Saccadic function in spasmodic torticollis

Impaired Inhibitory Control of Saccadic Eye Movements in Cervical Dystonia: An Eye-Tracking Study

BACKGROUND

The pathophysiology of cervical dystonia is still unclear. Recent evidence points toward a network disorder affecting several brain areas. The objective of this study was to assess the saccadic inhibition as a marker of corticostriatal function in cervical dystonia.

METHODS

We recruited 31 cervical dystonia patients and 17 matched healthy controls. Subjects performed an overlap prosaccade, an antisaccade, and a countermanding task on an eye tracker to assess automatic visual response and response inhibition.

RESULTS

Cervical dystonia patients made more premature saccades (P = 0.041) in the overlap prosaccade task and more directional errors in the antisaccade task (P = 0.011) and had a higher rate of failed inhibition in the countermanding task (P = 0.001).

CONCLUSIONS

The results suggest altered saccadic inhibition in cervical dystonia, possibly as a consequence of dysfunctional corticostriatal networks. Further studies are warranted to confirm whether these abnormalities are affected by the available therapies and whether this type of impairment is found in other focal dystonias. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Oculomotor functions in focal dystonias: A systematic review

Focal Dystonia (FD) is a chronic neurological disorder, which causes twisting and repetitive movements and abnormal postures induced by involuntary sustained contractions of agonist and antagonist muscles. Based on the hypothesis that several dystonia-related brain regions, including cerebellum, are implicated in oculomotor disturbances (OCD), a number of studies investigated oculomotor function in patients with dystonia. However, conceptual clarity with respect to the used assessment tools and interpretation of the findings is lacking in the literature. This is the first article to systematically review studies that assessed oculomotor function in patients with FD. In total, 329 publications, published until September 1, 2019, were identified through MEDLINE search. Twenty out of 329 studies, involving 232 subjects in total, met the inclusion criteria. Most of the studies reported oculomotor disturbances in patients with FD. Abnormalities included asymmetry in vestibulo-ocular reflex (VOR), disturbances in saccadic functions, and prolonged latencies of eye motion. Discrepancies in the results could be explained, at least partially, by the long period of time over which the reviewed studies were published, the different methods used for testing the eye movements, and the limited number of patients assessed since the majority of data derived from case reports or small-scale studies. Further prospective studies with larger subject numbers are needed, using advanced tools for the assessment of oculomotor function in focal dystonia.

Visual compensation in cervical dystonia

Introduction: Visual-spatial processing can be affected in people with cervical dystonia (CD). These impairments have almost exclusively been observed in laboratory studies, but the impact of visuo-spatial impairments on daily activities is unclear. Here, we investigated how people living with CD visually explore the environment. Method: 21 participants (10 patients and 11 controls) walked a designated course searching for targets while their eye movements were tracked by the means of eye tracking glasses. In addition, all participants performed an extensive battery of spatial neglect tests. Results: People with CD visually compensated for their lateral head position. That is, they made more eye movements towards the side opposite their lateral head position. No evidence for or against spatial asymmetries were detected in a range of behavioral measures (target detection, standard neglect battery tests) on a group level. Yet, single patients showed a neglect-like pattern. One patient with a right torticollis failed to detect most left-sided targets when walking. Conclusion: In general, participants seem to adequately compensate for the deviated head posture when judging spatial stimuli. However, the insufficient spatial exploration in one patient with a valid driving license should raise awareness in clinicians to potential visuo-spatial problems in people living with CD. An interesting question arising from our findings is whether people with CD and marked visuo-spatial deficits would benefit from scanning training, as is employed in stroke rehabilitation.

Reduced Frequency of Ipsilateral Express Saccades in Cervical Dystonia: Probing the Nigro-Tectal Pathway

Background

Cervical dystonia is a hyperkinetic movement disorder of unknown cause. Symptoms of cervical dystonia have been induced in animals in which the integrity of the nigro-tectal pathway is disrupted, resulting in reduced inhibition of the deep layers of the superior colliculus. This same pathway is believed to play a critical role in saccade generation, particularly visually guided, express saccades. It was hypothesized that individuals with cervical dystonia would present with a higher frequency of express saccades and more directional errors.

Methods

Eight individuals with cervical dystonia and 11 age- and sex-matched control participants performed three saccadic paradigms: pro-saccade, gap, and anti-saccade (120 trials per task). Eye movements were recorded using electro-oculography.

Results

Mean saccadic reaction times were slower in the cervical dystonia group (only statistically significant in the anti-saccade task, F(1, 35)  =  4.76, p  =  0.036); participants with cervical dystonia produced fewer directional errors (mean 14% vs. 22%) in the anti-saccade task; and had similar frequencies of express saccades in the gap task relative to our control population (chi-square  =  1.13, p  =  0.287). All cervical dystonia participants had lower frequencies of express saccades ipsilateral to their dystonic side (the side to which their head turns), (chi-square  =  3.57, p  =  0.059).

