Figure 5.3 Sagittal non-contrast T1-weighted MRI in the above patient shows mild descent of the cerebellar tonsils below the foramen magnum. Note very mild compression of the lower brainstem.

include brainstem and cerebellar tumors, strokes, degenerative diseases and hind-brain anomalies such as the Chiari malformation (see Chapter 7, Bouncing vision). Comitant esotropia associated with Chiari I malformation is believed to be related to a disturbance in the vergence mechanism rather than to dysfunction of the sixth nerves. In nearly all cases, it is accompanied by other symptoms and signs of hindbrain overcrowding. In light of the patient’s clinical findings, the tonsillar descent was re-interpreted as a symptomatic Chiari I malformation. Following sub-occipital decompression, her headaches and diplopia resolved, however intermittent paresthesias persisted.

Sphenoid sinus mucocele

Case: A 78-year-old retired teacher had a fourmonth history of progressive horizontal diplopia. Initially her double vision occurred only on extreme

Figure 5.4 Sagittal non-contrast T1-weighted MRI

in 78-year-old woman with a progressive sixth nerve palsy shows a large sphenoid sinus mucocele.

right gaze but over time it came to involve primary position as well, first at distance and then at near. Examination showed a partial right sixth nerve palsy and was otherwise unrevealing. An MR scan was reportedly normal except for a mucocele in the sphenoid sinus, which was assumed to be an incidental finding. Results of additional laboratory testing and a lumbar puncture were non-diagnostic and she was therefore referred for neuro-ophthalmic consultation. Review of her outside scan showed a large, round hyperintensity in the sphenoid sinus consistent with a mucocele. The mass was noted to extend posteriorly, eroding through to the back of the clivus where it was believed to be compromising the sixth nerve (Figure 5.4). She underwent transphenoidal excision of the lesion, histopathologically confirmed as a mucocele, and over the weeks following surgery her diplopia and sixth nerve palsy resolved completely.

Discussion: Sinus disease is so common in the normal population that it is frequently encountered on a radiologic study obtained for another purpose and dismissed as an incidental finding. For example, a study examining the incidence of sinus disease in a series of patients with optic neuritis found

no difference compared to a group of healthy controls (about 13% in each group). In the above case, however, the mucocele was not incidental. Its significance could be appreciated when the scan appearance was correlated with the clinical findings. The mucocele had grown posteriorly into the pathway of the sixth nerve as it ascends the clivus.

Because sinus disease is so common in certain geographic areas, it is important to have some guidelines to help decide when it has clinical significance. Particular attention should be given to sinus disease when there is bony erosion with extension into neighboring soft tissues. Sinusitis that invades the orbit can mimic any form of the idiopathic orbital inflammatory syndrome. Sinusitis that seeds the cavernous sinus can result in a life-threatening cavernous sinus thrombosis. In some cases, the patient demographics should arouse suspicion. For example, poorly controlled diabetics, especially those in ketoacidosis, immunosuppressed individuals, patients with hematologic malignancies and those on hemodialysis are especially susceptible to fungal sinusitis. A patient with any of these risk factors who develops an acute orbital inflammatory syndrome with evidence of paranasal sinusitis should be investigated promptly for mucormycosis, as this rapidly invasive infection has a high mortality rate. Such patients are also prone to aspergillosis, which may similarly spread from the adjacent sinuses into the orbit where it may present as a mass lesion.

Dolichoectatic basilar artery

Case: A 70-year-old retired radiologist had a twoyear history of painless horizontal diplopia. At first, his double vision had been intermittent but was now constant and caused him to feel off balance while walking. He reported no other neurologic or systemic symptoms. Past medical history included hypertension, coronary artery disease and a hemispheric TIA due to carotid stenosis, treated with endarterectomy several years earlier. His neurologic status had been stable since then. On examination, there was a moderate right abduction deficit

A

B

C

Figure 5.5 MRI in the above patient with a painless, chronic right sixth nerve palsy. This sequence of axial T2-weighted images shows the course of the basilar artery.

(A) The basilar artery, seen here as a low-intensity signal void (arrow) is positioned in the midline at the ventral medulla. (B) At the pontine level, the basilar artery is slightly enlarged and is clearly shifted to the right where it compresses and flattens the ventral aspect of the pons.

(C) At the midbrain, the basilar artery is seen at an oblique angle as it redirects its course back toward the midline.

causing an esotropia that worsened on right gaze. Abduction saccades in the right eye were slow. The remainder of his examination was unremarkable and a Tensilon (edrophonium chloride) test was negative.

A CBC, ESR, CRP, acetylcholine receptor antibodies and antiGQ1b antibodies were normal. An MRI of brain and orbits was interpreted as normal although the radiologist’s report noted tortuosity of the basilar artery. A lumbar puncture showed a normal opening pressure and CSF constituents. In light of the negative evaluation and progressive cranial nerve palsy, the patient was referred for neuroophthalmic consultation.

A critical review of the MR scan, in the context of the patient’s clinical findings, revealed that the ectatic and tortuous basilar artery was distorting the ventral surface of the pons, particularly on the right side at the exit zone of the sixth nerve (Figure 5.5). Based on a combination of the clinical and radiographic features, a diagnosis of dolichoectatic com-

pression of the sixth nerve was made. His esotropia progressed slightly over the next eight months then stabilized for the next three years. The patient was managed symptomatically with prisms and a follow-up MRI was unchanged.

Discussion: Dolichoectasia describes abnormal elongation, distention and tortuosity of blood vessels. The prevalence of intracranial dolichoectasia is estimated at 0.06 to 5.8%, most frequently involving the vertebrobasilar system in which the anomaly is sometimes referred to as a fusiform aneurysm of the basilar artery. The pathogenesis of dolichoectasia is not completely understood but the condition is strongly associated with arteriosclerosis and chronic hypertension. Histopathologically, there is diffuse loss of elastic tissue and atrophy of smooth muscle cells, structures which normally protect the arterial wall from the expansile effects of high systolic pressure.