4th Nerve Palsy
Title: 4th Nerve Palsy
Authors: Alex Wright, 4th Year Medical Student, University of Utah School of Medicine; Tanner Ferguson, 4th year medical student, University of South Dakota Sanford School of Medicine
The 4th cranial nerve (CN), or trochlear nerve, is a motor nerve solely responsible for innervating the superior oblique muscle. The superior oblique muscle has unique rotational actions for the eye that are dependent upon the position of the eye. This summary will provide a brief review of the function of the superior oblique muscle and the clinical significance of a 4th nerve palsy.1
The superior oblique’s unique insertion on the eye permits a “pulley” type action on the eye. In the eye’s primary position (fixating straight ahead), the primary action of the muscle is intorsion (rotation of 12 o’clock position of limbus towards nose). It also provides depression (particularly in adduction) and abduction.1
When a trochlear nerve palsy occurs, the clinical signs can differ depending on acute versus chronic. The most common cause of a 4th nerve palsy is trauma, followed by congenital and ischemic causes.2 Traumatic 4th nerve palsies may occur with a relatively mild blow to the head not associated with loss of consciousness or skull fracture. This is due to the long course that the 4th CN takes as it exits the pons posteriorly. If a patient has vasculopathic risk factors and has accompanying pain with the symptoms, ischemia is more likely. When it occurs in an acute setting, patients often complain of a binocular, vertical diplopia that is worse with downward or side gaze. Patients may also complain of difficulty reading or with near vision. Congenital cases typically present with a longstanding compensatory head tilt and occasionally have facial asymmetry from the head tilt (see image 1).
Clinical diagnosis of vertical diplopia can be localized with the Parks-Bielschowsky Three Step Test. Step one involves identifying which eye is higher in primary gaze (hypertropia). Step two is for establishing if the hypertropia is worse in right or left gaze and step three in right or left head tilt. For example, a right 4th CN palsy will present with a right hypertropia, worse in left gaze and right head tilt. A left 4th nerve palsy will present with a left hypertropia, worse in right gaze and left head tilt. Because the eye alignment is worse when tilting towards the lesion, many patients will demonstrate a compensatory head tilt away from the lesion.3 If you note a compensatory head tilt in a patient and suspect a CN IV palsy, it may be helpful to look at older photos to assess how long the patient’s head tilt has been present.
If a patient presents with a sudden onset of a CN IV palsy without a history of trauma and no cardiovascular risk factors, the patient may necessitate neuroimaging to further evaluate, particularly if other neurological signs/symptoms are present.4 The palsy can be caused by increased intracranial pressure or a cerebellar tumor. It is therefore important to look for papilledema and evaluate for signs of cerebellar disease in conjunction with neuroimaging.
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- Keane JR. Fourth nerve palsy Historical review and study of 215 inpatients. Neurology. 1993;43(12):2439-2439. doi:10.1212/WNL.43.12.2439.
- Prasad S, Volpe NJ, Tamhankar MA. Clinical reasoning: a 36-year-old man with vertical diplopia. Neurology. 2009;72(19):e93-e99. doi:10.1212/WNL.0b013e3181a55ee3.
- Tamhankar MA, Biousse V, Ying G-S, et al. Isolated third, fourth, and sixth cranial nerve palsies from presumed microvascular versus other causes: a prospective study. Ophthalmology. 2013;120(11):2264-2269. doi:10.1016/j.ophtha.2013.04.009.