IV. Patetico

innervatesthe superior

oblique muscleof the eye

IVIV Trochlear Nerve

© L. Wilson-Pauwels© L. Wilson-Pauwels

IV Trochlear Nerve

CASE HISTORY

Harpinder is a 53-year-old woman with long-standing diabetes and hypertension. She wasout for an afternoon walk when she experienced the onset of headache behind her righteye. It was a continuous dull pain and, although not severe, caused her to cut her walkshort.While walking home, she experienced intermittent double vision. Harpinder assumedthat she was just tired and expected it to pass. After arriving home, as she began to pre-pare the evening dinner, she noticed that her double vision worsened when she was look-ing down at the chopping board, but it improved if she tilted her head to the left or if shelooked up. Alarmed that something was wrong, Harpinder went to the hospital.

Initially, Harpinder was assessed by the resident. The resident found that Harpinder’seye movements were normal, and he could not account for her double vision. Review ofthe case with a staff neurologist gave some clues to her problem. Harpinder’s pupils wereequal in size and reactive to light, and there was no evidence of ptosis (upper eyeliddroop). Harpinder was able to move her eyes fully through the horizontal planes. However,when the neurologist tested Harpinder’s vertical eye movements, with her eyes abductedand adducted, he discovered that Harpinder could not look down with her right eye whenit was adducted. Harpinder also noticed that she had double vision in this position and thatthe image from her right eye was superior and parallel to the image from her left eye. Theneurologist diagnosed a right cranial nerve IV palsy and attributed it to infarction of thenerve due to diabetes.

ANATOMY OF THE TROCHLEAR NERVE

The trochlear nerve, the smallest of the cranial nerves, innervates only a single mus-cle in the orbit: the superior oblique muscle (Figure IV–1A). It has only a somaticmotor component (Table IV–1). The cell bodies of the lower motor neurons con-stitute the trochlear nucleus, which is located in the tegmentum of the midbrain atthe level of the inferior colliculus (Figure IV–1B). Like other somatic motor nuclei,the trochlear nucleus is close to the midline. Motor neurons from the trochlearnucleus innervate predominantly, if not exclusively, the contralateral superioroblique muscle. Axons arising from the trochlear nucleus course dorsally around theperiaqueductal grey matter and cerebral aqueduct and cross the midline (FigureIV–1C). The crossed axons emerge from the dorsal aspect of the midbrain just cau-dal to the inferior colliculus to form the fourth cranial nerve. The nerve curves ven-trally around the cerebral peduncle to pass between the posterior cerebral andsuperior cerebellar arteries lateral to the third cranial nerve. Cranial nerve IV runsanteriorly to pierce the dura at the angle between the free and attached borders ofthe tentorium cerebelli.

70

Trochlear Nerve 71

Inferiorcolliculus

Trochlearnucleus

CN IV

Superiormedullary

velum

Superiorcerebellarpeduncle

B

MID

BR

AIN

Figure IV–1 A, Apex of the right orbit illustrating the tendinous ring. B, Dorsal aspect of the brain stem. C, Somatic motortract from the trochlear nucleus in the brain stem to the superior oblique muscle.

C

Superioroblique muscle

A

Tendinousring

CN IV throughsuperior orbital

fissure

Trochlear nucleus

Posterior cerebralartery

Superior cerebellarartery

Superior orbitalfissure

Trochlear nerveCN IV

Levator palpebraesuperioris muscle

Superior rectus muscle

Superior oblique muscle

Midbrain—level of inferior colliculus

Trochlea

CN III

Trochlea

Tendon ofsuperior oblique muscle

Central retinalartery


Cranial nerve IV enters the cavernous sinus along with cranial nerves III, V1, V2,and VI. Within the cavernous sinus, the trochlear nerve is situated between nervesIII and V1 and lateral to the internal carotid artery (Figure IV–2). It leaves the cav-ernous sinus and enters the orbit through the superior orbital fissure, above thetendinous ring (see Figure IV–1A). The nerve then courses medially, close to theroof of the orbit, and runs diagonally above the levator palpebrae superioris muscleto reach its target, the superior oblique muscle. Here the nerve divides into three ormore branches that enter the superior oblique muscle along its proximal third.

Eye Movements

The tendon of the superior oblique muscle loops through a “pulley” of fibrocarti-lage (the trochlea) in the connective tissue of the superior anteromedial corner of theorbit and then turns posterolateral to insert into the sclera in the superolateral pos-terior quadrant of the eye (Figure IV–3). Contraction of the muscle, therefore, pullsthe posterolateral surface of the eyeball forward.

72 Cranial Nerves

Table IV–1 Nerve Fiber Modality and Function of the Trochlear Nerve

Nerve Fiber Modality Nucleus Function

Somatic motor Trochlear Innervation of the superior oblique(efferent) muscle of the eye

Figure IV–2 Slice through the right cavernous sinus showing the relationship of cranial nerve IV to other structures coursing through the sinus.

