We have spent the past few weeks discussing the role of the oculomotor tests as part of the videonystagmography (VNG) exam. This week, we will devote a few paragraphs to the smooth pursuit tracking test.
Smooth pursuit eye movements are used to maintain stable gaze on objects that are moving within the visual field. Focus is placed on the object of interest while the background is ‘‘blurred.’’ For example, if you hold your head still and watch a car drive by, you are using smooth pursuit eye movements to keep the car in the center of your line of vision. Accurate smooth pursuit requires a target (real or imagined) and matches the velocity (speed) of the target. Smooth-pursuit tracking can function alone or in conjunction with the VOR to assist in gaze stability during movement (if you are also moving your head while watching that same car).
Vestibular Testing: Smooth Pursuit
The smooth-pursuit test (also known as sinusoidal or oscillating tracking) is performed with the patient seated in front of a light bar. The patient is asked to “follow the lights” while holding their head perfectly still. Smooth pursuit is evaluated for symmetry (the difference between rightward and leftward scores) and gain (eye velocity versus target velocity). Analysis criteria take into consideration significant age and gender effects. Asymmetrical pursuit abilities are highly suggestive of central nervous system (CNS) disease.
Symmetrical Disorders of Smooth Pursuit should be interpreted with caution and should be corroborated by other signs of CNS disease before making a diagnosis of cerebellar dysfunction or central vestibular dysfunction. According to Dr. Tim Hain on his excellent website Dizziness-and-balance, common causes of symmetrical smooth pursuit abnormalities include:
• Advanced age
• Brainstem disorders
• Cerebellar disorders
• Cerebral cortical disturbances
• Congenital nystagmus
• Drug ingestion
• Visual disorders “
Whereas, Asymmetrical pursuit abnormalities are more closely associated with specific brainstem disorders and may be associated with:
• Acute parietal lobe disorder
• Acute frontal lobe disorder
• Lesion of pontine nuclei
• Superimposed nystagmus