Discussion

The finding of slower saccadic reaction times in cervical dystonia does not support the concept of reduced inhibition in the nigro-tectal pathway. Further research is required to confirm the observed relationship between the lateralization of lower frequencies of express saccades and direction of head rotation in cervical dystonia.

Keeping your head on target

The mechanisms by which the human brain controls eye movements are reasonably well understood, but those for the head less so. Here, we show that the mechanisms for keeping the head aimed at a stationary target follow strategies similar to those for holding the eyes steady on stationary targets. Specifically, we applied the neural integrator hypothesis that originally was developed for holding the eyes still in eccentric gaze positions to describe how the head is held still when turned toward an eccentric target. We found that normal humans make head movements consistent with the neural integrator hypothesis, except that additional sensory feedback is needed, from proprioceptors in the neck, to keep the head on target. We also show that the complicated patterns of head movements in patients with cervical dystonia can be predicted by deficits in a neural integrator for head motor control. These results support ideas originally developed from animal studies that suggest fundamental similarities between oculomotor and cephalomotor control, as well as a conceptual framework for cervical dystonia that departs considerably from current clinical views.

Trunk bradykinesia and foveation delays during whole-body turns in spasmodic torticollis

We have investigated how the abnormal head posture and motility in spasmodic torticollis interferes with ecological movements such as combined eye-to-foot whole-body reorientations to visual targets. Eight mildly affected patients and 10 controls voluntarily rotated eyes and body in response to illuminated targets of eccentricities up to ±180°. The experimental protocol allowed separate evaluation of the effects of target location, visibility and predictability on movement parameters. Patients’ latencies of eye, head, trunk and foot motion were prolonged but showed a normal modification pattern when target location was predictable. Peak head-on-trunk displacement and velocity were reduced both ipsi- and contralaterally with respect to the direction of torticollis. Surprisingly, peak trunk velocity was also reduced, even more than in previously studied patients with Parkinson’s disease. As a consequence, patients made short, hypometric gaze saccades and only exceptionally foveated initially nonvisible targets with a single large gaze shift (4 % of predictable trials as opposed to 30 % in controls). Foveation of distant targets was massively delayed by more than half a second on average. Spontaneous dystonic head movements did not interfere with the execution of voluntary gaze shifts. The results show that neck dystonia does not arise from gaze (head-eye) motor centres but the eye-to-foot turning synergy is seriously compromised. For the first time we identify significant ‘secondary’ complications of torticollis such as trunk bradykinesia and foveation delays, likely to cause additional disability in patients. Eye movements per se are intact and compensate for the reduced head/trunk performance in an adaptive manner.

Oculomotor function in multiple system atrophy: clinical and laboratory features in 30 patients

We reviewed the clinical and laboratory oculomotor features in 30 patients with probable multiple system atrophy (MSA), 22 with MSA-P and 8 with MSA-C. Six patients were also examined post mortem, MSA being confirmed in four and excluded in two (Parkinson's disease and progressive supranuclear palsy). Clinical examination showed the following abnormalities; excessive square wave jerks--21 of 30 patients; mild vertical supranuclear gaze palsy--8 of 30; gaze-evoked nystagmus--12 of 30 patients, three of whom had no extraocular evidence of cerebellar dysfunction; positioning downbeat nystagmus--10 of 25; mild or moderate saccadic hypometria--22 of 30; impaired ("broken up") smooth pursuit--28 of 30; reduced VOR suppression--16 of 24. Electro-oculography and caloric testing did not add significant extra information. In patients presenting with an akinetic-rigid syndrome it can be difficult to differentiate idiopathic Parkinson's disease from MSA-P and other causes of atypical parkinsonism. Our findings suggest that the presence of excessive square wave jerks, mild-moderate hypometria of saccades, impaired VOR suppression, spontaneous nystagmus or positioning downbeat nystagmus may be oculomotor "red flags" or clues to the presence of MSA. Further, the presence of clinically slow saccades, or moderate-to-severe gaze restriction, suggests a diagnosis other than MSA.

Positional down beating nystagmus in 50 patients: cerebellar disorders and possible anterior semicircular canalithiasis

OBJECTIVES

To clarify the clinical significance of positional down beat nystagmus (pDBN).

METHODS

A discussion of the neuro-otological findings in 50 consecutive patients with pDBN.

RESULTS

In 38 patients there was evidence of CNS disease (central group) but in 12 there was not (idiopathic group). In the CNS group, presenting symptoms were gait, speech, and autonomic dysfunction whereas in the idiopathic group patients mostly reported positional vertigo. The main neurological and oculomotor signs in the CNS group were explained by cerebellar dysfunction, including 13 patients with multiple system atrophy. In patients with multiple system atrophy with a prominent extrapyramidal component, the presence of pDBN was helpful in the differential diagnosis of atypical parkinsonism. No patient with pDBN had the Arnold-Chiari malformation, a common cause of constant down beat nystagmus (DBN). In the idiopathic group, the pDBN had characteristics which suggested a peripheral labyrinthine disorder: vertigo, adaptation, and habituation. In six patients an additional torsional component was found (concurrently with the pDBN in three). Features unusual for peripheral disorder were: bilateral positive Dix-Hallpike manoeuvre in nine of 12 patients and selective provocation by the straight head-hanging manoeuvre in two.

