A 6-year-old child presents with incomitant strabismus. On examination, the deviation increases when the child looks in the direction of action of the paralysed muscle. Which feature best distinguishes paralytic strabismus from comitant strabismus?

## Distinguishing Paralytic from Comitant Strabismus ### Key Diagnostic Feature **Key Point:** The single best feature that distinguishes paralytic (incomitant) strabismus from comitant strabismus is that the **angle of deviation varies with the direction of gaze — specifically, it increases in the direction of action of the paralysed muscle**. In comitant strabismus, the deviation remains constant in all directions of gaze. ### Comparison Table | Feature | Paralytic Strabismus | Comitant Strabismus | |---------|----------------------|---------------------| | **Deviation in different gazes** | **Increases in direction of paralysed muscle** | Constant in all directions | | **Primary vs Secondary Deviation** | Secondary > Primary | Secondary = Primary | | **Diplopia** | Present (especially in direction of paralysis) | Absent (suppression occurs) | | **Ocular movements** | Restricted (limitation of ductions) | Full ductions | | **Head posture** | Abnormal (face turn toward paralysed muscle) | Normal | | **Onset** | Sudden (nerve palsy, trauma) | Gradual (developmental) | ### Why Incomitance is the Defining Feature The fundamental definition of paralytic strabismus is **incomitance** — the deviation is not the same in all positions of gaze. Because one or more extraocular muscles are paretic, the eye cannot move fully in the direction of that muscle's action, so the misalignment is greatest when gaze is directed into the field of action of the paralysed muscle. This is the primary, defining characteristic that separates it from comitant strabismus (Kanski's Clinical Ophthalmology; Duane's Ophthalmology). Secondary deviation > primary deviation is a **consequence** of the incomitance (Hering's law of equal innervation), but it is a secondary sign, not the defining discriminator. The question stem itself describes the incomitant nature of the deviation, confirming that option B is the best distinguishing feature. ### Clinical Pearl **Clinical Pearl:** In clinical practice, the cover-uncover test performed in multiple gaze positions is used to demonstrate incomitance. A change in the angle of deviation across gaze positions immediately flags a paralytic (incomitant) cause and mandates neurological workup. Comitant strabismus, by contrast, shows the same angle in all positions, reflecting a supranuclear or developmental aetiology rather than a muscle/nerve palsy. **High-Yield:** Incomitance (deviation increasing in the direction of the paralysed muscle) is the pathognomonic hallmark of paralytic strabismus and is the single best discriminator from comitant strabismus on examination (Kanski's Clinical Ophthalmology, 9th ed.).