Trochlear nerve

The trochlear nerve (), also called the fourth cranial nerve or CN IV, is a motor nerve (a somatic efferent nerve) that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea.
The trochlear nerve is unique among the cranial nerves in several respects:
Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.
The human trochlear nerve is derived from the basal plate of the embryonic midbrain. The words "trochlea" and "trochlear" (, ) come from Ancient Greek "trokhiléa", “pulley; block-and-tackle equipment”.
Structure.
The trochlear nerve emerges from the back (dorsal) brainstem, just below the inferior colliculus. It circles from behind around the brainstem and runs forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery, and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process. It runs on the outer wall of the cavernous sinus. Finally, it enters the orbit through the superior orbital fissure and supplies the superior oblique muscle.
Nucleus.
The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.
The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem posteriorly. Thus a lesion of the trochlear nucleus affects the "contralateral" eye. Lesions of all other cranial nuclei affect the "ipsilateral" side.
Function.
The trochlear nerve provides motor supply to the superior oblique muscle of the eye,
The trochlear nerve carries axons of type GSE, general somatic efferent, which innervate skeletal muscle of the superior oblique muscle.
The superior oblique muscle ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. "Trochlea" means “pulley” in Latin; the fourth nerve is named after this structure.
Actions of the superior oblique muscle.
In order to understand the actions of the superior oblique muscle, it is useful to imagine the eyeball as a sphere that is constrained—like the trackball of a computer mouse—in such a way that only certain rotational movements are possible. Allowable movements for the superior oblique are (1) rotation in a vertical plane—looking down and up ("depression" and "elevation" of the eyeball) and (2) rotation in the plane of the face ("intorsion" and "extorsion" of the eyeball).
The body of the superior oblique muscle is located "behind" the eyeball, but the tendon (which is redirected by the trochlea) approaches the eyeball from the "front". The tendon attaches to the top (superior aspect) of the eyeball at an angle of 51 degrees with respect to the "primary position" of the eye (looking straight forward). The force of the tendon's pull therefore has two components: a forward component that tends to pull the eyeball downward (depression), and a medial component that tends to rotate the top of the eyeball toward the nose (intorsion).
The relative strength of these two forces depends on which way the eye is looking. When the eye is "adducted" (looking toward the nose), the force of depression increases. When the eye is "abducted" (looking away from the nose), the force of intorsion increases, while the force of depression decreases. When the eye is in the primary position (looking straight ahead), contraction of the superior oblique produces depression and intorsion in roughly equal amounts.
To summarize, the actions of the superior oblique muscle are (1) "depression" of the eyeball, especially when the eye is adducted; and (2) "intorsion" of the eyeball, especially when the eye is abducted. The clinical consequences of weakness in the superior oblique (caused, for example, by fourth nerve palsies) are discussed below.
This summary of the superior oblique muscle describes its most important functions. However, it is an oversimplification of the actual situation. For example, the tendon of the superior oblique inserts "behind" the equator of the eyeball in the frontal plane, so contraction of the muscle also tends to "abduct" the eyeball (turn it outward). In fact, each of the six extraocular muscles exerts rotational forces in "all three planes" (elevation-depression, adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the eye is looking. The relative forces change every time the eyeball moves—every time the direction of gaze changes. The central control of this process, which involves the continuous, precise adjustment of forces on twelve different tendons in order to point both eyes in exactly the same direction, is truly remarkable.
The recent discovery of soft tissue pulleys in the orbit—similar to the trochlea, but anatomically more subtle and previously missed—has completely changed and greatly simplified our understanding of the actions of the extraocular muscles. Perhaps the most important finding is that a two-dimensional representation of the visual field is sufficient for most purposes.
Clinical significance.
Vertical diplopia.
Injury to the trochlear nerve cause weakness of downward eye movement with consequent vertical diplopia (double vision). The affected eye drifts upward relative to the normal eye, due to the unopposed actions of the remaining extraocular muscles. The patient sees two visual fields (one from each eye), separated vertically. To compensate for this, patients learn to tilt the head forward (tuck the chin in) in order to bring the fields back together—to fuse the two images into a single visual field. This accounts for the “dejected” appearance of patients with “pathetic nerve” palsies.
Torsional diplopia.
Trochlear nerve palsy also affects torsion (rotation of the eyeball in the plane of the face). Torsion is a normal response to tilting the head sideways. The eyes automatically rotate in an equal and opposite direction, so that the orientation of the environment remains unchanged—vertical things remain vertical.
Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time. To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.
The characteristic appearance of patients with fourth nerve palsies (head tilted to one side, chin tucked in) suggests the diagnosis, but other causes must be ruled out. For example, torticollis can produce a similar appearance.
Causes.
The clinical syndromes can originate from both peripheral and central lesions.
Peripheral lesion.
A peripheral lesion is damage to the bundle of nerves, in contrast to a central lesion, which is damage to the trochlear nucleus. Acute symptoms are probably a result of trauma or disease, while chronic symptoms probably are congenital.
Acute palsy.
The most common cause of "acute" fourth nerve palsy is head trauma. Even relatively minor trauma can transiently stretch the fourth nerve (by transiently displacing the brainstem relative to the posterior clinoid process). Patients with minor damage to the fourth nerve will complain of “blurry” vision. Patients with more extensive damage will notice frank diplopia and rotational (torsional) disturbances of the visual fields. The usual clinical course is complete recovery within weeks to months.
Isolated injury to the fourth nerve can be caused by any process that stretches or compresses the nerve. A generalized increase in intracranial pressure—hydrocephalus, pseudotumor cerebri, hemorrhage, edema—will affect the fourth nerve, but the abducens nerve (VI) is usually affected first (producing horizontal diplopia, not vertical diplopia). Infections (meningitis, herpes zoster), demyelination (multiple sclerosis), diabetic neuropathy and cavernous sinus disease can affect the fourth nerve, as can orbital tumors and Tolosa–Hunt syndrome. In general, these diseases affect other cranial nerves as well. Isolated damage to the fourth nerve is uncommon in these settings.
Chronic palsy.
The most common cause of "chronic" fourth nerve palsy is a congenital defect, in which the development of the fourth nerve (or its nucleus) is abnormal or incomplete. Congenital defects may be noticed in childhood, but minor defects may not become evident until adult life, when compensatory mechanisms begin to fail. Congenital fourth nerve palsies are amenable to surgical treatment.
Central lesion.
Central damage is damage to the trochlear nucleus. It affects the "contralateral" eye. The nuclei of other cranial nerves generally affect "ipsilateral" structures (for example, the optic nerves - cranial nerves II - innervate both eyes).
The trochlear nucleus and its axons within the brainstem can be damaged by infarctions, hemorrhage, arteriovenous malformations, tumors and demyelination. Collateral damage to other structures will usually dominate the clinical picture.
The fourth nerve is one of the final common pathways for cortical systems that control eye movement in general. Cortical control of eye movement (saccades, smooth pursuit, accommodation) involves conjugate gaze, not unilateral eye movement.
Examination.
The trochlear nerve is tested by examining the action of its muscle, the superior oblique. When acting on its own this muscle depresses and abducts the eyeball. However, movements of the eye by the extraocular muscles are synergistic (working together). Therefore, the trochlear nerve is tested by asking the patient to look 'down and in' as the contribution of the superior oblique is greatest in this motion. Common activities requiring this type of convergent gaze are reading the newspaper and walking down stairs. Diplopia associated with these activities may be the initial symptom of a fourth nerve palsy.
Alfred Bielschowsky's head tilt test is a test for palsy of the superior oblique muscle caused by damage to cranial nerve IV (trochlear nerve).