Neuroimaging in Ophthalmology

This comprehensive review summarizes the best neuroimaging studies for specific neuro-ophthalmic indications, covering both computed tomography (CT) and magnetic resonance imaging (MRI) protocols with their diagnostic radiographic findings. Published in the Saudi Journal of Ophthalmology in 2012, this article provides essential guidance for ophthalmologists on when and how to order neuroimaging for their patients.

Background

Advances in neuroimaging over the past three decades have revolutionized the evaluation, management, and treatment of neuro-ophthalmic disorders. Non-invasive approaches for early detection and monitoring have decreased morbidity and mortality. However, the growing number of imaging modalities and sequences available makes it essential for ophthalmologists to understand which studies to order for specific clinical scenarios.

The two main imaging modalities used in neuro-ophthalmology are CT and MRI, but variations of these modalities -- including specialized sequences, contrast protocols, and targeted anatomic regions -- must be considered for each clinical indication. Critically, the ordering physician must communicate relevant clinical information to the interpreting radiologist to ensure the best diagnostic interpretation.

Study Design

This article is a narrative review that systematically covers the major neuro-ophthalmic clinical indications requiring neuroimaging. For each indication, the authors identify the preferred imaging modality, recommended sequences, contrast requirements, and key diagnostic radiographic findings. The review is organized by clinical presentation and includes a comprehensive summary table of all recommendations.

Overview of Imaging Modalities

Computed Tomography (CT)

CT scanning uses conventional X-ray technology with a rotating source and detectors that measure attenuation values in Hounsfield units. Key principles:

• Dense materials (bone) appear bright white; less dense materials (air) appear dark
• Iodinated contrast can improve sensitivity and specificity but is not always needed
• Contrast is not beneficial in thyroid eye disease, may obscure acute hemorrhage, and is contraindicated with iodine allergy
• CT advantages: Faster acquisition, superior bone detail, better for acute hemorrhage, sinus disease, and calcifications

Magnetic Resonance Imaging (MRI)

MRI relies on the interaction of hydrogen atoms with an intense magnetic field -- no radiation exposure is involved. Key sequences include:

• T1-weighted: Fluid is dark, fat is bright. Useful with gadolinium contrast for enhancing lesions
• T2-weighted: Fluid and water-containing tissues are bright, fat is dark. Useful for detecting pathologic edema
• FLAIR (Fluid Attenuated Inversion Recovery): Suppresses free CSF signal on T2, making periventricular demyelinating lesions more conspicuous
• Fat suppression: Essential for orbital imaging -- bright T1 fat signal can obscure both pathology and gadolinium enhancement in the orbit
• DWI/ADC (Diffusion Weighted Imaging / Apparent Diffusion Coefficient): Measures aberrant Brownian motion of water. Critical for differentiating acute ischemic stroke (restricted diffusion) from vasogenic edema (unrestricted diffusion)
• CISS/FIESTA: Specialized sequences for better cranial nerve visualization, useful for intrinsic nerve lesions such as oculomotor nerve schwannoma

Clinical Applications by Indication

Optic Neuropathy

For an unexplained unilateral or bilateral optic neuropathy (including optic atrophy):

• Preferred study: Pre- and post-contrast MRI of the head and orbits
• Key protocol: Fat suppression is highly recommended for showing intraorbital lesions
• Rationale: Can disclose compressive, inflammatory, or infiltrative etiologies along the entire course of the optic nerve

Optic Neuritis

For acute demyelinating optic neuritis presenting with painful, unilateral vision loss in young adults:

• Preferred study: MRI with pre- and post-contrast fat suppression of the orbits (for optic nerve enhancement) plus T2-weighted brain imaging with FLAIR for periventricular demyelinating lesions (including sagittal views of the corpus callosum)
• Prognostic significance: The Optic Neuritis Treatment Trial (ONTT) showed that patients without brain lesions had a 22% risk of multiple sclerosis at 15 years, versus 78% with multiple white matter lesions

Bitemporal Hemianopsia

For bitemporal hemianopsia from lesions of the optic chiasm:

• Preferred study: MRI of the sella with and without contrast
• Common causes: Pituitary adenoma, craniopharyngioma, meningioma, dysgerminoma (adults); optic pathway glioma (children); internal carotid artery aneurysm
• When to use CT: Acute setting for hemorrhage (pituitary apoplexy), or to demonstrate calcification in suprasellar lesions or hyperostosis in meningioma

Homonymous Hemianopsia and Cortical Blindness

For homonymous hemianopsia from retrochiasmal pathway lesions:

• Preferred study: Cranial MRI with and without contrast, attention to the contralateral retrochiasmal pathway
• Acute presentation: Initial non-contrast head CT may be obtained for suspected stroke or intracranial hemorrhage, but follow-up MRI is recommended
• DWI/ADC utility: Can differentiate reversible vasogenic edema in posterior reversible encephalopathy syndrome (PRES) from irreversible cytotoxic edema in ischemic stroke -- critical for prognosis

Cranial Nerve Palsy

For ocular motor cranial neuropathies (third, fourth, or sixth nerve palsy):

