same-side (ipsilateral) abducens nucleus to move the eyes outward, and it also influences signals to the opposite-side oculomotor nucleus to help coordinate both eyes together. This makes it essential for conjugate horizontal gaze and quick eye movements called saccades. The PPRF lies close to other eye-movement pathways and connects down the brainstem with nearby structures, including the nucleus prepositus hypoglossi.

Damage to the PPRF disrupts horizontal gaze on the side of the lesion. Unilateral lesions typically cause an inability to move the eyes horizontally to that side (ipsilateral horizontal gaze palsy) and impair saccades. Depending on the extent, other eye movements can be affected, and eye drifting or nystagmus may occur when looking in the opposite direction.

Most short-term PPRF lesions are due to small-vessel strokes from lipohyalinosis and high blood pressure. These small arteries are prone to blockage or microemboli, especially in people with vascular risk factors, heart valve issues, or arrhythmias. Within the PPRF, there are different neuron groups, including a population that links to the cerebellum to help refine eye movements.