Editing Trochlear nerve (section)

==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.<ref>Hoya K, Kirino T.  Traumatic "Trochlear Nerve Palsy Following Minor Occipital Impact". ''Neurol Med Chir'' 40:358-360, 2000</ref> 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=====
{{Main|Congenital fourth nerve 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 malformation]]s, 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 (eye)|accommodation]]) involves [[Conjugate gaze palsy|conjugate gaze]], not unilateral eye movement.

===Clinical assessment===
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 [[synergy|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 [[Bielschowsky's head tilt test|head tilt test]] is a test for palsy of the superior oblique muscle caused by damage to cranial nerve IV (trochlear nerve).