Synkinesis - Variations - Extra-ocular Muscle Synkinesis

Extra-ocular Muscle Synkinesis

The six muscles around the eye (extraocular muscles) are innervated by three different cranial nerves: Abducens (6th nerve), Trochlear (4th nerve), and Oculomotor (3rd nerve). After nerve trauma around the eye, a combination of any two of these three cranial nerves have been shown to be involved with extra-ocular synkinesis. More over, while the abducens and the trochlear nerve each innervate one specific muscle, the oculomotor nerve has many functions including eyelid retraction and pupil constriction. Thus, during synkinesis, a variable of these functions are involved. Examples include:

• On attempted abduction of an affected eye, the eye adducts and the eyelid retracts.
  • This is an interaction between the abducens nerve and a branch of the oculomotor nerve. Voluntary activation of the abducens nerve (eye abduction) causes involuntary activation of the oculomotor nerve (eye adduction and eyelid elevation).
• On attempted abduction, the eye’s unreactive pupil constricts
  • Another interaction, yet different, between the abducens nerve (eye abduction) and the oculomotor nerve (pupil constriction).
• On attempted adduction with eye depression, the eyelid retracts.
  • This is a case reported in which voluntary activation of the trochlear nerve (eye depression + eye abduction) is involuntarily activating a branch of the oculomotor nerve responsible for eyelid retraction.

Other less common variations of synkinesis involving the cranial nerves include:

• Trigeminal-Abducens Synkinesis
  • After physical trauma to the skull, the muscle involved in eye abduction can become reinnervated by the branch of the trigeminal nerve involved in innervating the muscles of mastication(chewing muscles). Thus, involuntary abduction of an involved eye will occur upon eating or chewing.
• Trigeminal-Facial Synkinesis
  • After surgical trauma, the muscles of mastication can become reinnervated by the facial nerve as opposed to the trigeminal nerve. This causes weakness in voluntary chewing; also, facial movements such as blinking cause the muscles to contract.

Synkinesis is also commonly used in referring to the involuntary convergence of the eyes that accompanies focusing for near. This accommodation-convergence synkinesis can result in esotropia, or eyes that turn in when the ratio between accommodation and convergence is unusually high.