Incomitent

The angle changes according to the direction of gaze

Subtypes

• Neurogenic: reduced velocity in the field of a paretic muscle

  • Greater for versions than ductions
  • Reduced saccadic velocity
  • Associated with sequelae if longstanding (see above):
    • Underaction of palsied muscle
    • Overaction of contralateral synergist (yoke)
    • Overaction of the ipsilateral antagonist due to contracture
    • Underaction of contralateral antagonist (by Hering’s law)

• Mechanical: limitation away from a restricted muscle

  • Equal for ductions and versions
  • Normal saccadic speed
  • Associated with IOP increase in the direction of limitation and globe retraction

Ocular neuromyotonia (ONM)

• Rare cause of episodic diplopia (lasting seconds to minutes) due to tonic spasms of the extraocular muscle
• Associated with skull-base radiation: onset may be years later
• Tends to involve same nerve every time: can affect III, IV or VI
• Triggered by activation of the involved nerve/muscle
• Responds well to carbamazepine or gabapentin

	Neurogenic	Restrictive
Ductions/versions	Ductions > versions	Ductions = versions
Saccades	Slower	Normal
Sequelae	Yes	No (or only contra synergist)
IOP change	No	Yes
Globe	Normal position	Retraction
Hess	Smaller field represents affected eye with increasing concomitance with time due to sequelae	Compression of fields in direction of limitation
Forced duction testing	Full passive movement (unless antagonist contracture)	Reduced passive movement

Myasthenic strabismus

• Consider myasthenia with variable, fatiguable ocular motility abnormalities
• Associated with ptosis, Cogan’s lid twitch and systemic involvement
• Seen in CPEO, TED, myositis
• Mechanical strabismus patterns