Introduction

Mucopolysaccharidosis (MPS) is a group of lysosomal storage disorders characterized by deficient activity of enzymes that degrade glycosaminoglycans (GAGs). Although the available therapies for MPS help with most of the symptoms, it’s not the case for the spinal manifestation of the skeletal disease in these patients, since the discs and bones are not as well vascularized as other tissues¹. Using sodium (²³Na) MRI it is possible to quantitatively and non-invasively evaluate GAG content in cartilage², therefore this technique could serve as a possible method in the assessment of intervertebral disc status, in the spine of MPS patients, representing how much a patient’s skeletal system is affected.The aim of this study was to find if ²³Na-MRI is able to detect sodium signal difference in cervical intervertebral discs between MPS II patients their sex- and age-matched controls.

Material and Methods

After giving written and oral consent, three MPS II patients and their sex and age-matched controls (27+/-11) had been measured using 7T MRI (Magnetom, Siemens Healthineers, Erlangen, Germany) with a 6-channel ²³Na-tuned surface spine coil (QED, Mayfield Village, Ohio).
A 3D gradient echo sequence with variable echo time train (vTE-GRE) optimized for ²³Na signal was acquired with the following parameters: resolution 2.5⨯2.5⨯8.0mm³, 14 averages, TR/TE 130/0.72ms at the center of the k-space, pulse bandwidth 120Hz/Px, FA 90°, acquisition time of 21 min ³. For B₁ field correction in ²³Na-MRI, a homogeneous cylinder phantom (0.9% NaCl) measurements were performed.
²³Na images were corrected by normalizing using images from phantom measurement. Polygonal ROIs were manually drawn around the discs and their average value was normalized to CSF acquired from an ROI placed in the fourth ventricle. Image processing and ROI positioning in the discs, was done using an in-house script written in MATLAB (Mathworks Inc, Natick, MA). Shapiro-Wilk normality test was used to define the normality of the data and then a paired t-test was used to compare patients with matched controls. Analysis of covariance was used to assess the possible covariance of age and/or disc position. Statistical analysis was performed in RStudio (RStudio, Boston, MA).

Results

Examples of sagittal ²³Na-MRI map from a MPS II patient and control (subject pair P1/C1 in Fig. 2) are displayed on the Figure 1. The average disc values calculated from ROIs drawn on ²³Na-MRI maps from all the discs (C2/C3 – C5/C6) of all subjects, can be seen on the Figure 2, along with the average values and standard deviations for each subject. The C6/C7 disc data were excluded from the analysis due to possible bias caused by breathing artifacts.
Analysis of covariance didn’t show a significant effect of age, or the disc placement to the acquired and normalized ²³Na values from disc ROIs (p = 0.225 for disc number and 0.872 for age).The normalized disc ²³Na MRI values in MPS II patients were found to be significantly lower than in their matched controls (p < 0.01).
Boxplots representing data from all cervical discs in MPS II patients and their controls can be seen on Figure 3. Boxplot pooling all the discs from subjects and controls respectively, can be seen on Figure 4.

Discussion and Conclusion

It is well known that MPS II patients exhibit evidence of intervertebral disc degeneration. A decrease signal intensity was found on T₂-weighted MRI images⁴. Accelerated intervertebral disc degeneration was found as well in animal MPS models¹.Young MPS VII dogs exhibit enlarged nucleus pulposus and increased water and GAG content in anulus fibrosus, in older age they did show advanced degenerative changes in intervertebral discs, including complete disc height collapse¹. Additionally, ²³Na has been shown to be able to detect signs of intervertebral disc degeneration². Our results support both previous findings. We have found significantly lower ²³Na values in cervical discs of MPS II patients compared to healthy controls. This indicates that sodium MRI has a potential as a novel quantitative and non-invasive marker for disc degeneration in MPS II patients.
Typical treatment options for MPS patients are bone marrow transplantation and enzyme replacement therapy. One of the challenges is the lower efficacy of these techniques in affecting the spine, where tissues are mostly avascular and need to use diffusion to deliver the treatment into the cells¹. Based on our results, ²³Na MRI in cervical discs might serve as a biomarker for monitoring new emerging therapies.
The major limiting aspect of our study was the ²³Na surface coil design, causing signal loss from more distant tissues. This can be better observed in the Figure 1, where the noise visibly increases at a certain level away from the coil in both the patient and the control. This increase in noise will affect data especially in subjects with more curved spine or higher BMI. The solution would be to improve the coil design for future studies.
To conclude, ²³Na MRI represents a potential quantitative method for assessment of cervical disc degeneration in MPS II patients and could serve as a potential marker used in monitoring of emerging therapies.

Acknowledgements

We acknowledge financial support by the OeNB (grant #13418). AS was supported by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 794986.