Timothy C. Hain, MD Page last modified: October 29, 2017
|The sixth nerve is shown on the left, emerging just above the anterior inferior cerebellar artery (AICA).|
The sixth cranial nerve innervates the lateral rectus muscle of the eye, which pulls the eye laterally. Injury to the 6th nerve generally causes horizontal double vision, worse at distance than at near, and worse when looking in the direction that requires use of the weaker muscle (i.e. left or right). A sensitive way to detect this is with the Lancaster test.
The 6th nerve derives from the 6th nerve nuclei located in the pons. Nerve fibers exit from the brainstem and travel upward, bending over the petrous portion of the temporal bone to enter the cavernous sinus. The fibers enter the orbit via the superior orbital fissure to innervate the lateral rectus muscle.
Supplemental material on the site DVD: Movie of right 6th nerve palsy.
Common causes of 6th nerve weakness include:
Disorders that can resemble 6th nerve palsy include:
In restriction syndromes, the eye is held back by the medial rectus. Thyroid disease, Duanes and and trauma are the most common source of these.
Duanes syndrome is a congenital oculmotor anomaly -- it is manifested by a pseudo-6th as well as ocular retraction and narrowing of the palpebral fissure on adduction (Rutstein, 1992). Rarely, patients with Duanes have congenital hearing loss and other congenital malformations (see Okihiro syndrome in the section on congenital hearing loss which includes malformation of the upper extremity as well). Restriction syndromes may be differentiated from ocular muscle weakness with the forced duction test.
6th neve palsy has been reported after lumbar anesthesia (Richer and Ritaccca, 1989), epidural anestheisia (Gupta et al), and after myelography.
In spasm of the near reflex, there is intermittent convergence, accomodation and miosis. The miosis on looking to the side is the hallmark of the disease.
Horizontal double vision can also be caused by damage to the medial rectus function. The medial rectus is innervated by the 3rd nerve. This situation is much rarer than 6th nerve palsy, and also usually is accompanied by other visual disturbances associated with damage to the oculomotor nerve (3rd nerve), which also innervates several other eye muscles as well as the pupil constrictors.
A suggested workup for 6th nerve palsy, adapted from Galetta and Lawton Smith (1989) is as follows:
- General physical, ophthalmologic and neurologic
- Blood pressure
- Ascultation for bruits over the eyes and mastoid
- Cocaine test
- Schirmer's test
- Forced duction
- Forced generations
- Sed Rate
- Fasting glucose
- RPR, FTA, thyroid function
- CT scan with and without contrast
- Orbital MRI
In special situations
- Nasopharyngeal biopsy
- Bone scan