In our previous cross-sectional study, we evaluated the relationship between TT and MTP as a strength metric in DMD, DM1, and ALS patients. There was higher TT in the DMD group, lower MTP in the DM1 group, and a significant correlation between TT and MTP in the ALS group [1]. In another study, we also revealed the distinctive features of dysphagia in DM1 and DMD patients using VFSS, hyoid bone movement during the pharyngeal phase of swallowing (excursion), or pharyngeal residue measurement [5], which also demonstrated distinct abnormalities in swallowing muscle function between DM1 and DMD. The study also showed the correlation between TT and BW, but not MTP, in the DM1 group [1], suggesting that, in DM1 patients, TT was associated with malnutrition and concomitant weight loss. However, long-term tongue muscle weakness is also thought to promote malnutrition, and further loss of muscle mass may lead to additional TT and BW reductions [6]. Therefore, we assessed TT, MTP, and BW at multiple times during disease progression in this study.
Van Den Engel-Hoek et al. reported that TT can be assessed, conveniently and reproducibly, in DMD patients using ultrasound [3, 7]. More recently, they reported increased tongue hypertrophy and dystrophic changes in masticatory muscles but did not report changes in tongue muscles [8]. Although we found “tongue pseudohypertrophy” in DMD patients by ultrasound assessment, there were no significant longitudinal changes as in the ALS group. We suggest that, once tongue muscle tissue is replaced by connective tissue or fat in the early stage of DMD [9], there is no further progression in term of TT.
Poor tongue strength is independently associated with shorter survival time in ALS patients [10], so measuring TT and MTS is critical for prognosis and adjustment of treatment. Tamburrini et al. examined tongue movement function among ALS patients using both ultrasonography and VFSS, but they did not conduct objective tongue atrophy assessment [11]. Nakamori et al. suggested an association between TT, disease progression, and tongue dysfunction [12], consistent with our previous study showing a significant correlation between TT and MTP in ALS [1]. The mean TT value of ALS patients in their study (41.9 ± 4.0 mm) was similar to our final measurement (40.0 ± 6.5 mm), while the mean TT value of their control volunteers (44.8 ± 3.0 mm) was closer to the initial value in our ALS group (42.8 ± 6.8 mm), strongly suggesting tongue atrophy progressed during the roughly 2-year follow-up period. Nakamori et al. also found reduced TT over 15 months. In accordance with their study, the ALS group also included patients with no clear TT reduction (Fig. 3a), possibly due to differences in time from onset or rate of disease progression between bulbar or limb onset types. Alternatively, we found no association between TT and BW, in contrast to the linear association between TT and BMI reported by Nakamori et al. In our previous study, bulbar onset patients showed a stronger correlation between TT and MTP than limb onset patients, suggesting these measures especially important for monitoring bulbar paralysis patients [1], especially during the early clinical stages.
This study has several limitations, most notably the differences in mean age among groups, which is known to influence MTP [13]. However, age matching of DMD, ALS, and DM1 is difficult due to the differences in age at onset and course. Nonetheless, these results suggest distinct dysphagia pathomechanisms and progression features of among neurogenic and myogenic disorder patients.
This longitudinal study revealed more rapid loss of tongue thickness and strength in ALS patients than DMD and DM1. Only the ALS patients also exhibited substantial weight loss over the observation period, a change strongly associated with shorter survival [14]. On the other hand, DM1 and DMD patients may show only slowly developing tongue dysfunction in the short term, although DM1 may have lower tongue strength and DMD patients enlarged tongues, respectively, both of which can contribute to dysphagia. These findings suggest that regular re-evaluation of TT and MTP could provide valuable information on tongue dysfunction progression in NMD.