Keywords
oculomotor nerve - synkinesis - aberrant regeneration - meningocele - idiopathic intracranial
hypertension
Introduction
The oculomotor nerve, or cranial nerve III (CN III), provides motor innervation to
the inferior oblique muscle, the levator palpebrae superioris muscle, and superior,
inferior, and medial rectus muscles.[1] It also allows for constriction of the pupil and accommodation of vision to near
objects via parasympathetic nerve fibers to the constrictor pupillae and ciliary muscles.[1] A CN III palsy classically presents with a “down-and-out” appearance of the affected
eye due to unopposed lateral rectus and superior oblique contraction, ptosis, and
mydriasis.[1]
Oculomotor synkinesis, or aberrant regeneration of CN III, can occur following CN
III palsy when some injured nerve fibers regenerate in a disordered fashion, innervating
a different muscle than prior to injury.[2] This results in synkinetic eye movements, with voluntary muscle contraction accompanied
by involuntary contraction of a different muscle due to stimulation by aberrant nerve
fibers. This most often occurs due to trauma, aneurysm, or mass lesion, most commonly
meningioma.[1]
[3] In one small study of 20 patients, the incidence of oculomotor synkinesis after
traumatic CN III palsy was 15%.[4]
We present a case of a patient who presented with a CN III palsy and oculomotor synkinesis
caused by a meningocele involving the oculomotor cistern (OMC), possibly secondary
to undiagnosed idiopathic intracranial hypertension (IIH).
Case Report
Our patient was a 29-year-old male without relevant past medical or surgical history
who initially presented in 2018 with 1 week of intermittent diplopia and left pupil
mydriasis and underwent emergent workup, given concern for a CN III palsy. CT angiography
of the head and MRI of the brain did not demonstrate evidence of aneurysm or other
acute findings.
Subsequently, he was seen in our university neuro-ophthalmology clinic. Physical exam
was notable for a left-sided CN III palsy with ptosis, mydriasis without afferent
pupillary defect, restricted motion in all directions except abduction, and double
vision on upward and downward gaze during adduction. He also had improvement in ptosis
with adduction and downward gaze, known as the pseudo-von-Graefe's sign, suggestive
of aberrant innervation of the levator palpebrae superioris by the fibers of CN III
innervating the medial rectus and inferior rectus muscles, concerning for oculomotor
synkinesis and prompting further workup.[3] He did not have papilledema on exam.
A month after the initial studies, the patient underwent repeat MRI of the brain with
constructive interference in steady state (CISS) sequence, which showed a meningocele
projecting into the left OMC, causing mass effect on the left oculomotor nerve with
nerve atrophy. He was managed expectantly. On subsequent surveillance MRIs, there
has been interval development of atrophy involving the cisternal segment of the left
oculomotor nerve ([Fig. 1]). His symptoms mildly improved without intervention in the 5 months after symptom
onset but have since plateaued, with persistent restricted motion, double vison, and
intermittent blurry vision in his left eye. He was initially followed up with a yearly
MRI of the brain for the first 2 years; thereafter, MRI of the brain every 2 years
and findings had been stationary.
Fig. 1 (A, B, and D) Axial and coronal CISS sequences showing a cystic lesion projecting into and enlarging
the posterior aspect of the left oculomotor cistern (arrowheads), causing mass effect
and lateral displacement of CN III (red arrows). The normal right CN III can be seen
in the R oculomotor cistern (yellow arrows). (C) Coronal T2-weighted sequence more anteriorly showing atrophy of the cisternal segment
of the left oculomotor nerve (red arrow) and a normal oculomotor nerve in the right
oculomotor cistern (yellow arrow).
Retrospectively, he had several imaging findings suggestive of IIH on initial presentation,
including empty sella, prominent arachnoid granulations, stenosis of the bilateral
sigmoid–transverse sinus junctions, and prominence of the perioptic subarachnoid space
with tortuosity of the optic nerves ([Fig. 2]). Additionally, the patient has a high body mass index (over 40), known to be associated
with IIH.
Fig. 2 (A) Sagittal T1-weighted post-contrast image demonstrating empty sella with osseous
remodeling of the sella turcica (*). (B) Axial CISS sequence demonstrating distention of the perioptic subarachnoid space
with tortuosity of the optic nerves (arrows). (C, D) Sagittal and coronal T1-weighted post-contrast images demonstrating stenosis of
the bilateral and left sigmoid–transverse sinus junctions, respectively (arrowheads).
A prominent arachnoid granulation is additionally noted.
Discussion
The Monro-Kellie doctrine posits that the total volume within the rigid skull is constant, comprising brain
tissue, cerebrospinal fluid (CSF), and blood. An increase in one component necessitates
a compensatory decrease in another to maintain normal intracranial pressure.[5] In IIH, compensatory mechanisms try to redistribute or reduce CSF volume to maintain
pressure homeostasis, leading to distention of optic nerve sheaths, flattening of
the posterior globe/papilledema, empty sella, compression/stenosis of the distal transverse
sinuses, and enlargement of any of the CSF-containing spaces or formation of meningocele,
in places such as Meckel's cave, geniculate ganglion, and OMCs.
The OMC is a CSF-filled dural cuff that envelops the oculomotor nerve (cranial nerve
III) as it traverses from the interpeduncular cistern into the lateral wall of the
cavernous sinus.[6]
In addition to trauma and aneurysms, mass lesions are a common cause of oculomotor
synkinesis; however, this is most commonly secondary to parasellar meningioma or pituitary
adenoma,[1]
[3] while oculomotor synkinesis secondary to a meningocele has not previously been reported.
The etiology of our patient's meningocele is unclear but most likely secondary to
IIH, which is associated with meningoceles in up to 10% of patients.[7] He meets three out of four imaging criteria of IIH (empty sella, transverse venous
sinus stenosis, distended and tortuous optic nerve sheath but not flattening of the
posterior globe) and intermittently experienced associated symptoms of IIH, including
intermittent blurred vision and diplopia.[7] While he did not present with papilledema, the absence of papilledema in IIH can
occur after spontaneous CSF leak from the spine or skull base, reducing intracranial
pressure and potentially resulting in a meningocele. Unfortunately, the patient was
not asked about rhinorrhea at initial presentation, which would support a possible
CSF leak, and he has not previously undergone lumbar puncture, which is necessary
for a diagnosis of IIH.
Conclusion
Our patient presented with a CN III palsy and oculomotor synkinesis caused by a meningocele
involving the OMC and possibly secondary to underlying IIH.
