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Woman evaluated for bilateral sixth nerve palsies
Examination of extraocular motility revealed bilateral complete abduction deficits with an esotropia in primary gaze.
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A 54-year-old woman admitted to the neurology service was referred to the neuro-ophthalmology department for evaluation of bilateral sixth nerve palsies. She was transferred to Tufts Medical Center from an outside hospital with 10 days of progressively worsening occipital headaches, nausea and vomiting. She complained of blurred vision in both eyes for 3 days before presentation but denied diplopia. She elected to patch one eye as she found this improved her symptoms. She had no history of head trauma. Her ocular history was significant only for refractive error. She was healthy and did not take any medications. Family and social histories were unremarkable.
Examination
On examination, the patient’s best corrected visual acuity was 20/50 in the right eye and 20/30 in the left eye. Pupils were round, equal and reactive without an afferent pupillary defect. Confrontation visual fields and Hardy-Rand-Rittler color plates were full in both eyes. Examination of extraocular motility revealed bilateral complete abduction deficits (Figure 1) with an esotropia in primary gaze, estimated to be greater than 60 ∆D by the Krimsky method. Anterior segment examination was remarkable only for 2+ nuclear sclerosis bilaterally. Dilated fundus exam was normal and without disc edema (Figure 2). MRI of the brain showed diffuse meningeal enhancement (Figure 3).
Figure 1. External photographs showing extraocular motility with complete bilateral abduction deficits.
Figure 2. Color disc photographs of the right and left eyes showing healthy optic nerve heads without disc edema.
Images: Muakkassa NW, Hedges TR
Figure 3. MRI obtained with gadolinium, coronal section, showing diffuse meningeal enhancement.
What is your diagnosis?
Bilateral sixth nerve palsies
The differential diagnosis for a unilateral sixth nerve palsy is broad, but the differential narrows substantially for acquired bilateral sixth nerve palsies without other associated cranial neuropathies. The list includes lesions of the posterior cranial fossa and clivus where both abducens nerves run together in close proximity at the skull base, basilar artery aneurysms, dolichoectasia, intracranial hypertension or hypotension, traumatic sixth nerve palsies, bilateral carotid-cavernous fistulas and cavernous sinus thrombosis. Specific lesions of the posterior cranial fossa include meningioma, chondroma or nasopharyngeal carcinoma. Cavernous sinus lesions or thrombosis would be unusual because they would be expected to cause multiple cranial neuropathies bilaterally, including optic neuropathy.
In this patient, the absence of papilledema excluded intracranial hypertension from the differential. The absence of conjunctival injection, normal IOP and lack of proptosis on exam made a carotid-cavernous fistula less likely as well. MRI did not reveal any lesions or tumors in the posterior fossa or clivus region, ruling out neoplastic etiologies. However, the MRI did reveal diffuse meningeal enhancement, making intracranial hypotension the most likely on a shortened list of possibilities.
Diagnosis and management
Gregory D. Lee
Nora W. Muakkassa
We were highly suspicious of spontaneous intracranial hypotension as the etiology of the bilateral sixth nerve palsies. CT of the head and CT angiography of the head and neck showed normal arterial circulation without evidence of aneurysms either of the basilar artery or cavernous portion of the carotid artery. They did reveal small bilateral subacute subdural hematomas, which further supported a diagnosis of intracranial hypotension. The opening pressure on lumbar puncture was unmeasurable. Cerebrospinal fluid (CSF) analysis was unremarkable except for an elevated protein level. An MRI of the cervical, thoracic and lumbar spine was performed to locate the suspected CSF leak, although none was identified. A “blind” epidural blood patch was performed. One day after the procedure, the patient noted significant improvement in her symptoms, and 3 days later, she had almost complete resolution of her headaches, nausea and vomiting. Three months later, her extraocular motility had improved substantially, with her esotropia decreased to 16 ∆D, for which she was fitted with a Fresnel prism.
Discussion
Intracranial hypotension usually occurs as a result of lumbar puncture, epidural anesthesia or trauma. Less commonly there is no identifiable etiology, and it is termed spontaneous intracranial hypotension (SIH). SIH is a poorly understood disease and often is misdiagnosed. Typically in SIH, there is no identifiable leak on CT or MRI. Presentation can vary widely from mild headache to more severe symptoms such as cranial neuropathies, quadriplegia or even coma. The incidence is unknown, although it has been reported to be higher in women with a peak incidence around age 40 years. As in our patient, nausea, vomiting and photophobia can occur due to meningeal irritation. Of all the cranial nerves, the abducens nerve is thought to be the most susceptible to both intracranial hypertension and hypotension secondary to its course through the base of the skull. One theory involves downward herniation of the brainstem resulting in stretching and compression of both abducens nerves. Another theory describes the “strangulation” of the nerves by adjacent vessels.
It is a difficult diagnosis to make because the symptoms are often nonspecific, and patients often undergo treatment for secondary findings, such as subdural hematomas. MRI of the brain may reveal pachymeningeal enhancement, subdural fluid collections, engorged veins, brain sag or pituitary hyperemia. Subdural fluid collections are thought to occur due to increased potential space due to CSF hypovolemia.
Diffuse enhancement of all of the meninges on gadolinium-enhanced MRI has been reported in up to 83% of patients and is likely the most useful finding to suggest this diagnosis. Opening pressure on lumbar puncture may be normal, low or unmeasurable. CSF analysis often shows elevated protein levels due to meningeal irritation. Patients usually undergo spinal MRI to identify a potential leak. If a leak is identified, patients are typically treated with an epidural blood patch (EBP) in the location of the leak. Even if no leak is identified, patients are usually treated with a “blind” EBP. Whether or not there is an identifiable leak, most patients’ symptoms improve after EBP, although some patients require more than one treatment. It is unclear exactly why this treatment is effective. One proposed mechanism suggests that the injection of blood into the epidural space increases the pressure in the CSF space and occludes the leak; another mechanism suggests that injection of blood leads to clotting of microscopic leaks. If a large leak is identified, it may require surgical closure. Treatment usually results in resolution of symptoms and reversal of neurologic deficits.
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For more information:
Nora W. Muakkassa, MD, and Thomas R. Hedges III, MD, can be reached at New England Eye Center, Tufts University School of Medicine, 750 Washington St., Box 450, Boston, MA 02111; 617-636-4219; website: www.neec.com.
Edited by Gregory D. Lee, MD, and Nora W. Muakkassa, MD. They can be reached at New England Eye Center, Tufts University School of Medicine, 750 Washington St., Box 450, Boston, MA 02111; 617-636-4219; website: www.neec.com.