Revisiting Parakinesia Brachialis Oscitans: You Yawn, I Move! A Scoping Review

Baseline Clinical Characteristics

Across the included studies, thirty-one individual cases of PBO were described (Table 1), and aggregated data are summarized in Table 2 (extended summary available in Supplementary Table 3). Most patients were men (67.7%, n = 21), with a mean age at presentation of 56.1 years (SD 13.6; range 20–76). Weakness without any other neurological deficit was reported in three cases (9.7%), while the remaining patients presented with associated neurological features, most frequently dysarthria (32.3%) and facial weakness (32.3%), followed by aphasia (19.4%), sensory abnormalities (9.7%), headache (9.7%), limb incoordination (6.5%), reduced level of consciousness (6.5%), vertigo (3.2%), visual field deficit (3.2%) and seizures (3.2%). Grading of motor strength using the MRC scale was explicitly reported in only six cases, with upper-limb grades ranging from 0 to 4.

Clinical Description and Duration of Parakinesia brachialis oscitans

PBO manifested in the upper limb alone in 29 cases (93.5%), while combined upper and lower limb involvement was observed in the remaining two cases. Limb involvement was left-sided in 19 cases (61.3%) and right-sided in 12 (38.7%), and was contralateral to the lesion in all 31 patients. PBO was present at the initial presentation in nine cases (29%) and within 1–7 days in an additional 11 cases (35.5%). Onset between more than one week and one month was reported in five cases (16.1%), and beyond one month in two cases (6.5%). Latency was not specified in four cases (12.9%). The longest reported latency was four months. One case reported botulinum toxin use for spasticity, followed by eventual resolution of PBO. Complete resolution was documented in ten patients (32.3%), occurring within one week in four cases and after more than one week in four, and at an unspecified interval in two. Improvement without resolution was reported in one patient (3.2%), and PBO persisted at last follow-up in five (16.1%). The longest reported persistence of PBO was three years. Outcome duration was not specified in 15 cases (48.4%).

Lesion Etiology and Neuroanatomical Localization

Anatomical involvement could be determined in 27 of the 31 cases; in the remaining four the report named only a vascular territory. Involvement was most frequent in the internal capsule (12 cases, 44.4%) and the corona radiata or subcortical white matter in 11 (40.7%), followed by the basal ganglia in eight (29.6%), the cortex in six (22.2%), the brainstem in four (14.8%) and the thalamus in one (3.7%). A vascular territory was named by the source authors in only 12 of the 26 patients with an ischemic lesion (46.2%), and was the middle cerebral artery in eleven and the posterior and middle cerebral arteries together in one. These frequencies describe lesion involvement rather than independent anatomical localization, because categories were not mutually exclusive and several cases had multifocal lesions. The predominant underlying etiology was acute ischemic stroke (83.9%). Less frequent causes were intracerebral hemorrhage (9.7%) and multiple sclerosis or other inflammatory etiologies (6.5%).

Neuroanatomical and Pathophysiological Mechanisms

PBO has been reported almost exclusively in the setting of acute or subacute focal brain injury, involving the descending motor pathways, such as in the internal capsule. In this sample, the lesions were predominantly supratentorial and subcortical with the internal capsule involved in 44.4% and the corona radiata and adjacent subcortical white matter in 40.7% of the 27 patients. This distribution is compatible with the hypothesis that interruption of corticospinal output, rather than a single-site injury, is the shared substrate, but a case-based literature cannot establish which structures are necessary or sufficient. Reports of PBO resolving as strength improves are consistent with a state-dependent phenomenon, although this cannot be concluded from the present data: PBO persisted for one to three years in 5 patients (16.1%), complete resolution was documented in only 10 (32.3%), and outcome was not reported at all in 15 (48.4%). Observed resolution proportions should not be interpreted as estimates of the natural history of PBO because follow-up was unavailable or incompletely reported in nearly half of cases and follow-up duration varied substantially.

Lim and Neo [9] have proposed at least two hypotheses to explain how yawning triggers this release phenomenon: the emotional motor system and the proprioceptive loop theories (Figure 2). Neither has been tested directly in patients with PBO. The emotional motor system theory highlights that yawning is a coordinated somatic expression of emotional disengagement, leading to co-activation of bulbar and motor neurons. These neurons allow involuntary, stretch-like limb movements via intact reticulospinal pathways when the corticospinal tract is injured [10, 11]. The proprioceptive loop theory proposes that yawning induces a contraction of the respiratory muscles, which sends afferent signals through the ventral spinocerebellar tract to the spinal cord, cerebellum, and the lateral reticular nucleus in the medulla. This triggers an efferent response that travels along extrapyramidal pathways back to the cervical spinal cord (C4–C8), which may produce involuntary upper-limb movement [1]. However, this explanation does not fully account for lower limb involvement reported in some cases, a limitation raised by de Lima et al [5]. Lastly, some studies indicate that yawning is associated with monoaminergic activation in the brainstem, which may facilitate residual motor responses in patients with corticospinal tract injuries [12, 13]. Lim and Neo proposed that dopaminergic agents may support recovery in patients with PBO in whom spinoreticular-cerebellar pathways are intact. However, this benefit has not been translated into post-stroke rehabilitation studies [9]. Moreover, it remains unclear why not all patients with focal brain injury and limb weakness develop PBO.

