The abducens nerve controls the lateral rectus muscle of the eye and allows for sideways movement of the eye. Damage to the peripheral nerve causes double vision due to the unopposed medial rectus muscle. Damage to the abducens nucleus in the brainstem causes internuclear ophthalmoplegia, affecting both eyes simultaneously with impaired lateral movement. Cortical lesions affecting conjugate gaze are discussed elsewhere.

1. TheThe abducens nerveabducens nerve (the(the sixth cranial nervesixth cranial nerve , also, also
called thecalled the sixth nervesixth nerve or simplyor simply VIVI) is a motor nerve) is a motor nerve
(a “somatic efferent” nerve) that controls the movement(a “somatic efferent” nerve) that controls the movement
of a single muscle, theof a single muscle, the laterallateral rectusrectus musclemuscle of the eyeof the eye

2. Peripheral anatomyPeripheral anatomy
The abducens nerve leaves the brainstem at the junction of theThe abducens nerve leaves the brainstem at the junction of the ponspons
and theand the medullamedulla, medial to the, medial to the facial nervefacial nerve. In order to reach the. In order to reach the
eye, it runs upward (superiorly) and then bends forward (anteriorly).eye, it runs upward (superiorly) and then bends forward (anteriorly).
The nerve enters theThe nerve enters the subarachnoidsubarachnoid spacespace when it emerges from thewhen it emerges from the
brainstem. It runs upward between the pons and thebrainstem. It runs upward between the pons and the clivusclivus, and, and
then pierces thethen pierces the duradura matermater to run between the dura and the skull.to run between the dura and the skull.
At the tip of the petrousAt the tip of the petrous temporal bonetemporal bone it makes a sharp turn forwardit makes a sharp turn forward
to enter theto enter the cavernous sinuscavernous sinus. In the cavernous sinus it runs. In the cavernous sinus it runs
alongside thealongside the internal carotid arteryinternal carotid artery. It then enters the orbit through. It then enters the orbit through
thethe superior orbital fissuresuperior orbital fissure and innervates the lateral rectus muscleand innervates the lateral rectus muscle
ofof the eye.the eye.

3. Central anatomyCentral anatomy
The abducens nucleus is located in the pons, on the floor of theThe abducens nucleus is located in the pons, on the floor of the
fourth ventriclefourth ventricle, at the level of the, at the level of the facialfacial colliculuscolliculus. Axons from the. Axons from the
facial nerve loop around the abducens nucleus, creating a slightfacial nerve loop around the abducens nucleus, creating a slight
bulge (the facial colliculus) that is visible on the dorsal surface of thebulge (the facial colliculus) that is visible on the dorsal surface of the
floor of the fourth ventricle. The abducens nucleus is close to thefloor of the fourth ventricle. The abducens nucleus is close to the
midline, like the other motor nuclei that control eye movements (themidline, like the other motor nuclei that control eye movements (the
oculomotoroculomotor andand trochleartrochlear nuclei).nuclei).
Motor axons leaving the abducens nucleus run ventrally and caudally throughMotor axons leaving the abducens nucleus run ventrally and caudally through
the pons. They pass lateral to thethe pons. They pass lateral to the corticospinalcorticospinal tracttract (which runs(which runs
longitudinally through the pons at this level) before exiting the brainstem atlongitudinally through the pons at this level) before exiting the brainstem at
the pontomedullary junctionthe pontomedullary junction

4. Clinical syndromesClinical syndromes
Complete interruption of the peripheral sixth nerve causesComplete interruption of the peripheral sixth nerve causes diplopiadiplopia
(double vision), due to the unopposed action of the(double vision), due to the unopposed action of the medialmedial rectusrectus
musclemuscle. The affected eye is pulled medially. In order to see without. The affected eye is pulled medially. In order to see without
double vision, patients will rotate their heads so that both eyes aredouble vision, patients will rotate their heads so that both eyes are
looking sideways. On formal testing, the affected eye cannot abductlooking sideways. On formal testing, the affected eye cannot abduct
past the midline – it cannot look sideways, toward the temple. Partialpast the midline – it cannot look sideways, toward the temple. Partial
damage to the sixth nerve causes weak or incomplete abduction ofdamage to the sixth nerve causes weak or incomplete abduction of
the affected eye. The diplopia is worse on attempted lateral gaze, asthe affected eye. The diplopia is worse on attempted lateral gaze, as
would be expected (since the lateral gaze muscle is impaired).would be expected (since the lateral gaze muscle is impaired).

5. Nuclear lesionsNuclear lesions
 Damage to the abducensDamage to the abducens nucleusnucleus does not produce an isolateddoes not produce an isolated
sixth nerve palsy, but rather a horizontalsixth nerve palsy, but rather a horizontal gaze palsygaze palsy that affects boththat affects both
eyes simultaneously. The abducens nucleus contains two types ofeyes simultaneously. The abducens nucleus contains two types of
cells: motor neurons that control the lateral rectus muscle on thecells: motor neurons that control the lateral rectus muscle on the
same side, and interneurons that cross the midline and connect tosame side, and interneurons that cross the midline and connect to
the contralateral oculomotor nucleus (which controls the medialthe contralateral oculomotor nucleus (which controls the medial
rectus muscle of the opposite eye). In normal vision, lateralrectus muscle of the opposite eye). In normal vision, lateral
movement of one eye (lateral rectus muscle) is precisely coupled tomovement of one eye (lateral rectus muscle) is precisely coupled to
medial movement of the other eye (medial rectus muscle), so thatmedial movement of the other eye (medial rectus muscle), so that
both eyes remain fixed on the same object.both eyes remain fixed on the same object.
 The control ofThe control of conjugate gazeconjugate gaze is mediated in the brainstem by theis mediated in the brainstem by the
medial longitudinal fasciculus ,a nerve tract that connects the threemedial longitudinal fasciculus ,a nerve tract that connects the three
extraocular motor nuclei (abducens, trochlear and oculomotor) intoextraocular motor nuclei (abducens, trochlear and oculomotor) into
a single functional unit. Lesions of the abducens nucleus and thea single functional unit. Lesions of the abducens nucleus and the
MLF produce observable sixth nerve problems, most notablyMLF produce observable sixth nerve problems, most notably
internuclear ophthalmoplegia (INO).internuclear ophthalmoplegia (INO).

6. Supranuclear lesionsSupranuclear lesions
 The sixth nerve is one of the final common pathways forThe sixth nerve is one of the final common pathways for
numerous cortical systems that control eye movement innumerous cortical systems that control eye movement in
general. Cortical control of eye movement (saccades, smoothgeneral. Cortical control of eye movement (saccades, smooth
pursuit, accommodation) involvespursuit, accommodation) involves conjugate gazeconjugate gaze, not, not
unilateral eye movement. Disorders ofunilateral eye movement. Disorders of conjugate gazeconjugate gaze areare
discussed elsewherediscussed elsewhere

7. Supranuclear lesionsSupranuclear lesions
 The sixth nerve is one of the final common pathways forThe sixth nerve is one of the final common pathways for
numerous cortical systems that control eye movement innumerous cortical systems that control eye movement in
general. Cortical control of eye movement (saccades, smoothgeneral. Cortical control of eye movement (saccades, smooth
pursuit, accommodation) involvespursuit, accommodation) involves conjugate gazeconjugate gaze, not, not
unilateral eye movement. Disorders ofunilateral eye movement. Disorders of conjugate gazeconjugate gaze areare
discussed elsewherediscussed elsewhere