Paralytic Strabismus: Third, Fourth, and Sixth Nerve Palsy

Paralytic Strabismus 817
secondary function: intorsion of the abducted eye on attempted downgaze. The
torsional movement that indicates intact superior oblique function is best appreciated
by observing a conjunctival vessel (Fig. 12).
Differential Diagnosis
The most common cause of acquired fourth nerve palsy is trauma. 31,49,50 The trochlear
nerve is the longest and thinnest of all the cranial nerves, coursing along the
free edge of the tentorium through the prepontine cistern, where it is vulnerable
to crush or shearing injury. Fracture of the base of the skull is an alternative cause.
In cases of bilateral traumatic fourth nerve palsies, both nerves are often injured at
the anterior medullary vellum, where they decussate. 49 Traumatic fourth nerve
palsies may occur after minor head injuries without loss of consciousness or skull
fractures.
Decompensated congenital fourth nerve palsy is also common and may present in
adulthood. There is often a long-standing head tilt, which may be observed on inspection
of prior photographs, and an insidious onset of intermittent vertical diplopia. Characteristic
features of congenital fourth nerve palsy include inferior oblique overaction,
large vertical fusional amplitude, and minimal torsional diplopia. The precise cause of
congenital fourth nerve palsy is unclear but may include hypoplasia of the nucleus,
birth trauma, anomalous muscle insertion, muscle fibrosis, structural abnormalities
of the tendon, or inferior oblique muscle abnormalities. 51 Decompensation later in
life probably relates to breakdown of vertical fusion leading to symptomatic diplopia,
rather than progressive superior oblique dysfunction. 52
Microvascular ischemia may cause fourth nerve palsy, typically in patients more
than 50 years of age with vascular risk factors. There is often periorbital aching pain
on presentation, which can be severe. There is an excellent chance of spontaneous
recovery within several months.
Fig. 12. A 75-year-old man (also shown in Fig. 20) with right third nerve palsy from pituitary
apoplexy with spared superior oblique function. (A) A conjunctival vessel is observed (arrow)
in primary gaze. (B) On attempted downgaze, the conjunctival vessel is observed to move
from the 2 o’clock position to the 3 o’clock position (arrow), showing intorsion of the eye
by an intact superior oblique muscle.