Figure 2

The common denominator in both theories is an imbalance of the descending control pathways. Töpper et al. described involuntary stretching during yawning as an automatic motor pattern normally suppressed by an intact corticospinal tract, reappearing when that tract is interrupted, characteristically as spasticity and pathological reflexes return [10]. This timing is not universal: in a prospective series of 75 patients with internal capsule stroke, yawning-associated limb movements were more frequent in those with hypotonia than hypertonia (87% versus 61%) [2]. Blin et al. reached a comparable conclusion pharmacologically: apomorphine-induced yawning reproduced the same stereotyped arm movement in their patient, which they attributed to release of basal ganglia projections onto brainstem motor centers, while a simulated yawn and passive head and neck movement did not [14]. Töpper et al. noted that an intact basal ganglia projection is nonetheless not a prerequisite, since yawning with associated limb stretching has been observed after bilateral midbrain infarction [10]. A comparable release of an automatic motor pattern has been demonstrated experimentally in chronic hemiparetic stroke, where head rotation modulates flexion synergy torques in the paretic limb, reproducing the asymmetric tonic neck reflex and implicating increased reliance on ipsilateral reticulospinal pathways once corticospinal drive is lost [15]. Consistent with a proximally weighted brainstem drive, yawning-associated movements in hemiplegic limbs occur far more often at proximal than distal joints [2]. PBO is compatible with an altered interaction between impaired corticospinal control and relatively preserved brainstem and extrapyramidal pathways, although the specific circuit remains unidentified.

Clinical Implications of Parakinesia Brachialis Oscitans

PBO is brief and occurs only with a yawn, so whether it is detected depends largely on someone being present when the patient yawns; it is likely documented most often in stroke units and inpatient rehabilitation wards. The small number of published cases therefore does not accurately estimate the prevalence of the phenomenon. Although not specifically in PBO, a prospective observational study found yawning-associated movements of the hemiplegic limbs in 78.6% of the 75 patients with internal capsule stroke, a figure close to that of an earlier survey of 40 hemiplegic patients [2, 7]. Recognition of the stereotyped, yawn-locked pattern and return of the limb to rest is important for an accurate diagnosis and differentiation from other involuntary movements of a paretic limb (Supplementary Table 4) or yawning-related movement disorders (Supplementary Table 5) [16]. Importantly, the phenomenology reported under the term PBO is itself broader than the classic description: de Lima et al. reported yawning-induced upper limb movement in one patient in whom only the lower limb was paretic and in another in whom no hemiparesis was present, describing both as variants [5].

Appropriate counseling for patients and caregivers is also important to avoid leading to false expectations of functional improvement, which has not been established. Although initially noted that PBO does tend to diminish as motor recovery occurs [1], associated movements in the hemiplegic arm were subsequently judged to have no prognostic value [2]. A recent retrospective study has now examined whether PBO carries prognostic information: among patients with acute ischemic stroke matched on baseline severity, those with PBO had higher motor recovery scores at three months than those without, and PBO resolving within one month was associated with higher scores than persistent PBO [17]. Although promising, PBO cannot yet be used as a prognostic marker until prospective data are available.

Strengths, Limitations, and Future Directions

This scoping review represents, to our knowledge, the most comprehensive synthesis of published PBO cases to date. We provided a structured overview of the characteristic presentation, lesion localization, timing, and reported course of this phenomenon. Its limitations arise from the evidence base and from the review process. The evidence base is small and unevenly reported: motor severity, imaging detail and follow-up were documented inconsistently, and lesion site was frequently given only as a vascular territory, which restricted anatomical coding to 27 of the 31 patients. Sex distribution, laterality, latency and outcome are reported as observed frequencies, without inference about sex or hemispheric predominance. Because case reports favor unusual or well-documented presentations, the published literature is unlikely to reflect either the true frequency of PBO or the range of its outcomes. Within the review process, inclusion was restricted to English-language reports, which excluded three reports and may have introduced a language bias to which a small evidence base is particularly vulnerable; no formal critical appraisal was undertaken, consistent with PRISMA-ScR guidance.

Future studies should emphasize prospective case follow-up, standardized reporting, and advanced neuroimaging approaches, such as diffusion and functional connectivity techniques, to better elucidate the neural substrates of yawning-associated motor release. While no treatments have been shown to modify PBO, rehabilitative strategies that reinforce voluntary motor control, such as mirror therapy, may warrant exploratory investigation.