Reply to "Comment: Another Piece of the Puzzle of Anomalous Connectivity in Joubert's Syndrome"

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We thank Dr. Chang his interest and feedback on our report discussing the conventional and diffusion tensor imaging (DTI) findings of a 2-year-old child with the clinical presentation of Joubert's syndrome (JS) and matching brainstem imaging findings.[1]

We carefully read the comments and issues raised by Dr. Chang.[2]

We like to provide some additional context regarding the thoughtful points that were raised in the letter.

Dr. Chang mentioned in his letter that generic issues are frequently encountered with DTI and deterministic tractography analysis, including concerns about our analysis, which has some limitations, majorly, apparent “false” positive tracts in areas of crossing fibers and the false appearance of a disruption of neural tracts secondary to partial volume effects.

However, there is a disconnect between Dr. Chang's raised issues and the actual information presented in our manuscript. For example, our manuscript did not show midline crossing fibers connecting the bilateral middle cerebellar peduncles (MCPs) as suggested by Dr. Chang.

We would like to emphasize that it is important to note that our DTI data provide average water diffusion property within a pixel which is an indirect indicator of white matter tracts.[3] Hence, our interpretation of the tractography findings in this patient with JS is reasonable. However, we cannot definitively confirm that some of these anatomical tracts are missing/aberrant. First, as stated in our manuscript, the tractography results in our study were obtained from a routine clinical protocol. Second, while Dr. Chang raised concerns about our deterministic approach, there is no mentioning about the additional probabilistic approach that we used/applied.

As a matter of fact, we were able to complement our deterministic approach with added probabilistic tractography results which resulted in additional information as discussed in our manuscript.

As stated by Dr. Chang, partial volume effects and false positive connection are factors that need to be considered in any DTI study. However, these limitations cannot fully explain the absence of the corticospinal tract (CST) fibers in the anterior brainstem.

Our findings derived from both deterministic and probabilistic tractography suggest that the white matter bundle anterior to the interpeduncular fossa represent ectopic, midline crossing fibers with no direct connection to the bilateral MCPs but with a midline and posterior component, reaching/merging with the bilateral medial lemnisci (ML). Moreover, probabilistic tractography demonstrated a small component of the left CST fibers embedded within the ectopic anterior mesencephalic structure.

In addition, we discussed another plausible explanation where the anterior mesencephalic cap dysplasia represents the middle component of transverse pontine fibers. Therefore, the abnormal appearance of white matter tracts in the brainstem in our patient may reflect a combination of an ectopic decussation of the motor tracts (as suggested by Arrigoni et al)[4] with aberrant posterior course of the CST, abnormal transverse pontine fibers, or a combination of these.

We have also discussed other plausible explanations and talked about our limitations.

Finally, as discussed in our conclusion, future studies with a dedicated high-resolution multiangle DTI of the brain stem may shed more light on this complex and unique brainstem malformation. Finally, we thank the Editor-in-Chief of Neuropediatrics for the opportunity to write a reply to Dr. Chang's “Letter to the editor.”

Publication History

Article published online:
21 July 2022

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