This observational cross-sectional study was conducted with the approval of the Ethics Committee of St.
James’s Hospital.
All patients gave written informed consent to the study.
We selected 30 patients with nystagmus secondary to MS,
who attended the neurology outpatient service.
All patients with an established diagnosis of MS who had nystagmus on clinical examination were considered to be eligible.
Exclusion criteria were as follows: any patient who was not voluntarily willing to undergo the scan or had other objections,
pregnant patients,
patients under the age of 18,
and patients with mental disabilities or learning disabilities.
Demographic information and information on type of MS and type of nystagmus were recorded.
All thirty patients were imaged using the following MRI protocol on a 3T MRI scanner (Achieva,
Philips Medical Systems,
The Netherlands) using an 8-channel phased array head coil as follows:
(1) Dual-Echo (Proton Density and T2-weighted) turbo spin echo sequence with TR/TE1/TE2 = 2000/10/80 ms,
turbo factor = 10,
spatial resolution = 0.45 x 0.45 x 4 mm3,
30 slices with 1 mm gap
(2) T2-weighted fluid attenuated inversion recovery (FLAIR) sequence with TR/TI/TE = 11000/2800/125 ms,
turbo factor = 31,
spatial resolution = 0.45 x 0.45 x 4 mm3,
30 slices with 1 mm gap
(3) T1-weighted 3D volumetric fast gradient echo sequence with TR/TE=25/1.8 ms,
flip angle = 30°,
SENSE parallel imaging factor = 2,
spatial resolution = 1 x 1 x 1 mm3
(4) Diffusion tensor imaging (DTI) sequence was performed on 22 patients with a 32-diffusion direction protocol and TR/TE = 12800/55 ms,
SENSE parallel imaging factor = 2,
SPIR fat suppression,
spatial resolution = 2 x 2 x 2 mm3 with no gap between slices,
and b = 1000 s/mm2.(8)
The scans were analyzed for visible white matter lesions by two independent blinded readers.
Those who had DTI analysis were analyzed for white matter tract integrity.
Diffusion weighted images were corrected for geometric distortions and subject motion prior to performing FA computation and tractography analysis.
Geometric distortions in the DTI images were removed before performing the tractography analysis through the use of a B0 fieldmap acquired immediately after the DTI data.
Regions of interest were placed in the frontal,
temporal and occipital eye fields; fibers were tracked down to the brainstem.
Deterministic streamline tracking was performed by following the principle eigenvector orientation in both directions from initial seed points similar to Conturo et al (1) with an FA threshold of 0.125,
step size of 0.4mm and an angular threshold of 180.
In areas where there was visual evidence of fiber tract breakdown,
FA values were assessed as a metric of diffusion anisotropy and consequently fiber integrity.
FA is a scalar value between zero and one that describes the degree of anisotropy of a diffusion process.
A value of zero indicates that diffusion is isotropic,
meaning it is unrestricted in all directions.
A value of one indicates that diffusion occurs only along one axis and is fully restricted along all other directions.
FA is a measure often used in diffusion imaging where it is thought to reflect axonal diameter and myelination in white matter.
FA is calculated from the eigen values (λ1,
λ2,
λ3) of the diffusion tensor.
Though the maximum FA value of 1 is clinically unattainable,
values as high as 0.7-0.8 are observed in white matter.
FA values falling below 0.2 along the tracts examined were considered significant for the purpose of our study.