CONCLUSION

It is argued that some patients with idiopathic pDBN have benign paroxysmal positional vertigo (BPPV) with lithiasis of the anterior canal. The torsional component may be weak, because of the predominantly sagittal orientation of the anterior canal, and may not be readily seen clinically. Nystagmus provocation by bilateral Dix-Hallpike and straight head-hanging may be explained by the vertical upwards orientation of the ampullary segment of the anterior canal in the normal upright head position. Such orientation makes right-left specificity with the Dix-Hallpike manoeuvre less important than for posterior canal BPPV. This orientation requires a further downwards movement of the head, often achieved with the straight head-hanging position, to provoke migration of the canaliths. The straight head-hanging manoeuvre should be carried out in all patients with a history of positional vertigo and a negative Dix-Hallpike manoeuvre.

Adaptive changes of saccadic eye-head coordination resulting from altered head posture in torticollis spasmodicus

We asked whether and how the abnormal head posture in torticollis patients affects saccadic gaze shifts and impairs the associated head movements. We wanted to learn to what extent observed changes directly result from the disease or reflect compensatory mechanisms, secondary to the altered head posture. We compared the results of patients with those of normal subjects. When patients viewed a centric target, their heads were a priori deviated in the direction of the torticollis, with orbital eye position showing a compensatory offset in the opposite direction. These abnormal eye and head positions were re-established when patients returned from an eccentric gaze position by means of a centripetal gaze shift, independently of its direction and magnitude, unlike in normal subjects who always recentred eyes and head. In normal subjects the share of the head in the total gaze shift amounted to about 70%, whereas in patients it contributed only 30%, necessitating correspondingly larger orbital eye displacements and eccentricities. Moreover, patients' head movements were asymmetric; they were larger when gaze was shifted into, or returned from the hemifield contralateral to the torticollis direction compared with gaze shifts in the ipsilateral hemifield. The eyes displayed a reversed asymmetry. Patients showed a significant increase in gaze latency and head versus eye delay as well as in the number of corrective saccades. However, head velocity was normal in four out of seven patients. Moreover, all patients made normal eye saccades (peak velocity, duration, gaze error), except for the increase in latency, which also occurred when gaze was shifted without head movements. Thus, patients' saccadic eye-head coordination showed abnormalities which mainly concerned the involved head movements. We suggest that the observed changes do not reflect a direct involvement of the disease upon the gaze shift mechanism, but can be interpreted as adaptive changes that compensate for the altered head posture. We formalized this view in the form of a dynamic model.

Microvascular decompression for spasmodic torticollis

Twenty patients with spasmodic torticollis (ST) were treated by microvascular decompression (MVD) of the spinal accessory nerves, the upper cervical nerve roots and the brainstem. Thirteen were female and seven male. Median age was 47 years (range 39 to 70 years). Median duration of symptoms was 5 years (range 4 months to 17 years). Ten had right horizontal; nine, left horizontal; and one, retrocollis ST. Twenty-two operations were performed on twenty patients, suboccipital craniectomy and C1 laminectomy in 18 and retromastoid craniectomy in 4 operations. The most common compressing blood vessels were the vertebral artery and/or the posterior inferior cerebellar artery. No nerve section was performed. Three patients died of unrelated conditions, 3, 5 1/2, and 6 years postoperatively, respectively. Minimum follow-up period in the rest of the cases is 5 years (range 5 to 10 years). Thirteen (65%) were cured, four (20%) improved with minimal spasm, one (5%) improved with moderate spasm, and two (10%) improved minimally or unchanged. In most cases the cure or improvement was noticed gradually over 6 months to two years following the operation. There was no operative mortality. Postoperative morbidities included transient cerebrospinal fluid leakage through the surgical incision in one case and an apparent multiple small vessel stroke involving periventricular white matter in one reoperation case with full recovery. MVD for ST is a nondestructive benign procedure with high probability of cure or significant improvement.

Cervico-ocular function in patients with spasmodic torticollis

The cervico-ocular (COR) and active and passive vestibulo-ocular reflexes (VOR) were measured in seven patients with spasmodic torticollis (ST) and six normal controls. The COR was found to be weak or absent in both groups. The VOR gain was similar in the two groups but five patients had a significant asymmetry of the response. There was no evidence of abnormal cervico-vestibular interaction during active head rotation. The study suggests that the VOR asymmetry frequently found in ST cannot be explained on the basis of an abnormal cervical input.