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How to know which muscle is affected in Diplopia?

Diplopia is worse in the direction of action of the weak eye muscle.

Lateral rectus muscle weakness causes diplopia worse on looking to the side of the weak muscle and is worse at distance.

Medial rectus muscle weakness causes diplopia that is worse for near than for distance vision and is worse to the contralateral side.

Superior oblique weakness causes diplopia that is worse on looking downward to the side opposite the weak muscle and causes difficulty with tasks such as reading, watching television in bed, descending a staircase, and walking on uneven ground.

Yolk muscles work in unison, so that the images fall on corresponding parts of the retina. Diplopia indicates weakness of one of the yolk muscles.

For example : For looking to right side, the yolk muscles acting are right lateral rectus and the left medial rectus. On looking up and out to the right side, they are the right SR and left 10.

For Horizontal diplopia:
Find out whether diplopia is maximum to the right or left.
For example : if diplopia is maximum on looking to the left, it can be either left lateral rectus or right medial rectus (yolk muscles).
If the diplopia is worse on looking at far objects, it likely to left lateral rectus. If diplopia is more while reading (convergence), it is more likely to be right medial rectus.

For Vertical diplopia
Problems comes mainly with vertical diplopia. Vertical diplopia is caused by weakness of a depressor or elevator. If diplopia is worse in downgaze, the weak muscle is a depressor.
Either inferior rectus or superior oblique, if the diplopia is worse in upgaze, it is an elevator.
Either superior rectus or inferior oblique, If one image is tilted, then the weak muscle is more likely an oblique than a vertically acting rectus.

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