Post-Rotatory Nystagmus: A Dive Deeper
Building on the VOR (vestibulo-ocular reflex) circuit, this section explores a specific eye movement phenomenon called post-rotatory nystagmus.
The Experiment
- Students in a medical school class participate in an experiment.
- They are rotated in a chair for 30-60 seconds, then stopped abruptly.
The Outcome: Nystagmus in Action
- After stopping, the students exhibit a jerking eye movement known as nystagmus.
- This nystagmus is termed “post-rotatory” because it occurs after rotation.
- It’s a beating nystagmus, with the eyes repetitively moving in one direction (fast phase) followed by a corrective movement in the opposite direction (slow phase).
Understanding the Mechanism
- During rotation, the fluid within the inner ear canals (cupula) bends due to the motion.
- Stopping the rotation causes the cupula to return to its original position, but with a relative motion in the opposite direction.
- This triggers a false positive for the VOR circuit, resulting in a corrective eye movement (slow phase) in one direction.
- Once the eye reaches a new position, the system resets, leading to another fast corrective movement (nystagmus cycle).
Visualizing the Movement
- The nystagmus appears as a series of slow drifts followed by quick, jerky eye movements.
- A video demonstrates this in a medical student after a clockwise chair rotation.
- The student experiences a left-beating nystagmus, with fast eye movements to the left (corrections).
Nystagmus: Not Always a Cause for Concern
- This post-rotatory nystagmus is a normal physiological response.
- It indicates a healthy vestibular system and its ability to maintain gaze stability.
- The nystagmus eventually subsides (around 30 seconds) as the system resets.
- During the nystagmus, the person experiences a sensation of the visual world moving (oscillopsia).
Other Examples of Healthy Nystagmus
- Optokinetic nystagmus: This occurs when following repetitive visual patterns, like a picket fence while riding in a vehicle.
- The eyes track the pattern (slow phase) then make quick corrective movements (fast phase) to maintain fixation.
The Lecture Moves On
- The lecture concludes by exploring how the VOR function might change under various conditions.