To maintain optimal visual acuity, we use voluntary saccades to move the eyes rapidly from one point of visual fixation to the next (1). Loss of voluntary saccades may occur with brainstem strokes, although other types of eye movements are usually affected. Cases of selective saccadic palsy as a complication of cardiac surgery have been previously described (2-4). However, the pathogenesis of the disorder is unclear, and few reports have clearly defined the nature of the ocular motor deficit. Here, we report a patient with selective saccadic palsy after a cardiac operation and characterize and quantify the defect of the ocular motor disorder.
A 43-year-old man with chest pain underwent repair of an aortic dissection. The procedure was complicated by massive bleeding. On awakening from anesthesia, the patient had difficulty shifting his gaze and complained of vertical diplopia in primary position, which worsened in downward gaze. He also had difficulty walking because of an inability to alter his direction of gaze voluntarily.
On neurologic examination, the patient was alert and fully oriented. He had no dysarthria or dysphagia. He showed mild dysmetria of upper extremities and truncal ataxia with a tendency to fall.
Abnormalities on neuro-ophthalmic examination were limited to his eye movements. The patient had a right hypertropia, worse on looking down and to his left. Three-dimensional recording of eye motion with oculography (SMI, Teltow, Germany) was performed (see Video, Supplemental Digital Content 1, http://links.lww.com/WNO/A8). Horizontal and vertical saccades were profoundly slowed and hypometric. Optokinetic responses were absent. However, smooth pursuit, vergence, horizontal and vertical VOR, and the ability to hold the eye in an eccentric position were preserved. Eye movements and the ability to hold steady gaze were evaluated during attempted fixation of visual targets located centrally or eccentrically (±30° horizontally and ±20° vertically). There was no spontaneous or gaze-evoked nystagmus. Horizontal saccades were generated by a target moving pseudorandomly on a light bar with a range of ±16.7°. The range of target amplitude was 5°-30°. The patient had slowed and hypometric voluntary horizontal saccades (Fig. 1). Vertical saccades were completely paralyzed. Quick phases of nystagmus (reflexive saccades) were also reduced or absent during optokinetic nystagmus. Smooth pursuit (Fig. 1) and vergence eye movement were preserved. The vestibulo-ocular reflex (VOR), evaluated during passive or active head rotations, was normal in the horizontal and vertical planes while optokinetic stimuli elicited no horizontal or vertical movements (see Video, Supplemental Digital Content 1, http://links.lww.com/WNO/A8).
Bithermal caloric tests showed bilateral weak responses (less than 4° per second) (Fig. 2). Sinusoidal harmonic accelerations (peak velocity: 50° per second; frequency range: 0.02-0.32 Hz) showed decreased gains of the VOR and visual-enhanced VOR.
No brainstem abnormalities were detected on MRI (Fig. 3). Somatosensory and brainstem auditory-evoked potentials and audiogram were normal.
During 2 years of follow-up, our patient experienced no improvement in either his saccadic palsy or his truncal ataxia with gait instability.
Our patient showed abnormalities of saccades and quick phases of nystagmus including slowing, hypometria, and limited range. In contrast, smooth pursuit, VOR, vergence, and the ability to hold the eye in an eccentric position were preserved. Review of several anatomic structures within the brainstem may help explain this dichotomy. Premotor burst neurons and omnipause neurons lie within the reticular formation of the brainstem. Excitatory burst neurons generating horizontal saccades are located in the paramedian pontine reticular formation (PPRF), at the level of the abducens nuclei, and extend rostrally (1,5). Experimental bilateral lesions in the PPRF selectively abolish horizontal saccades, leaving other eye movements intact (6). Excitatory burst neurons for vertical and torsional saccades and quick phases are located in the rostral interstitial nuclei of the median longitudinal fasciculus (riMLF) (1,7). Bilateral lesions of riMLF abolish vertical and torsional rapid eye movements (8). However, it would be difficult to conceive a process causing selective impairment of burst neurons for horizontal and vertical saccades located in the pons and midbrain.
The omnipause neurons (OPNs) are another important component of the brainstem saccade generator and lie close to the midline in the raphe interpositus nucleus (1,9). OPNs are glycinergic and project to burst neurons in both the pontomedullary reticular formation, which directly controls horizontal fast eye movements, and the riMLF, which contains vertical eye movement-related burst neurons. OPNs are tonically active and make inhibitory connections with burst neurons directly controlling motoneurons for vertical and horizontal eye movements but pause before saccades in any direction. Chemical lesions of OPNs cause saccades to become slow (10). Recently, it has been proposed that OPNs also have a neuromodulatory function, to increase the responsiveness of saccade-related neurons when they receive a trigger signal (11-13). Therefore, the effect of OPNs on burst neurons may be 2-fold: inhibition when no saccade is planned, but enhancement of glutaminergic mechanisms when a saccade is triggered. So, a possible explanation of selective saccadic palsy in our patient is that OPNs might be damaged, in which case, both horizontal and vertical saccades would be expected to be slow.
The small vertical deviation in our patient suggests that structures adjacent to the brainstem reticular formation concerned with saccade generation may also have been affected. Yet, the pathogenesis of our patient's mild dysmetria and gait instability remains unclear. These findings could be explained by cerebellar damage as would decreased saccadic acceleration and deceleration and saccadic hypometria (14). However, MRI failed to demonstrate any abnormalities of the cerebellum.
It is likely that ischemia to the brainstem reticular formation was responsible for saccadic palsy in our patient. Hypotension, intraoperative hypothermia, or microemboli are all potential contributing factors (2). Over a 2-year period, our patient showed no clinical improvement. This is consistent with other reports that patients with this ocular motor disorder may remain permanently visually disabled after cardiac surgery (2,3).
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