• Preferred study: Cranial pre- and post-contrast MRI following the course of the involved nerve(s)
• Specialized sequences: CISS and FIESTA can better demonstrate cranial nerves and intrinsic nerve lesions
• Sixth nerve palsy: CT in conjunction with MRI may better show bone or sinus disease affecting the clivus
• Acute third nerve palsy with pupil involvement: Emergency non-contrast CT to exclude subarachnoid hemorrhage, followed by CTA for posterior communicating artery aneurysm

Nystagmus

For unexplained nystagmus:

• Preferred study: Pre- and post-contrast cranial MRI with attention to the brainstem
• Localizing subtypes: See-saw nystagmus localizes to midbrain or parasellar region; downbeat and periodic alternating nystagmus to the cervicomedullary junction; convergence retraction nystagmus to the dorsal midbrain; spasmus nutans in children may be associated with optic pathway glioma

Proptosis

For proptosis (abnormal anterior protrusion of the globe):

• Most common cause in adults: Thyroid eye disease (TED) -- CT orbit without contrast is recommended (iodinated contrast may potentiate thyrotoxicosis; CT is superior for bone evaluation before and after orbital decompression)
• Acute onset proptosis: CT orbit is recommended for initial evaluation (orbital cellulitis, orbital abscess, orbital inflammatory disease, retrobulbar hemorrhage)
• Chronic lesions: Orbital fat-suppressed post-contrast MRI for more detailed soft tissue characterization
• In children: Unilateral proptosis is commonly due to orbital cellulitis; bilateral proptosis raises concern for neuroblastoma or leukemia

Horner Syndrome

For Horner syndrome (ipsilateral ptosis and miosis with anisocoria greater in the dark):

• Preferred study: MRI of the head and neck to the second thoracic vertebra (T2) with and without gadolinium, plus MRA of the neck
• Acute painful Horner syndrome: T1-weighted MRI of the neck with contrast and fat suppression to look for the "crescent sign" of internal carotid artery dissection -- a crescent of hyperintense signal on T1 surrounding the hypointense normal carotid artery flow void
• Postganglionic Horner syndrome: MRI of the head and neck to the level of the superior cervical ganglion (C4) with MRA. Isolated postganglionic lesions are often benign

Papilledema

For papilledema (optic disc swelling from increased intracranial pressure):

• Preferred study: MRI of the brain and orbits with and without contrast, fat suppression of the orbit, and MRV of the head
• MRI findings: May reveal meningeal disease, hydrocephalus, space-occupying lesions, or signs of idiopathic intracranial hypertension (empty sella, fluid in optic nerve sheath, posterior globe indentation)
• MRV: May show sinus stenosis or venous sinus thrombosis
• Acute/emergent setting: CT may be useful as the initial study

Summary of Imaging Recommendations

Clinical Indication	Preferred Imaging	Contrast	Key Notes
Optic neuropathy	MRI head and orbit	Yes	Fat suppression for intraorbital lesions
Optic neuritis	MRI head and orbit	Yes	FLAIR for demyelinating lesions; prognostic for MS
Bitemporal hemianopsia	MRI sella	Yes	CT for pituitary apoplexy or calcification
Homonymous hemianopsia	MRI head	Yes	DWI/ADC for stroke vs. PRES
Cranial nerve palsy (III, IV, VI)	MRI head, skull base	Yes	CTA for acute third nerve palsy with pupil involvement
Nystagmus	MRI brainstem	Yes	Localize nystagmus subtype
Horner syndrome (preganglionic)	MRI head/neck to T2 + MRA	Yes	Look for carotid dissection crescent sign
Horner syndrome (postganglionic)	MRI head/neck to C4 + MRA	Yes	Often benign; rule out dissection
Thyroid eye disease	CT orbit	Avoid iodine	Superior for bone anatomy and decompression planning
Orbital cellulitis	CT orbit and sinuses	Depends	Add MRI/CTA if cavernous sinus thrombosis suspected
Orbital tumor	CT or MRI orbit	Yes	MRI superior for intracranial extension
Papilledema	MRI brain/orbits + MRV	Yes	Rule out venous sinus thrombosis

Clinical Significance

This review provides a practical framework for ophthalmologists ordering neuroimaging studies. Key practical takeaways include:

1. MRI is the workhorse for most neuro-ophthalmic indications, but always specify the region of interest, sequences needed, and whether contrast is required
2. Fat suppression is not optional for orbital MRI -- without it, intraorbital pathology can be completely obscured by normal orbital fat signal
3. CT retains critical roles in emergencies (acute hemorrhage, trauma), bone pathology, sinus disease, and thyroid eye disease
4. DWI/ADC sequences add prognostic value -- they can be ordered for any patient with suspected stroke-related visual loss to distinguish reversible from irreversible injury
5. Clinical correlation is essential -- always provide the radiologist with the differential diagnosis, suspected lesion location, and relevant clinical findings

Citation

Kim JD, Hashemi N, Gelman R, Lee AG. Neuroimaging in ophthalmology. Saudi J Ophthalmol. 2012;26(4):401-407.

References