The intracranial structures responsible for horizontal eye movement include the supranuclear centres, essentially the paramedian pontine reticular formation (PPRF) and the internuclear fibres of medial longitudinal fasciculus (MLF), as well as the abducens nucleus. PPRF, located rostral to the abducens nucleus, receives input from visual areas and sends excitatory neurons to the ipsilateral abducens nucleus. The abducens nucleus contains both motor neurons to ipsilateral lateral rectus muscle, and internuclear neurons which project to contralateral MLF to oculomotor subnucleus for medial rectus muscle [1]. The simultaneous contraction of ipsilateral lateral rectus and contralateral medial rectus results in horizontal conjugate gaze. The abducens nerve is at the facial colliculus, together with PPRF and MLF, all located within the pontine tegmentum [2].
Our patient was presented with abrupt onset of disconjugate horizontal gaze palsy, which fits into the classical clinical description of one-and-a-half syndrome. One-and-a-half syndrome is a combination of ipsilateral conjugate horizontal gaze palsy and ipsilateral internuclear ophthalmoplegia [3]. The monocular nystagmus is a ‘corrective’ jerk nystagmus towards the direction of MLF lesion. The absence of vestibulo-ocular reflex in both eyes is indicative of a lesion at the PPRF and the adjacent abducens nucleus due to disruption of signal from medial vestibular nucleus to the abducens nucleus. The presence of contralateral exotropia is a form of paralytic pontine exotropia due to ipsilateral PPRF and MLF lesions [4].
Furthermore, our patient had ipsilateral hemifacial palsy suggestive of lower motor neuron lesion of facial nerve. The lesion affecting the facial nerve is primarily located either in the motor nucleus in the facial colliculus, ventrolateral to the abducens nucleus, or its motor fascicle, which projects posteromedially and wraps around the abducens nucleus before exiting the pons laterally. The sparing of secretomotor and sensory branches of the facial nerve, and preservation of hearing are strongly suggestive of a lesion proximal to the cisternal segment, before where all three nerve roots join together and travel alongside with vestibulocochlear nerve in the cerebellopontine angle [5].
In this case, one-and-a-half syndrome with ipsilateral hemifacial palsy, give a clinical diagnosis of eight-and-a-half syndrome [6]. The diagnosis is further reinforced by the brain MRI findings, performed within 72 hours of onset. The MRI showed acute infarct over right pontine tegmentum, as evident by restricted pattern in DWI and ADC sequences and hyperintensity lesion in T2 sequence [6,7]. Therefore, the patient had a posterior circulation ASPECT score of 2.
In this case, the eight-and-a-half syndrome is most likely caused by acute ischemic lacunar infarct of the pons, in view of the abrupt onset and the underlying risk factors such as uncontrolled diabetes mellitus and essential hypertension. In fact, the most common cause for eight-and-a-half syndrome is vascular aetiology such as infarction, primarily caused by vertebrobasilar disease [6,8,9]. The sudden and abrupt nature of symptoms onset are often associated with infarct or ischemia. In our case, the region supplied by the lateral group of arteries were affected. There is no gender, race or age preference for the development of this syndrome. On the other hand, modifiable factors such as hypertension, diabetes mellitus and hypercholesterolemia are highly associated with the syndrome [6,9]. The other causes for eight-and-a-half syndrome include demyelinating disease such as multiple sclerosis of the pons, and space-occupying lesions at the pons, such as brainstem glioma, pontine cavernoma, pontine tuberculoma and cavernoma [8].
Pontine infarct can be life-threatening due to involvement of bulbar and respiratory muscles. Besides, there is a risk of developing obstructive hydrocephalus. Furthermore, NIHSS is not so great of an assessment tool for posterior circulation stroke. Study found that posterior circulation stroke patients presenting with low NIHSS have 23% increased odds of disability at 3 months than anterior circulation stroke patients. Previous studies have reported that > 75% of patients with posterior circulation stroke present with a baseline NIHSS score of 0 to 5 [10].
In this case, the limbs motor and sensory functions were intact, due to the fact that the long descending and ascending tracts were unaffected. Corticospinal and spinothalamic tracts are located at the basis pontis, ventral to the nuclei and fascicles responsible for ophthalmic and facial movements. The vascular supplies are directly from the perforators of the basilar artery, which is different from the vascular supply of the pontine tegmentum.
Similar to all type of strokes, anti-platelet therapy is necessary for secondary prevention of stroke. Identification and optimization of the risk factors are the mainstay of prevention. Neurorehabilitation is mostly warranted for better clinical outcome [10]. Prognosis of eight-and-a-half syndrome secondary to pontine infarct depends on the severity of neurological deficit and the ability to recover, with facial palsy often the first to show improvement [6,11]. Overall, the clinical outcome is better than that of a supratentorial infarct.