Original ArticleIdentification of Stria Medullaris Fibers in the Massa Intermedia Using Diffusion Tensor Imaging
Introduction
The stria medullaris thalami (SM) is the major afferent pathway to the lateral habenula (LHb). Input to the LHb is varied with a strong contribution from limbic and frontal lobe regions, specifically the anterior cingulate region, anterior insula, and dorsal orbitofrontal cortex.1, 2 It has been postulated that this pathway forms part of a “negative emotion processing” circuit.3, 4 Output from this circuit modulates the ventral striatum mesolimbic dopaminergic pathway as well as the serotonergic and noradrenergic neurotransmission.5, 6 Overactivity of this pathway potentiates learned helplessness, depression, and anxiety.
Modulation of the SM–LHb pathway by deep brain stimulation (DBS) has been proposed as a possible therapy for treatment-resistant depression.5, 6, 7 To date, DBS of the LHb has been described in only 2 patients but with promising results.6, 7 Because of its size and location, direct targeting of the LHb, although feasible, requires meticulous planning to both effectively target the structure and avoid side effects.7 For this reason, broader targeting of the SM, a structure not visible on conventional magnetic resonance imaging (MRI), may be more suitable for DBS surgery.
We recently demonstrated the utility of probabilistic DTI in defining the SM reliably for identification and DBS-targeting purposes.8 Interestingly, during subsequent tractography analysis, presence of decussating SM tracts was noted within the massa intermedia (MI), otherwise known as the interthalamic adhesion, in some patients. Given that this finding has not been previously described within the literature, we sought to formally investigate the presence as well as decussation pattern of SM fibers crossing the midline via this structure.
Section snippets
Subjects
Subjects were 12 patients undergoing preoperative imaging for DBS surgery to treat Parkinson disease. These subjects were chosen because our institution's pre-DBS MRI protocol includes high-resolution T1 and diffusion-weighted sequences necessary for tractography analysis. A significant proportion of subjects required sedation during MRI acquisition, which minimized movement artifact and image distortion. Tractography analysis was performed on 10 subjects with an identifiable MI as visualized
Results
In all 12 subjects, the SM was reliably visualized with the aforementioned technique of seeding both the right and left LHb. Results are listed in Table 1. Tractography results for several subjects are shown in Figure 2, Figure 3, Figure 4. Roughly 25,000 tracts were sent out per LHb (5000 for each of about 5 seeded voxels), and the colorized voxels are those with at least 250 tracts passing through them.
The presence of SM fibers within the MI was seen in 7 of the 10 subjects with MI. In 3
Discussion
The MI or interthalamic adhesion is a band bridging the medial thalami and is absent in as many as 20%–30% of individuals based on neuroanatomic studies.18 Little is known of its significance, especially in regard to functional pathways, which presumably explains its definition as an adhesion instead of a formal commissure. Several studies have shown a greater rate of absence of MI and changes in size in patients with schizophrenia spectrum disorders, and a meta-analysis by Trzesniak et al.
Conclusions
We provide evidence of crossing SM fibers within the MI through the use of probabilistic DTI. Given its anatomic location as a bridging pathway between thalami, further studies are necessary to assess its functional role within the limbic network.
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.