Crucial Questions in Imaging

State-of-the-art medical imaging facilities are now widely available. Imaging, however, remains expensive, is a potential drain on medical resources, and is not without risks. Accordingly, it is crucial that the clinician address the following issues while planning the use of an imaging study as part of a neuro-ophthalmic workup:

when to order what to order modality

location how to order

specify lesion and region of interest discuss with radiologist before ordering review with radiologist after imaging

When to Order

The decision to order an imaging study should be based on clinical localization and the expectation of particular findings. Further, the information acquired by imaging should have some influence on patient management or provide a more accurate prognosis of the disease’s natural history. Finally, the same information should not be available by simpler or less expensive means.

Transient visual phenomena without residual deficit are most frequently related to large-vessel disease and thereby require assessment of the vascular structures. Suspicion of a neoplastic mass lesion is a common reason for ordering an imaging study. The workup of a patient with decreased vision should include imaging of the course of the optic nerve if there is evidence of optic neuropathy that is not otherwise clinically explained. When unilateral disc edema is present, the differential diagnosis should include anterior ischemic optic neuropathy (AION), papillitis, and intraorbital compression. Imaging is not required in classic nonarteritic AION. In acute optic neuritis, a cerebral MRI should be ordered to look for evidence of cerebral white matter signal abnormalities typical of multiple sclerosis; these abnormalities are best demonstrated on T2-weighted or FLAIR images. In most cases of acute optic neuritis, specific orbital MRI sequences (eg, postcontrast T1-weighted with fat saturation, or T2-weighted) will also disclose signal abnormalities within the affected optic nerve (Fig 2-9D). A CT scan has very limited value in demyelinating disease. (See Chapter 14 for further discussion of multiple sclerosis and imaging studies.) When central visual function is affected without disc edema, or when field defects respect the vertical midline (as can occur with parachiasmal or retrochiasmal pathology), appropriate imaging studies should be obtained.

Bitemporal visual field defects localize to the chiasm or parasellar region and necessitate neuroimaging. Some of the most frequent chiasmal-compressive masses include

pituitary adenoma (Fig 2-14) with suprasellar extension meningioma

craniopharyngioma chiasmatic glioma aneurysm

Figure 2-14 MRI scans from a 45-year-old man who presented with a bitemporal defect, impotence, and an elevated prolactin level consistent with the clinical diagnosis of a prolactinoma. A, Coronal T1-weighted MRI postcontrast image shows a large pituitary adenoma stretching and compressing the chiasm (visible as a gray ribbon superior to the adenoma [arrow]). B, MRI appearance several months after treatment with a dopamine agonist shows near resolution of the prolactinoma; the enhancing structure in the suprasellar space is a normal-appearing infundibulum (arrow), with the

chiasm above. (Courtesy of Eric Eggenb erger, DO.)

Rarer compressive lesions in the parasellar region include metastases, chordomas, dysgerminomas, lymphomas, histiocytoses, epidermoids, Rathke cleft cysts, and granulomatous inflammatory disease.

Homonymous visual field defects imply pathology in the retrochiasmal visual pathway. Such defects are most commonly vascular in origin but should be imaged unless clearly associated with a prior stroke syndrome.

When a patient reports binocular double vision, anatomic localization is again crucial, and the pattern of misalignment becomes all-important. As with afferent system dysfunction involving the optic nerve, both CT and MRI studies may provide the necessary information regarding the extraocular muscles. MRI is somewhat more sensitive to the various changes associated with inflammation or infiltration, but CT delineates the size of the extraocular muscles well, particularly on direct (ie, not reformatted) coronal images (Fig 2-1B).

When the pattern of deviation fits an ocular motor cranial nerve palsy (CN III, CN IV, or CN VI), the decision about imaging depends on the clinically suspected pathophysiology and whether the palsy is isolated. The acute onset of an isolated ocular motor cranial nerve palsy in a patient in the age group at higher risk of vasculopathy (usually >50 years), especially when associated with a history of diabetes mellitus, hypertension, or vascular disease, is most likely due to microvascular ischemia, and emergent imaging may not be required. However, a pupil involving CN III palsy requires immediate imaging. With simultaneously occurring ocular motor cranial nerve palsies, especially when the fifth nerve is involved, a cavernous sinus location is most likely.

Skew deviation is a supranuclear lesion producing a vertical misalignment that does not fit the pattern of CN III or CN IV palsy. MRI of the posterior fossa should be obtained.

Certain common clinical situations are associated with negative results on imaging, such as AION (nonarteritic or arteritic), isolated microvascular ischemic ocular motor cranial neuropathy, and eye pain in a patient with normal findings on neuro-ophthalmic examination. However, when pain is associated with findings such as ptosis and miosis (Horner syndrome), carotid artery dissection

should be suspected; MRI/MRA or CT/CTA through the carotid artery is often sufficient for confirmation.

Chi SL, Bhatti MT. The diagnostic dilemma of neuro-imaging in acute isolated sixth nerve palsy. Curr Opin Ophthalmol. 2009;20(6):423–429.

Lee AG, Hayman LA, Brazis PW. The evaluation of isolated third nerve palsy revisited: an update on the evolving role of magnetic resonance, computed tomography, and catheter angiography. Surv Ophthalmol. 2002;47(2):137–157.

Pisaneschi M, Kapoor G. Imaging the sella and parasellar region. Neuroimaging Clin N Am. 2005;15(1):203–219.

Weber AL, Caruso P, Sabates NR. The optic nerve: radiologic, clinical, and pathologic evaluation. Neuroimaging Clin N Am. 2005;15(1):175–201.

What to Order

With some important exceptions (see Table 2-1), MRI is usually more valuable than CT in detecting a lesion and narrowing the differential diagnosis. The specific choice of imaging modality—including the sequence, orientation, and direction—depends on a combination of the suspected location and the expected pathology. In suspected large-vessel disease, MRA, CTA, and digital angiography may also be considered.

Imaging should be done of the brain and the orbits. Orbital imaging provides details of the optic nerves and the surrounding tissues that are often not detected with brain imaging alone.

When pathology is localized to the orbit, either CT or MRI can provide useful information. Orbital fat produces excellent contrast with the other orbital components on CT and MRI, and both modalities give excellent anatomical localization (see Fig 2-1, 2-2, 2-5).

More important than the choice of imaging modality is the selection of orientation and specific sequences. Direct coronal images are useful in most orbital disorders (Fig 2-15). With MRI, direct coronal imaging is not a problem, but CT may require specific positioning (eg, neck extension) that may be difficult to achieve with older patients. MRI of the orbit should be done with fat-saturation techniques designed to eliminate the high-intensity T1 signal from fat (see Fig 2-7, 2-9D).