76 Chapter 4: Radiographic errors

Figure 4.16 Close-up view of the scan shown in

Figure 4.15 shows a small enhancing mass at the entrance to Dorello’s canal on the left (arrow), consistent with a meningioma.

eyes are either aligned or they are not. Occasional patients, however, describe a definite history of gradual progression, as in the case under discussion. When due to a sixth nerve palsy, such progressive diplopia is initially present only on extreme lateral gaze, eventually progressing to involve center gaze, first at distance and then increasingly at near. Very small angle misalignments are often experienced as blurring rather than doubling of images. When just at the threshold of fusion, diplopia is often intermittent, then, when fusional capacity is exceeded, diplopia becomes constant. For many people, the most frequent visual task that they perform at distance is driving, and so patients with early or mild sixth nerve weakness often experience intermittent horizontal diplopia only during this activity.

The most common mechanism of a sixth nerve palsy in this patient’s age group is ischemia, either due to a vasculopathic cranial mononeuropathy or a brainstem stroke. The significance of his history

of slow progression is that it rules out an ischemic mechanism. The story in this case so strongly suggests a compressive lesion that a critical view of the scan with attention to the course of the sixth nerve is mandatory. The usual method for doing this is to start at the brainstem and work out to the orbit, tracing the pathway of the involved nerve. The abducens nerve exits the brainstem at the pontomedullary junction and ascends the subarachnoid space along the clivus. It then passes near the inferior petrosal sinus and slides under the petrosphenoid ligament, entering Dorello’s canal where it is tethered to the dura. The nerve proceeds through the cavernous sinus to the superior orbital fissure, entering the orbit to innervate the lateral rectus muscle. Axial T1-weighted images without and with contrast are most useful for evaluating the course of the cisternal and petrous portions of the abducens nerve, whereas coronal images provide critical information in the parasellar and orbital segments. Using this technique in the above case, and armed with the conviction (based on the history) that a mass lesion was present, the responsible lesion was identified.

Diagnosis: Petrous ridge meningioma

Tip: Any patient with a slowly progressive sixth nerve palsy should be assumed to harbor a mass lesion.

Midline and bilateral abnormalities

Radiographic abnormalities are most easily identified when the changes are unilateral or asymmetric. Disorders that involve midline structures and those that are bilateral and symmetric may escape detection. The next three cases illustrate this principle.

Bilateral idiopathic sixth nerve palsy

Case: A 62-year-old school psychologist experienced abrupt onset of horizontal diplopia on left gaze. She had no head or eye pain and no other neurologic deficits or recent systemic symptoms.

She took medications for hypertension and hypercholesterolemia and was otherwise in good health. Examination one week after onset showed an isolated left abduction deficit. An MR scan was reportedly normal and a presumptive diagnosis of vasculopathic (ischemic) sixth nerve palsy was made (see Chapter 11, Acute isolated sixth nerve palsy). On re-examination one week later she had developed an abduction deficit on the right side as well. There were no other neurologic deficits and no papilledema. An edrophonium chloride (Tensilon) test was said to be equivocal. A lumbar puncture showed an opening pressure of 160 mm of water with normal chemistries, cell count, cytology and flow cytometry. A number of blood tests for vasculitis and systemic inflammatory disease, a chest X-ray and urinalysis were all normal.

Is a diagnosis of vasculopathic sixth nerve palsy still tenable here?

While vasculopathic cranial neuropathy is the most common cause of sixth nerve deficit in individuals over age 50 years, such palsies are virtually always unilateral. A diagnosis of bilateral vasculopathic or idiopathic sixth nerve palsy indicates that the actual cause has not yet been identified.

In light of her negative work-up, the patient was referred for neuro-ophthalmic consultation. Examination showed normal afferent visual function and optic disc appearance. There was a 40 diopter esotropia in primary position with no abduction of either eye past midline and with marked slowing of lateral rectus saccades bilaterally. A second Tensilon test was unequivocally negative.

What are the most common causes of bilateral sixth nerve palsy and what mechanism is most likely in this case?

Processes that frequently affect both sixth nerves include structural lesions at the skull base, disorders of intracranial pressure (too high or too low) and meningitis (See Table 4.3). The two abducens nerves travel in close proximity in the subarach-

Table 4.3 Causes of bilateral sixth nerve palsy

Trauma Meningitis

Increased intracranial pressure

Low pressure syndromes (post-lumbar puncture, myelography)

Skull base tumor Pituitary apoplexy Giant cell arteritis

Wernicke’s encephalopathy Miller Fisher syndrome

noid space, where they are vulnerable to meningeal infiltration (inflammatory or neoplastic), and along the clivus, where they are prone to compression by tumorous expansion. The sixth nerves are particularly vulnerable to changes in intracranial pressure because they are tethered at their entrance into Dorello’s canal, so that shifts in brain position due to pressure cause traction. The appropriate workup for bilateral sixth nerve palsy emerges from these anatomic considerations, and should include high quality neuro-imaging with attention to the skull base, and lumbar puncture, with measurement of intracranial pressure as well as cytologic CSF examination.

Re-evaluation of this patient’s MRI disclosed mild expansion of the clivus (Figure 4.17). A transphenoidal biopsy of the lesion revealed a large B-cell lymphoma. A metastatic survey showed no evidence of malignancy elsewhere and she was treated with chemotherapy and external beam radiation. Over the next six months, abduction improved in each eye, but she still had a 16 diopter esotropia in primary position for which she underwent extraocular muscle surgery. Post-operative ocular alignment was excellent and she remained clinically and radiographically stable three years later.

Discussion: Radiographic investigation of the skull base can be challenging. Standard CT sequences have poor resolution in this region due to signal degradation at soft tissue to bone interfaces. MR images are not hampered by this limitation but do

not visualize calcium, and so some bony abnormalities may be missed. If a skull base tumor is suspected, adequate radiographic investigation may require both an MRI and a CT with bone windows to fully define the site of origin of the tumor and its soft tissue extension.

Lesions of the clivus can be particularly difficult to appreciate because it is a midline structure and thus comparison of the affected with the normal side is not an option. The normal adult clivus contains red marrow and fatty marrow in varying proportions. On a pre-contrast T1-weighted image, red marrow appears as low-signal intensity (dark) and fatty marrow as high-signal intensity (bright) (Figure 4.17D). In young adults, the clivus has a predominantly low signal with spotty amounts of bright signal; with advancing age, the amount of low signal intensity decreases. In most adults over age 50, the clivus has a predominantly bright signal due to a preponderance of yellow marrow. When invaded by tumor, the clivus instead has a low-signal intensity, and fol-

lowing contrast administration the tumor portion enhances intensely. Normal clival tissue enhances mildly, if at all. In most cases, the clivus is best assessed on mid-sagittal cuts.

Lesions that develop in the clivus include chordoma, chondrosarcoma, plasmacytoma, giant cell tumors, hemangiomas, lymphomas, adenocystic and nasopharyngeal carcinomas and Paget’s disease. The clivus is also a site for bony metastases (Figure 4.18). In an older man with prostate cancer, a painless, progressive sixth nerve palsy must be considered a clival metastasis until proven otherwise. In addition to unilateral or bilateral sixth nerve palsy, lesions of the clivus can cause bulbar symptoms due to cranial nerve palsy, brainstem compression and endocrinopathies due to pituitary insufficiency.

Diagnosis: Clivus tumor

Tip: Investigation of unexplained bilateral sixth nerve palsy should include a critical evaluation