Applied Radiology

EDITOR'S NOTE

Evaluation of visual disturbance (blindness or ophthalmoplegia) is a difficult clinical problem. Although the ophthalmoscopic examination may be useful for evaluation of monocular blindness, evaluation of visual field defects involving both eyes and abnormalities of tracking requires an imaging study that extends beyond the retina into the orbit, the brainstem, and the remainder of the brain. MRI is by far the best imaging study for this evaluation. As discussed in this issue of Applied Imaging, the specific MR imaging technique used depends on the specific pattern of blindness or diplopia involved. This information must be provided to the radiologist in order to maximize the chance of determining the cause of visual disturbance.

--William G. Bradley, Jr., MD, PhD, FACR

Visual Disturbance

This issue of Applied Imaging focuses on the MR imaging aspects of visual disturbance, including problems involving loss of visual acuity as well as those leading to diplopia. Although blindness can originate in the globe itself, generally such lesions are accessible ophthalmoscopically and will, therefore, not be addressed here. Rather, we will focus our attention on three different anatomic regions that are visualized easily by MRI and that present with three different clinical patterns of blindness: monocular blindness (prechiasmatic optic nerve), bitemporal hemianopsia (central optic chiasm), and homonymous hemianopsia (retrochiasmatic). ¹ Tracking abnormalities can be divided into those that are referable to the orbit, the brainstem, or the cranial nerves. ²

Monocular Blindness

Excluding the eyeball itself, monocular blindness generally involves the optic nerve. The optic nerve (and the orbit in general) are best evaluated by imaging in the coronal plane to avoid partial volume averaging. Sequences are chosen to demonstrate either increased water content or abnormal enhancement (Figure 1). ³ The former is best accomplished with a T ₂ -weighted image and some form of fat suppression. At high field, this can be a spectroscopic "FatSat" pulse; at any field strength, a short TI inversion recovery (STIR) sequence can be used (which is chosen to null fat based on its short T ₁ relaxation time.)

To demonstrate abnormal enhancement following administration of gadolinium, the ideal sequence is a fat saturated, T ₁ -weighted image, which will show enhancing lesions of the optic nerve against a dark background. Ideally, this is accomplished with a spectroscopic fat saturation pulse. Unfortunately this can only be performed at high field; STIR cannot be used with contrast because gadolinium shortens the T ₁ of the lesion to around the same value as that of fat, both of which would then be nulled. The primary lesions that can be detected in this category include optic neuritis, optic nerve glioma, and optic nerve sheath meningioma.

Optic neuritis (Figures 2A and 2B) can be detected ⁴ on the basis of either increased water content (on T ₂ -weighted images) or enhancement following administration of gadolinium (on T ₁ -weighted images). In either case, fat suppression is desirable, as noted above. Optic neuritis tends to present with abnormal increased signal intensity on T ₂ -weighted images and abnormal enhancement on T ₁ -weighted images, generally without increased size. ⁴ If optic neuritis is suspected on the basis of clinical and imaging findings, it is mandatory to perform MR imaging of the brain as well to detect multiple sclerosis, which may or may not be evident clinically. While relatively advanced MS can be detected with conventional MR imaging, the most sensitive technique to detect early MS is a thin-slice, sagittal fluid attenuated inversion recovery (FLAIR) that shows the "subcallosal striations" ⁵ so highly associated with early cases of MS (Figure 2C).

Generally, gliomas of the optic nerve are diagnosed on the basis of increased size, with or without enhancement, as they tend to be low-grade lesions (Figure 3). Such lesions may extend posteriorly to involve the optic chiasm and optic tracts, particularly in patients with neurofibromatosis type I. ⁶

Optic nerve sheath meningiomas also present with increased size; however, they invariably enhance with gadolinium. Such lesions tend to be more eccentric than optic nerve gliomas, reflecting their origin from the optic nerve sheath. Meningiomas tend to be isointense to brain on both T ₁ - and T ₂ -weighted sequences, whereas optic nerve gliomas tend to be brighter than brain on T ₂ -weighted images and darker than brain on T ₁ -weighted images. ³

Bitemporal Hemianopsia

Because of the optic lens, the temporal portion of the visual field is mapped onto the nasal portion of the retina and vice versa. Since the nasal fibers of the retina decussate in the middle of the optic chiasm, they are the most susceptible to damage from midline sellar masses. This leads to loss of the temporal portion of both visual fields, ie, bitemporal hemianopsia. ¹

The optic chiasm is best evaluated in the coronal plane with thin (at least 3 mm) T ₁ - and T ₂ -weighted images. Gadolinium is useful to demonstrate certain inflammatory processes and tumors.

The most common lesion to produce bitemporal hemianopsia is the pituitary macroadenoma (Figure 4). Since these lesions tend to arise in the midline, they initially impact the center of the optic chiasm. ⁷ This occasionally occurs following initiation of bromocryptine treatment with bleeding into the tumor and secondary enlargement. Thus, it is important to perform MR imaging to detect those macroadenomas which, while not abutting the optic chiasm initially, could potentially expand to abut the optic chiasm if bleeding occurs.

