1. Comitant

 A. Refractive-myopic refractive error cause of deviation (low AC/A ratio)
 B. Nonrefractive-refractive error not cause of deviation
 C. Anomalous insertion of horizontally acting muscles
 D. Abnormal check ligaments
 E. Faulty innervational development
 F. Autosomal-dominant trait
 G. Idiopathic

2. Noncomitant

 A. Convergence insufficiency
 B. Divergence excess
 C. Duane type II, III
 D. Internuclear ophthalmoplegia
 E. Myasthenia gravis
 F. Third nerve palsy
 G. Thyroid myopathy

3. Pattern exotropia

 A. V exotropia-deviation greater in upward than in downward gaze
  (1) Underaction superior oblique
  (2) Overaction inferior oblique
 B. Exotropia-deviation greater in downward than in upward gaze
  (1) Underaction inferior oblique muscle
  (2) Overaction superior oblique muscle

von Noorden GK. Binocular vision and ocular motility: theory and management of strabismus. St. Louis: CV Mosby.

Wright KW. Pediatric ophthalmology and strabismus. St. Louis: CV Mosby, 1995.