paresis
Paresis is a partial loss of action of a muscle, in which the action is impaired but not abolished. Complete loss of action is paralysis, and the term palsy is used either for both or as a synonym for paralysis.1,2 Clinicians often use paretic and paralytic interchangeably. The two cannot always be told apart on clinical grounds, because an apparently paralyzed muscle may regain some function after surgery or botulinum toxin injection of its antagonist.2,3
Paresis of an extraocular muscle produces an incomitant deviation, which must be distinguished from comitant strabismus if the weak muscle is to be identified and treated.1,3 A recent paresis shows a motor deficiency in the field of action of the affected muscle, diplopia, a larger deviation on fixation with the affected eye, and often a compensatory head posture.3 An acquired paresis may signal a condition that affects the patient's general health.3
More detail
A paretic deviation passes through stages. At first the weak muscle underacts, its direct antagonist overacts, and the largest deviation lies in the field of action of the paretic muscle. In a recent right superior oblique paresis, for example, the right hypertropia is greatest in the left lower field of gaze.3
Overaction and later contracture of the antagonist then become the main feature, and they may persist long after the paresis has subsided. Finally the deviation spreads into all fields of gaze and becomes increasingly comitant, a change called spread of comitance. In most patients comitance spreads within weeks, months or even years, so a long-standing paresis can be hard to identify.3
Sources (9)
- Pickwell's Binocular Vision Anomalies, ch. 17
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 8
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 20
- Brazis, P.W. 2011. Localization of lesions of the oculomotor nerve: recent concepts. Mayo Clinic Proceedings, 86(10), pp.1029-1035.
- Ehlers, J.P., Shah, C.P. and Fenton, G.L. eds. 2008. The Wills eye manual: office and emergency room diagnosis and treatment of eye disease. 5th ed. Philadelphia: Lippincott Williams & Wilkins.
- Lavin, P.J.M. 2005. Pupillary dilatation in oculomotor nerve palsy. Archives of Ophthalmology, 123(4), pp.555-556.
- Prasad, S. and Volpe, N.J. 2010. Paralytic strabismus: third, fourth, and sixth nerve palsy. Neurologic Clinics, 28(3), pp.803-833.
- Rucker, J.C. and Tomsak, R.L. 2005. Binocular diplopia: a practical approach. The Neurologist, 11(2), pp.98-110.
- Rutstein, R. and Daum, K. 1997. Anomalies of Binocular Vision: Diagnosis & Management. St. Louis: Mosby.
Reviewed October 2026