Editing Trochlear nerve (section)

====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.