Lesions of the trochlear nerve (CN IV)
Lesions of the trochlear nerve (CN IV)
Features of a Trochlear (Fourth) Nerve Palsy
- Failure to intort the eye (superior oblique): the affected eye cannot look down and in.
This lesion suggests that there must be damage to the contralateral brainstem; i.e. in the crude diagram above there must be a right midbrain lesion.
Causes of unilateral CN IV lesions:
- Head injury (most common)
- Brainstem lesion - unlikely stroke, more likely tumour.
- A central lesion affects the contralateral eye: the trochlear nerve is the only cranial nerve that innervates structures contralateral to its nucleus.
Causes of bilateral CN IV lesions:
- Freakishly rare
- Diabetes-associated neuropathy
- Myasthenia gravis
References
The LITFL summary of cranial nerve lesions is without peer in terms of useful information density.
Walker, H. Kenneth, W. Dallas Hall, and J. Willis Hurst. "Clinical methods." 3rd edition.(1990).Chapter 60 Cranial Nerves III, IV, and VI: The Oculomotor, Trochlear, and Abducens Nerves - by J. Donald Fite and H. Kenneth Walker.
Twenty five years out of date, but still relevant.
These authors, in turn, reference even more ancient vellum:
Leigh RJ, Zee DS. The neurology of eye movements. Philadelphia: FA Davis, 1983.
Miller NR. Walsh and Hoyt's clinical neuro-ophthalmology. Vol 2. 4th ed. Baltimore: Williams and Wilkins, 1985.