Optokinetic Eye Movements

The optokinetic (OPK) tape is an essential part of the functional neurologist's toolkit. It can be used to evaluate optokinetic nystagmus, which provides a window of assessment into the functionality of the frontal lobe, parietal lobe, cerebellum and other brain regions involved in the generation and control of optokinetic eye movements. The optokinetic tape can also be used to rehabilitate functional deficits within these brain areas, which also makes it a powerful treatment modality.

OPK nystagmus is made up of two eye movements:

  1. Smooth pursuit - initiated primarily by the parietal lobe ipsilateral to the direction of the pursuit. 

  2. Saccade - initiated primarily by the frontal lobe contralateral to the direction of the saccade.

The pursuit and saccade should be equal in amplitude and opposite in direction.

Diagram a)

Horizontal OPK

Diagram a) illustrates horizontal OPK nystagmus elicited by movement of the tape from the patient's left to right. A rightward pursuit initiated by the right parietal cortex is followed by a leftward saccade initiated by the right frontal lobe. The left cerebellum helps to control the accuracy of the leftward saccade.


Diagram b)

Diagonal OPK

Diagram b) shows diagonal OPK nystagmus from up left to down right (patient's perspective). The midline cerebellum has been shown to significantly contribute to diagonal eye movements, especially pursuits. A down right pursuit (activates right parietal lobe & left cerebellum) is followed by an up left saccade (activates R frontal lobe & left cerebellum).

 

Diagram c)

OPK & brain

Diagram c) describes different directions of optokinetic stimulation in relation to the brain regions it activates. Direction-specific OPK stimulation can be used both to test and rehabilitate a particular side and region of the brain. 

 

OPK testing:
When testing optokinetic eye movements, the practitioner should observe for:

  • Speed of nystagmus.
  • Smoothness of movement.
  • Whether the amplitude of the pursuits and saccades is equal or whether the eyes gradually drift to the right or left.
  • Fatigability of the response.

If a deficit is found, other tests for the brain region in question should be performed to confirm or refute your diagnosis - never rely on isolated findings.

For example, if the OPK saccade component appears to be deficient, testing volitional (rather than reflexive) saccades along with other tests of frontal lobe function, such as anti-saccades and rapid repeated index-finger-to-thumb opposition, will aid the diagnostic process. Cerebellar function should also be tested since the cerebellum helps to control the accuracy of saccades.

If on the other hand the OPK pursuit component appears to be failing, testing different directions of pursuits along with other tests of parietal lobe function, such as graphaesthesia, sterognosis, sensory discrimination and joint position sense, would be indicated.

Instructions to the patient:
Optokinetic eye movements are reflexive in nature so should be elicited by the patient simply looking at the moving OPK tape - the initial instruction to the patient should therefore be 'look at the tape'. Some patients however will fixate their vision straight ahead in response to this instruction and will thus suppress the OPK reflex. In this case the patient should be instructed to watch each successive red square as it appears in front of them.

Prescribing optokinetics as a home therapy:
Optokinetic stimulation powerfully stimulates the brain and is therefore an invaluable therapeutic modality. Some patients however are not able to metabolise stimulation of such intensity very well - in which case you will see rapid fatigue of the OPK response or the patient may feel sick/ dizzy after OPK testing. 

When prescribing OPK stimulation as a home therapy, be sure to test what speed and duration of stimulation the patient can cope with and then give 2/3 of that to do at home.