CN III

CN IV

CN VI

Internal carotidartery

V1

V2

Pituitarygland

Sphenoidalsinus


A B

C D

E F

ABDUCTED POSITION

AT REST IN ACTION

NEUTRAL POSITION

AT REST IN ACTION

ADDUCTED POSITIONAT REST IN ACTION

Trochlear Nerve 73

Figure IV–3 Movements of the right eye produced by the action of the superior oblique muscle in different eye positions (viewedfrom above). For rotation around axes, see Figure IV–4. A, At rest, adducted position. B, When the eye is adducted (toward thenose, rotation around the Y axis), the pull of the superior oblique muscle (pink arrow) causes downward movement of the cornea(black arrow). C, At rest, neutral position. D, When the eye is in a neutral position (looking straight ahead, rotation around bothY and Z axes), the pull of the superior oblique muscle (pink arrow) causes both downward movement of the cornea and intorsion(black arrow). E, At rest, abducted position. F, When the eye is abducted (away from the nose, rotation around the Z axis), thepull of the superior oblique muscle (pink arrow) causes intorsion (black arrow).


74 Cranial Nerves

The actual movement caused by the superior oblique muscle depends on the ini-tial position of the eye (Table IV–2 and Figure IV–4).

CASE HISTORY GUIDING QUESTIONS

1. Why did Harpinder have double vision?

2. Why did Harpinder’s double vision improve when she tilted her head to the left?

3. What other lesions could affect the trochlear nerve?

4. Why did the resident not identify Harpinder’s eye movement problem?

1. Why did Harpinder have double vision?

Harpinder’s right superior oblique muscle is not functioning. As a result, its antag-onist muscle, the inferior oblique, extorts and slightly elevates her right eye (FigureIV–5). Consequently, the visual fields are projected onto different areas ofHarpinder’s right and left retinae and she perceives two distinct images.

Table IV–2 Eye Movements Mediated by Cranial Nerve IV

Nerve Muscle Primary Action Subsidiary Action

CN IV Superior oblique Downward gaze Intorsion, abduction

Adduction(toward nose) medial rectus muscle CN III

Figure IV–4 Right eye movements around the “X,” “Y,” and “Z” axes. Movements driven by cranial nerve IV are highlighted in pink.

Downward gazeinferior rectus CN III and

superior oblique muscles CN IV

Extorsioninferior rectus and inferior oblique muscles CN III

"Z"

AX

IS

"Y" AXIS

"X"

AX

IS

Upward gazesuperior rectus and

inferior oblique muscles CN III

Abduction(away from nose)

lateral rectus muscle CN VI

Intorsionsuperior rectus CN III and superior oblique muscles CN IV


Trochlear Nerve 75

2. Why did Harpinder’s double vision improve when she tilted her head to the left?

Normally, as the head tilts from side to side, the eyes rotate in the opposite direc-tion (see Figure IV–5A). Because Harpinder’s right superior oblique muscle wasparalysed, her right eye was extorted and slightly elevated (see Figure IV–5B, upperfigure). When she tilted her head to the left, her normal left eye intorted and herdouble vision improved (see Figure IV–5B, lower figure).

When she tiltedher head to the left, her eyes rotated in theopposite direction(right eye extorted,left eye intorted)

Harpinder's normal right eyebefore stroke

After the stroke,Harpinder'sextorted andslightly elevatedright eye causeddouble vision

To compensate,Harpinder tiltsher head to the left(unaffected side)and her left eyeintorts to improveher double vision

Figure IV–5 Ocular rotation. A, Before Harpinder's stroke; B, after Harpinder's stroke.

A B


76 Cranial Nerves

3. What other lesions could affect the trochlear nerve?

The trochlear nerve can be damaged anywhere from its nucleus in the midbrain toits termination in the orbit. The intramedullary axons can be damaged in the mid-brain by infarction, tumor, or demyelination. The nerve can be damaged in the sub-arachnoid space by trauma, tumor, and meningitis. Within the cavernous sinus,aneurysm of the internal carotid artery, cavernous sinus thrombosis, tumors, andinflammation can all damage cranial nerve IV. The nerve can also be affected byischemia; this is most commonly seen in patients with diabetes or hypertension.

4. Why did the resident not identify Harpinder’s eye movement problem?

The superior oblique muscle has two major actions: intorsion of the eye (ie, rotationaround the anteroposterior axis) and downward gaze (see Figure IV–4). Since the eyeis radially symmetric, rotation about the anteroposterior axis is very difficult todetect (but watching the movement of the conjunctival vessels is sometimes help-ful). A better test of the superior oblique muscle is to examine downward gaze whenthe eye is adducted (Figure IV–6). The resident did not test Harpinder’s vertical eyemovements with her right eye adducted and, thus, missed the abnormality.

CLINICAL TESTING

Cranial nerve IV is tested in conjunction with cranial nerves III and VI through theassessment of eye movements (see Chapter 13). (See also Cranial Nerves Examinationon CD-ROM.)

Figure IV–6 Harpinder is unable to move her right eye downward.


Trochlear Nerve 77

ADDITIONAL RESOURCES

Brodal A. Neurological anatomy in relation to clinical medicine. 3rd ed. New York:Oxford University Press; 1981. p. 532–77.

Glimcher PA. Eye movements. In: Zigmond MJ, Bloom FE, Landis SC, et al, editors.Fundamental neuroscience. San Diego: Academic Press; 1999. p. 993–1009.

Porter JD. Brainstem terminations of extraocular muscle primary sensory afferent neuronsin the monkey. J Comp Neurol 1986;247:133–43.

Spencer RF, McNeer KW. The periphery: extraocular muscles and motor neurons.In: Carpenter RHS, editor. Eye movements. Boca Raton: CRC Press Inc.; 1991.p. 175–99.

IV. Patetico

Documents

Transcript of IV. Patetico