Another common lesion that can produce bitemporal hemianopsia is the meningioma. ⁸ Usually, parasellar meningiomas (Figure 5) can be distinguished from macroadenomas on the basis of their signal intensity, meningiomas tending to be closer to brain in signal intensity than pituitary macroadenomas. Both enhance with gadolinium; however, meningiomas tend to enhance more homogeneously than macroadenomas. ⁸ In addition, the use of gadolinium may allow a dural tail to be seen, which is highly characteristic of meningiomas. When meningiomas and macroadenomas invade the cavernous sinus, a narrowed carotid artery is more likely to be seen with meningiomas (which are tough tumors) than with macroadenomas (which tend to be softer). ⁹

Aneurysms arising from the distal internal carotid arteries or the anterior circle of Willis can occasionally point toward the optic chiasm, leading to bitemporal hemianopsia. ⁸ It is absolutely mandatory to be able to distinguish aneurysms from macroadenomas as the latter are typically approached transphenoidally. Obviously, mistaking an aneurysm for a macroadenoma would be catastrophic. Unfortunately, aneurysms have a wide range of appearances based on whether they are patent or thrombosed. If patent, a flow void should be seen within the aneurysm and flow artifacts should be noted emanating from it along the phase-encode axis. ¹⁰ If clotted, the hemorrhage in the aneurysm can be bright or dark depending on its age and the type of sequence and contrast used. ¹¹ In some cases, MR angiography may be useful.

Homonymous Hemianopsia

Lesions producing homonymous hemianopsia may involve the following retrochiasmatic structures (from anterior to posterior): optic tract, lateral geniculate body of the thalamus, optic radiation, or calcarine cortex. The closer the lesion is to the primary visual (calcarine) cortex in the occipital lobes, the more "congruent" the homonymous visual field defects. ¹ Thus, occipital lobe infarcts lead to similar sized abnormalities in both contralateral visual fields, whereas tumors of the optic tracts or lateral geniculate bodies produce less perfectly matched visual field defects. ¹

With acute onset of homonymous hemianopsia, stroke in the distribution of the contralateral posterior cerebral artery is the most likely etiology. When available, echo planar diffusion imaging is the most sensitive MR imaging technique to make this diagnosis within the first few hours (Figure 6A). Subsequently, hyperintensity is noted on axial T ₂ -weighted or FLAIR images in the typical vascular territory of the posterior cerebral artery (Figure 6B).

Tumors of the upper brainstem that involve the optic tracts, lateral geniculate bodies, or proximal optic radiations are best seen using axial T ₂ -weighted or FLAIR MR images, on which the lesions are usually bright. While contrast may have a role in detecting pilocytic and higher grade astrocytomas, enhancement is not usually a feature of tumors of the upper brainstem. ¹²

Ophthalmoplegia

Eye motion is controlled by six extra-ocular muscles, which in turn are controlled by three cranial nerves: CN III, CN IV, and CN VI. ² Lesions involving these cranial nerves, their brainstem nuclei, or the corticobulbar tracts can lead to abnormalities of tracking. In addition, the motion of the eyes is coordinated by a white matter tract located anterior to the aqueduct and fourth ventricle known as the "medial longitudinal fasciculus" (MLF). ¹³ Lesions of the MLF produce a particular type of diplopia known as an "internuclear ophthalmoplegia" (INO). ¹³ Such lesions are readily visualized on T ₂ -weighted axial images of the brainstem, which show hyperintensity anterior to the aqueduct or fourth ventricle (Figure 7). Multiple sclerosis is by far the most common cause of internuclear ophthalmoplegia. ²

Diplopia can be caused by tumors of the orbit (Figure 1), the brainstem (Figure 8), the cranial nerves per se, or masses that impinge upon the cranial nerves, the latter being more common. ⁸ For example, since the third cranial nerve passes between the superior cerebellar and posterior cerebral arteries, aneurysms of either of these arteries can impinge upon CN III, leading to diplopia, specifically a third nerve palsy. Similarly, parasellar masses extending into the cavernous sinus can impinge upon one or more of these cranial nerves as they are en route to the superior orbital fissure. ³ Primary masses of the cranial nerves are generally schwannomas.

The most common cause of isolated palsies of the third, fourth, or sixth cranial nerves is microvascular infarction, most commonly seen in diabetics. ¹⁴ Unfortunately, such lesions are not currently treatable, nor are they currently visible by MRI.

Summary

Following the ophthalmoscopic examination, MRI is the preferred imaging modality to evaluate blindness. When ordering an MRI study, it is important to differentiate monocular blindness, bitemporal hemianopsia, and homonymous hemianopsia, as different MR imaging techniques are used to evaluate these three distinct clinical presentations.

Similarly, evaluation of visual tracking abnormalities is best accomplished with MRI to detect lesions of the orbit, the cranial nerves, which supply the extraocular muscles, or their brainstem nuclei. While MRI may not depict all causes of cranial nerve palsy leading to ophthalmoplegia, it should detect the vast majority of treatable causes of ophthalmoplegia, ie, masses impinging upon the cranial nerves (including aneurysms).

Clinical Quiz: True or False

1. MRI is indicated for all patients presenting with visual disturbances, eg, blindness or ophthalmoplegia.

2. The same MRI technique is used regardless of the specific visual disturbance.

3. Usually, monocular blindness is produced by lesions of the occipital lobe.

4. The most likely cause of bitemporal hemianopsia is a suprasellar mass.

5. In a patient with known neurofibromatosis, monocular blindness is most often caused by an optic nerve sheath meningioma.

1. False. Patients with cardiac pacemakers and ferromagnetic intracranial aneurysm clips cannot be scanned by MRI. These patients are better scanned by CT.

2. False. The specific MR imaging technique used depends on the specific type of visual disturbance: monocular blindness and bitemporal hemianopsia being best evaluated in the coronal plane and homonymous hemianopsia in the axial plane.

3. False. Lesions of the occipital lobe result in homonymous hemianopsia, ie, visual field defects affecting both eyes.

4. True.

5. False. In neurofibromatosis type I patients (the most common form), the classic lesion causing blindness is the visual pathway glioma.