A new study published in the Proceedings of the National Academy of Sciences evaluated muscle activation and movement patterns in blinking behaviors, and uncovered the importance of fine neural control in eyelid motion.
Give me some background.
Eyelid movements are controlled by contractions from the orbicularis oculus (OO), which is a unique skeletal muscle with a circular geometry and diffuse innervation.
The diffuse innervation is thought to allow for spatiotemporal differentiation in activation of different parts of the OO, enabling nuanced motion of the overall eyelid structure.
However: It is not currently understood how segmental activation and muscle excursion of the OO produces distinct eyelid movements during different behaviors, the study authors noted.
- In fact: Until recently, eyelid motion has largely been modeled in a single dimension (open–close).
Now talk about the study.
Investigators analyzed eyelid neuromechanics using segmental intramuscular electromyography (EMG) and a three-dimensional motion-capture system on the eyelids of eight volunteers to track movement in ultraslow motion for five eyelid behaviors, including:
- Spontaneous blinks
- Voluntary blinks
- Reflexive blinks
- Soft closure
- Forced closure
Why: To understand how activation differs segmentally across the OO and how patterns of activation change to produce different behavior-specific eyelid kinematics.
Findings?
Broadly: The research team found that each eyelid behavior is characterized by unique patterns of OO activation and resultant eyelid kinematics specific to that behavior’s purpose.
In addition: There was significant variation in muscle activation and muscle excursion patterns across the eyelids that could not reasonably be explained by a conventional single-segment model of the OO (i.e., open–close).
How did the eyelid behaviors vary in OO activation and eyelid kinematics?
- Spontaneous blinks: Characterized by early lateral-to-medial motion of the eyelid, with a small amount of reverberation (i.e., overshoot of the upper eyelid beyond its complete closure position) and incomplete closure
- Soft closure: Had a different pattern of activation intensity across the eyelid compared to spontaneous blinks and the eyelid consistently closed more fully than in spontaneous blinks
- Also: The medial OO activated more consistently earlier in spontaneous blinks than soft closure
- Voluntary blinks: Deviated in the medial direction early in closure and then returned laterally
- Reflexive blinks: Moved further and faster than the other behaviors and exhibited a larger reverberation phase, higher contraction velocities, and more complete closure than other behaviors
Forced closure: Followed a similar trajectory to soft closure, but soft closure deviated laterally at the closed end of the motion
Expert opinion?
While previous studies have shown that a small electrical pulse can stimulate the OO to move, designing a system to mimic natural eyelid behaviors has not yet been possible.
“What we now have is a good roadmap to such a device, including where exactly to place electrodes, how to time them, and how strong the pulse should be,” explained Daniel Rootman, MD, one of the study authors.
- Dr. Rootman added that these guidelines could help pave the way for the development and clinical testing of such a device, “with the ultimate goal of providing real relief for patients.”
Any limitations to consider?
A few …
- The study was performed on young, healthy participants with characteristic “double eyelid” morphology in an upright seated position
- Further studies are necessary to understand the impact of age, pathology, eyelid configuration, and posture on eyelid neuromechanics
- EMG from the levator palpebrae superioris (LPS) muscle was not recorded due to concern for the safety of the eye
Take home.
These findings demonstrate the role of segmental activation in eyelid motion, highlighting the importance of precise neural control in producing natural eyelid behavior.
Next steps: This research can act as a starting point for robust mechanistic models of eyelid function to further develop diagnostic tools, prostheses, and therapies for eyelid paralysis.