Journal of Neuro-Ophthalmology:
Isolated Sixth Nerve Palsy From Hemorrhage of a Pontine Cavernous Malformation
Mallery, Robert M. MD; Klein, Joshua P. MD, PhD; Pless, Misha L. MD
Section Editor(s): McCulley, Timothy J. MD
Department of Neurology (RMM, MLP), Massachusetts General Hospital, Boston, Massachusetts; and Department of Neurology (RMM, JPK), Brigham and Women's Hospital, Boston, Massachusetts.
Address correspondence to Misha L. Pless, MD, 15 Parkman Street, WACC Suite 835, Boston, MA 02114; E-mail: firstname.lastname@example.org
The authors report no conflicts of interest.
Abstract: A 32-year-old woman who developed binocular horizontal diplopia was found to have an isolated fascicular sixth nerve palsy secondary to hemorrhage of a cavernous malformation within the left pontine tegmentum. There was sparing of the paramedian pontine reticular formation and absence of a horizontal gaze palsy. The natural history of cavernous malformations and a mechanism by which hemorrhage of these vascular lesions may produce minimal neurologic signs, including isolated ocular motor cranial nerve palsies, is discussed. Magnetic resonance imaging (MRI) that includes susceptibility-weighted sequences leads to their accurate diagnosis.
A healthy 32-year-old woman awoke with binocular horizontal diplopia and a sensation of pressure in her head. On examination, there was a left esotropia measuring 20 prism diopters in primary gaze and limited abduction of the left eye. Ductions and torsional eye movements were otherwise intact, and the remainder of the neurologic examination was normal. Given her young age and lack of vascular risk factors, brain magnetic resonance imaging (MRI) was obtained that demonstrated a 5 × 5-mm2 lesion within the left pontine tegmentum, consistent with a cavernous malformation (Fig. 1). The lesion was deemed inoperable, and her clinical deficit improved spontaneously over 6 months. Brain MRI at that time showed resolution of hemorrhage and edema (Fig. 2).
A lesion within the pontine tegmentum classically causes an ipsilateral, conjugate, horizontal gaze palsy because of either a failure of saccadic initiation by the excitatory burst neurons within the paramedian pontine reticular formation or loss of their axonal connections to the ipsilateral sixth nerve nucleus. In contrast, our patient presented with an isolated abduction deficit of the left eye, presumably from involvement of the left sixth nerve fasciculus. The remarkable absence of neighborhood signs in this case provides evidence that hemorrhage of cavernous malformations within eloquent areas of the brainstem may present with relatively minor neurologic deficits, including isolated ocular motor cranial nerve palsies.
Cavernous hemangiomas are vascular malformations that occur in approximately 0.4%–0.8% of the general population and typically cause initial symptoms in the third or fourth decade (1–4). They are well-circumscribed lesions that may reach several centimeters in diameter and consist of thin-walled, sinusoidal-like blood vessels. The risk of hemorrhage is estimated at 0.1%–3.1% per lesion per year, and the risk increases if there has been a previous hemorrhage (1). Their prevalence at various sites in the brain is proportional to brain volume, and the pons is thus a site of predilection in the brainstem accounting for approximately 62%–75% of brainstem cavernous malformations (2,4). The risk of hemorrhage from an infratentorial location has been reported to be 1.2–5.5 times greater than the risk from a cortical lesion (5).
The fact that cavernous malformations do not contain internal neural tissue may explain why hemorrhage of these lesions can be asymptomatic or result in only minor neurologic impairment. Although cavernous malformations distort surrounding neural tracts, there is no interruption of connectivity or function. When hemorrhage occurs primarily within the lesion itself and spares the surrounding brain parenchyma, there may be minimal effect on adjacent neural structures.
Detection of cavernous malformations has been facilitated by the use of MRI and protocols that include susceptibility-weighted sequences. In our patient, a cavernous malformation with subacute hemorrhage appeared hyperintense on T1 and T2 sequences (Fig. 1). Cavernous malformations are distinguished from other vascular malformations by lack of enhancement with administration of contrast (Fig. 1B). Subacute hemorrhage is often accompanied by surrounding edema (Fig. 1C). Susceptibility-weighted sequences are highly sensitive for blood products, demonstrating hypointensity in the area of the lesion (Fig. 1D). Chronically, with resolved hemorrhage, the lesion may appear isointense or hypointense on T1 sequences (Fig. 2A, B) with a peripheral rim of T2 hypointensity, consistent with hemosiderin (Fig. 2C). With recurrent hemorrhage leading to accumulation of blood products of varying ages, the lesion takes on the typical “popcorn” appearance of mixed signal intensity on TI and T2 sequences. Hypointensity on susceptibility-weighted imaging persists in the chronic phase because of hemosiderin deposition (Fig. 2D) or dystrophic calcification.
Our case illustrates a good prognosis for neurologic recovery from a first-time hemorrhage. The patient's symptoms resolved over a 6-month period, suggesting remyelination of the sixth nerve fasciculus as hemorrhage and edema resolved. However, cavernous malformations carry a high risk for recurrent hemorrhage and must be considered as a potential cause of an “idiopathic” ocular motor cranial nerve palsy.
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