Assessing structure and function of the afferent visual pathway in multiple sclerosis and associated optic neuritis

Abstract

The afferent visual pathway is commonly affected in MS. Assessment of the afferent visual pathway using clinical, imaging and electrophysiological methods not only provides insights into the pathophysiology of MS, but also provides a method of investigating potential therapeutic measures in MS. This review summarises the various assessment methods, in particular imaging techniques of the visual pathway. Retinal nerve fibre layer (RNFL) thickness is usually reduced following an episode of optic neuritis. Techniques such as optical coherence tomography, scanning laser polarimetry, and confocal scanning laser ophthalmoscopy are used to quantify RNFL thickness. MRI of the optic nerve is not routinely used in the diagnosis of MS or optic neuritis, but is valuable in atypical cases and in research. T2- weighted images of the optic nerve usually show the hyperintense lesion in optic neuritis and gadolinium enhancement is seen in the acute attack. Quantifying atrophy of the optic nerve using MRI gives an indication of the degree of axonal loss. Magnetization transfer ratio (MTR) of the optic nerve provides an indication of myelination. Diffusion tensor imaging (DTI) of the optic nerve and optic radiation provide information about the integrity of the visual white matter tracts. Functional MRI following visual stimulation is used to assess the contribution of cortical reorganisation to functional recovery following optic neuritis. Investigations including logMAR visual acuity, Sloan contrast acuity, Farnsworth- Munsell 100-hue colour vision tests and Humphrey perimetry provide detailed quantitative information on different aspects of visual function. Visual evoked potentials identify conduction block or delay reflecting demyelination. These collective investigative methods have advanced knowledge of pathophysiological mechanisms in MS and optic neuritis. Relevant ongoing studies and future directions are discussed.

The investigation of acute optic neuritis: a review and proposed protocol

“…More-extensive optic nerve lesions are associated with poor clinical outcomes such as slow and incomplete recovery. 81,82 Depending on the severity of inflammation, contrast enhancement can be observed on (fat-suppressed) T1-weighted images in about 94% of patients with acute optic neuritis. In established optic atrophy from previous damage, T2 high signal changes will also be seen, but these changes are associated with thinning of the nerve and do not change over time.…”

“…For assessment of the optic nerve, the MRI protocol must include fat-suppressed T2-weighted images (for example, short tau inversion recovery or frequency-specific selective partial inversion recovery) 81 and contrast-enhanced (fatsuppressed) T1-weighted images (Box 3). The most cost-effective approach is to use a standard dosage of gadoliniumbased contrast medium.…”

Transorbital Sonography in Acute Optic Neuritis: A Case-Control Study

“…4,5 Some TOS studies have shown that the optic nerve may be enlarged in acute ON, [6][7][8][9][10][11] though the validity of this technique for the diagnosis of ON has not yet been recog-nized. 12 No sonographic data are available to distinguish enlargement of the nerve itself from that of its sheath, though an enlargement of the sheath in acute ON has been described with MR imaging. 13 Previous studies did demonstrate good accuracy and reliability of sonographic quantification of optic nerve sheath diameter (ONSD).…”

Visual pathway abnormalities were found in most multiple sclerosis patients despite history of previous optic neuritis

“…Conventional MRI has insufficient sensitivity and specificity to reveal the true extent of pathological changes occurring in MS, as normal appearing white matter may harbor different abnormalities, as seen in non-conventional MRI and pathological works 25 .…”