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.
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
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
Tracking abnormalities can be divided into those that are referable
to the orbit, the brainstem, or the cranial nerves.
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).
The former is best accomplished with a T
-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
To demonstrate abnormal enhancement following administration of
gadolinium, the ideal sequence is a fat saturated, T
-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
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
on the basis of either increased water content (on T
-weighted images) or enhancement following administration of
gadolinium (on T
-weighted images). In either case, fat suppression is desirable, as
noted above. Optic neuritis tends to present with abnormal
increased signal intensity on T
-weighted images and abnormal enhancement on T
-weighted images, generally without increased size.
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"
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.
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
- and T
-weighted sequences, whereas optic nerve gliomas tend to be
brighter than brain on T
-weighted images and darker than brain on T
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.
The optic chiasm is best evaluated in the coronal plane with
thin (at least 3 mm) T
- and T
-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
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
Another common lesion that can produce bitemporal hemianopsia is
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.
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
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.
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.
If clotted, the hemorrhage in the aneurysm can be bright or dark
depending on its age and the type of sequence and contrast used.
In some cases, MR angiography may be useful.
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.
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.
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
-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
-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.
Eye motion is controlled by six extra-ocular muscles, which in
turn are controlled by three cranial nerves: CN III, CN IV, and CN
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).
Lesions of the MLF produce a particular type of diplopia known as
an "internuclear ophthalmoplegia" (INO).
Such lesions are readily visualized on T
-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
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.
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
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.
Unfortunately, such lesions are not currently treatable, nor are
they currently visible by MRI.
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
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
3. Usually, monocular blindness is produced by lesions of the
4. The most likely cause of bitemporal hemianopsia is a
5. In a patient with known neurofibromatosis, monocular
blindness is most often caused by an optic nerve sheath
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.
5. False. In neurofibromatosis type I patients (the most common
form), the classic lesion causing blindness is the visual pathway