Response to Geroin et al.: Tapping into the mechanisms of abnormal functional postural control

We welcome the work by Geroin et al. [1] demonstrating the use of the shoulder touch test— similar to the shoulder tap test, which we previously described [2]— in patients with functional movement disorders (FMDs). In the original description, the shoulder tap test was abnormal in all patients with isolated functional gait disorders (FGDs). In their larger cohort of patients with FMDs, however, Geroin et al. found the shoulder touch test to be abnormal in only 48% of patients, all of whom had an abnormal pull test prior to the touch test. We highlight some methodological differences between our study and that of Geroin and colleagues that may account for this, beyond the "lighter" tactile stimulus applied to the shoulders. First, unlike our previous sample of patients with isolated FGDs, Geroin et al. studied patients with a range of FMDs. Although 84% had a gait disorder phenotype, this was not necessarily the primary presenting feature. The lower frequency of abnormal shoulder touch responses in some patients suggests the abnormality may not generalize across all FMDs and may instead be a feature of postural hypervigilance (that accompanies a functional gait disorder). Second, it would be helpful to clarify the wording of instructions given prior to the shoulder touch test. In the original description of this sign, we stated, “on the count of 3, if you are moved backwards, take a step to correct yourself” [2]. The emphasis on "if" may influence the subsequent postural response, particularly in more susceptible individuals primed to generate abnormal anticipatory mechanisms [2], although this may prove difficult to standardize. Finally, whereas we performed the shoulder tap test before delivering a pull test, Geroin et al. tested the shoulder tap test after having carried out a pull test, and only when the pull test was abnormal. Like our instructions, this may have primed individuals who are already susceptible to postural hypervigilance. We have observed several patients where a shoulder tap test is abnormal in the presence of a normal pull test. This discrepancy can be of clinical utility to confirm the presence of postural hypervigilance, and a route into discussing how automatic motor responses (reactive postural adjustments during a pull test) are intact. That the shoulder tap and touch tests are normal in the parkinsonian populations studied thus far [1, 2] does not signify that these clinical results are specific to a functional gait disorder; we have clinically observed abnormal shoulder taps in people with parkinsonism who also manifest fear of falling. In such individuals, a positive pull test may be construed as an intrinsic postural instability, whereas the presence of a positive shoulder tap test would instead suggest that such imbalance is at least in part functional in origin. Functional neurological disorders are commonly recognised in Parkinson's disease [3]. In summary, we welcome the replication and extension of the use of the shoulder tap test, but further work may be necessary to (i) best delineate the underlying mechanisms of the abnormal responses and (ii) identify the optimum stimuli for a given patient population.