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The symptoms of paramedian thalamic and midbrain infarct include ocular motor disturbances mainly in the vertical plane.1 We here describe a patient with the additional feature of an unusual horizontal eye movement synkinesis.
A 60 year old overweight man, with diabetes and mild hypertension, suddenly fell into a coma that lasted for 4 hours and was followed by slight right hemiparesis, recent memory impairment, hypersomnia, and vertical gaze impairment.
On admission to our centre, about 10 days after symptom onset, the patient still presented fluctuating drowsiness from which he could be easily aroused, normal cognitive functions with mild attention disturbance, slight right facial weakness, and mild incoordination at the finger-to-nose test with his right arm.
The most important findings involved ocular motor function. Both pupils were normal in diameter and reacted normally both to light and to convergence. The cover test did not disclose any eye misalignment. During attempted fixation, the patient showed saccade oscillations (usually square and macrosquare wave jerks)—that is, back to back involuntary horizontal saccades with an amplitude ranging from about 2 to about 10 degrees and with an intersaccadic interval of about 200 ms, that brought the eyes away from and back to the fixation point, at an approximate rate of three every 2 seconds. Clinical examination of eye movements in the horizontal plane and visually guided reflexive saccades recorded by the infrared reflection technique were both normal, whereas the amplitude range of vertical saccade and smooth pursuit eye movements covered only a few degrees of upward gaze. Vertical amplitude range was slightly greater for the vestibulo-ocular reflex in the pitch (yes-yes) plane. Moreover, when the patient attempted to make a downward saccade, he showed a gaze deviation to the left (figure). This synkinesis was more evident when the examiner lifted the patient’s lids, thus preventing lid synkinesis during downgaze. Attempted upward saccades did not produce any horizontal gaze deviation. Finally, the patient showed normal Bell’s phenomenon.
An EEG showed frontal, bilateral theta and theta/delta activity and sporadic drowsiness, and MRI (figure) disclosed a bilateral thalamomesencephalic infarct which involved predominantly the right side.
Horizontal gaze deviation on attempted downward saccades disappeared after about 15 days, whereas vertical gaze impairment and hypersomnia were unchanged 1 year later. Subsequent polisomnographic testing disclosed sleep apnoea.
The clinical features of our patient are those reported for thalamic infarct involving the rostrointerstitial nucleus of the medial longitudinal fasciculus (riMLF). By contrast, the leftward gaze deviation elicited by the attempt to make a downward saccade is at variance with all previous descriptions.
The triggering of a saccade requires not only the activation of the exitatory burst neurons (EBNs), but also the deactivation of the omnipause neurons (OPNs), which provide tonic inhibition of both horizontal and vertical EBNs.
Accordingly, any attempt to activate a lesioned riMLF should be associated with maximal OPN inhibition. However, OPNs discharge for saccade in any direction and are not strictly direction selective,2 as shown by horizontal oscillations during vertical saccades detectable in normal subjects.3 These oscillations suggest that during vertical saccades the inhibition of OPNs disinhibits both vertical and, to a lesser extent, horizontal EBNs.
In our patient, the horizontal gaze deviation was always directed to the left rather than in both directions as during oscillations. Many ocular motor structures, including those located in the midbrain,4 trigger a purely vertical (downward) saccade only when stimulated bilaterally, so as to nullify horizontal components with different direction depending on the stimulation side. This probably occurs for the riMLF too, as it shows ipsilateral projections to the abducens nucleus.5 In our patient, the projections to the left nucleus were probably spared by the fact that the lesion predominantly affected the right side.
Overall, our patient’s horizontal ocular motor synkinesis is unusual, and probably derives from a strong inhibition of OPNs, which in turn frees the horizontal EBNs, and from an unbalanced activation of the left abducens neurons via riMLF projections spared from the lesion, although it is not possible to exclude the possibility that the unbalanced activation of abducens neurons originated from frontal or parietal cortical areas or from the superior colliculus rather than from riMLF projections.
This hypothesis is strengthened by the reinforcement of the leftward eye deviation when the examiner kept the patient’s lids lifted. Since this manoeuvre prevents lid synkinesis, it results in what resembles an attempted forced lid closure which, on the basis of blink induced eye oscillations,6 is likely to be an additional stimulus for OPN inhibition. Moreover, although they occur in various conditions, saccade oscillations during fixation are in keeping with a reduction of OPN inhibition level.7
In conclusion, our patient presented an ocular motor synkinesis that should be listed among those occurring in thalamomesencephalic infarcts. This sign is unusual and it is likely to be overlooked, but it is fully explicable both by neurophysiology and by anatomical connections of the saccade system.