Our patient presented with SSN accompanying INO. Moreover, the infarction of pontine corresponded to the anatomic location of the lateral MLF, which was rare to see; that is the unusual point in this case. Most importantly, our case illustrates that the acute onset of this constellation of signs is highly suggestive of pontine infarction.
SSN can be seen in disorders of visual pathways, especially the optic chiasm. SSN has been reported in the literature with parasellar lesions such as pituitary macroadenoma [
2], brainstem infarcts, multiple sclerosis (MS), and congenital absence of the optic chiasma. The interstitial nucleus of Cajal (INC) is located in the anterolateral superior colliculus and regulates the rotation of the eyeball. The pathophysiology of SSN is considered related to the disturbance of the graviceptive pathway between the vestibular nucleus and INC. The INO is characterized by adduction paresis of the ipsilesional eye and dissociated abducting nystagmus of the contralesional eye on attempted gaze to the contralesional side, a complex ocular motility disorder caused by damage to the MLF. SSN is rarely associated with INO. Our patient, who had many cerebrovascular risk factors, had SSN and INO as sole manifestations of infarction in the bilateral dorsomedial pons; hence, an acute ischemic stroke affecting the pontine junction was strongly suspected at the beginning. Subsequent magnetic resonance examination results confirmed this assumption. Trial of ORG 10172 in Acute Stroke Treatment (TOAST) classification of this case is considered to be large-artery atherosclerosis. The region of the pons in our report contained the MLF that extended through the brainstem and lay near the midline just ventral to the fourth ventricle in the pons [
3]. The neural integrators (NI) are localized in the brainstem, adjacent to the extraocular motor neuron nuclei used to stabilize eye position in eccentric position for comfortable single vision. Hence, general dysfunction of NI will lead to symptoms such as blurred vision, vestibular imbalance, and vertigo [
4]. INC is a small collection of neurons located adjacent to MLF, which function as NI for vertical and torsional eye movements. Visual, vestibular, and ocular motor interaction also occurs in the INC. It is a significant unit of the “eye movement neural integrator” which combines ocular velocity signals and encodes them into position commands [
5]. Kim and Lee [
6] reported the case of a patient with bilateral INO and vertical gaze-evoked nystagmus (GEM) as sole manifestations of paramedian pontine infarction. They suggested that vertical gaze-holding failure due to the involvement of the MLF and/or paramedian tract (PMT) neurons might be related to vertical GEM. Choi
et al.’s [
7] findings demonstrated that INO can be accompanied by various oculomotor abnormalities including SSN. However, the previous case had unilateral MLF whereas our present case had bilateral MLF that is rare to see. Some researchers indicated that SSN was ascribed to unilateral inactivation of INC (the torsional eye-velocity integrator), with sparing of the torsional fast-phase generator, the rostral interstitial nucleus of MLF [
8]. Bilateral dorsomedial infarcts of the pons are uncommon and the neuro-ophthalmic presentation can be quite variable. The mechanisms are still unclear. We suspect that MLF lesions may lead to different types of dissociated torsional–vertical nystagmus, depending on the patterns involving the pathways from contralateral vertical semicircular canals and probably from the otoliths of the contralateral labyrinth. The association of the SSN and INO is rare; whenever present, the etiology was demyelinating or neoplastic. However, the constellation of acute onset of SSN and INO in a patient with the risk factors for cerebrovascular diseases highlighted the likelihood of acute brainstem stroke.