why is secondary deviation more than primary deviation in incomitant squint ?
In incomitant squint (also known as strabismus), the deviation of the eye (misalignment) is different in different directions of gaze or positions of the eye. This occurs due to variations in muscle function and muscle balance in different directions.
When discussing primary and secondary deviations in incomitant squint, we are referring to the misalignment of the eyes in the primary position (looking straight ahead) and when the eyes are moved away from the primary position, respectively.
Here’s why the secondary deviation can be greater than the primary deviation in incomitant squint:
- Muscle Imbalance and Weakened Muscles: In incomitant squint, there is often a significant muscle imbalance, weakness, or paresis (partial paralysis) of certain extraocular muscles. These weak muscles may not be able to generate normal levels of force to move the eye effectively in all directions.
- Compensatory Mechanisms and Adaptation: The brain and ocular muscles may adapt to the muscle imbalance to maintain binocular vision or minimize diplopia (double vision). In the primary position, the brain may employ compensatory mechanisms, such as activating other muscles or altering the muscle tone, to reduce the deviation and maintain binocular vision.
- Restrictions and Mechanical Factors: In incomitant squint, there could be mechanical factors, restrictions, or fibrosis affecting certain muscles or muscle paths. This can limit the range of movement of the affected eye, leading to a smaller primary deviation in the primary position.
- Unopposed Muscle Action: When the eye is moved away from the primary position, the weakened muscle’s function is further compromised. Meanwhile, the unopposed action of the stronger or normal antagonist muscle becomes more pronounced, causing a greater misalignment or secondary deviation.
In summary, the secondary deviation can be greater than the primary deviation in incomitant squint due to factors such as muscle imbalance, compensatory mechanisms, mechanical restrictions, and the unopposed action of stronger muscles. The imbalance and limitations become more evident when the eyes move away from the primary position.