One And a Half Syndrome
In 1967, Miller Fisher described a pattern of ophthalmoplegia in patients with various pontine lesions, known as “one-and-a-half syndrome” (OAHS). In this syndrome, patients present with a combination of an ipsilateral conjugate horizontal gaze palsy (referring to the ‘one’ horizontal gaze palsy) and an ipsilateral internuclear ophthalmoplegia (INO) (referring to the ‘half’ of a horizontal gaze palsy) .
Any disease causing a pontine lesion can account for the findings seen in the OAHS, including vascular, inflammatory, traumatic, infiltrative, demyelinating and neoplastic. Of these, the most commonly reported etiology in adults is vascular, including mainly ischemic brainstem infarction, pontine hemorrhages, basilar artery aneurysms and arteriovenous malformations . Demyelinating lesions (such as multiple sclerosis) and brainstem malignancies (primary or metastatic) including gliomas, metastatic melanoma, ependymoma of the fourth ventricle and astrocytoma of the cerebellum can also cause OAHS. More rarely, infectious causes including neurocysticercosis, brainstem tuberculomas, and brainstem encephalitis have also been reported .
OAHS presents with an ipsilateral conjugate horizontal gaze palsy and limited adduction of the ipsilateral eye (INO). The ipsilateral eye usually remains fixed straight ahead but has diminished or no ability to move in right or left horizontal gaze. In some cases, the contralateral eye can be exotropic, with abduction nystagmus during attempts of lateral eye movements . In OAHS vertical gaze is preserved.
Patients will typically present with diplopia, blurred vision and oscillopsia. The neurological exam may show many different patterns of nystagmus including gazed-evoked, upbeat nystagmus, horizontal and rotatory ipsilateral gaze nystagmus, spontaneous contralateral gaze nystagmus. Skew deviation and convergence disorders can also occur .
In discussing horizontal eye movements, there are three main structures within the dorsal pontine tegmentum that are relevant, namely the Paramedian Pontine Reticular Formation (PPRF), the internuclear fibers of the Medial Longitudinal Fasciculus (MLF) and the abducens nuclei .
The PPRF and the abducens nuclei act as the final pre-nuclear and nuclear centers for horizontal eye movement after integration of nerve impulses from visual areas of the frontal and parietal lobes, respectively . Therefore, upon the initiation of a horizontal gaze, excitatory neurons within the PPRF project to the ipsilateral abducens nuclei, located within the colliculus facialis at the lower part of the pons . In turn, the abducens nuclei project to two distinct sets of horizontal gaze fibers. First, the ipsilateral abducens nucleus triggers motor fibers that control the ipsilateral lateral rectus muscle. Second, the abducens nuclei also activates the contralateral MLF, which carries a set of excitatory interneuronal fibers that decussate at the same topographic level of their parent cell bodies. These fibers ascend to the mid-brain and terminate on neuronal cell bodies at the medial rectus subdivision of the oculomotor nuclear complex .
Since the OAHS is a combination of an ipsilateral horizontal gaze palsy and an ipsilateral INO, understanding of the relevant neuroanatomy and potential lesion locations is critical. Given the pathway described earlier, there seems to be four possible lesion locations to account for the horizontal gaze palsy in the OAHS, including (1) damage to both the ipsilateral abducens nuclei and the PPRF, (2) damage to the ipsilateral abducens nuclei only, (3) damage to the ipsilateral PPRF only, and (4) damage to the ipsilateral abducens nerve root fiber together with the contralateral MLF in the event of two separate lesions .
In the event of unilateral damage to the PPRF, one of two main findings can be observed, depending on the exact location of the lesion. When the PPRF lesion is located rostrally to the abducens nucleus, ipsilateral paralysis of the horizontal conjugate gaze occurs, including both saccadic and pursuit eye movements. Despite this paralysis, vestibular-induced reflex horizontal eye movements are preserved. In contrast, when the PPRF lesion is at the level of the abducens nucleus, there is loss of all ipsilateral saccadic and pursuit voluntary eye movements along with vestibular-induced reflex horizontal eye movements . Finally, damage to the ipsilateral abducens nucleus results in the arrest of all ipsilateral horizontal eye movements, including voluntary and reflexive movements .
Simultaneously, and in addition to the horizontal gaze palsy, lesions involving the ipsilateral MLF along any segment of its ascending path account for the ipsilateral INO observed in the OAHS. Apart from horizontal movements due to convergence, the internuclear neurons of the MLF are required for any type of conjugated horizontal movement. As such, in the case of an ipsilateral INO, there is paralysis of the ipsilateral eye adduction on attempted contralateral horizontal gaze coupled with a horizontal jerk nystagmus in the contralateral abducting eye . Nevertheless, it is to be noted that convergence, pupillary constrictor reflex activity and vertical eye movements are typically preserved as long as lesions do not involve the midbrain .
The diagnosis of OAHS is made through a combination of clinical and radiological findings. Clinically, a full ocular motility exam, including assessment of convergence, is performed. In addition, a thorough neurological exam with particular attention to the cranial nerves may further assist in lesion localization. Ocular manifestations of myasthenia gravis (MG) can imitate OAHS (i.e., a pseudo-OAHS), but adduction deficits would not be overcome with attempted convergence in MG. In the case of suspected MG or thyroid disease, MG antibody tests or thyroid function tests can be of clinical consideration respectively . Finally, to further elucidate the etiology underlying these symptoms, neuro-imaging is indicated, with MRI being the modality of choice to examine and localize underlying brainstem lesions .
While the main management of OAHS is directed at the underlying cause, various interventions exist for symptomatic management of disabling diplopia, oscillopsia or blurred vision in unresolved cases. Conservative symptomatic management includes eye patching or single eye occlusion to manage diplopia. Prisms can also be utilized to minimize diplopia . Although unlikely to restore ocular movement, surgical intervention using extraocular muscle recession and adjustable sutures can be considered to improve binocular vision, head posture and cosmesis .
Alternatively, injection of botulinum toxin to selected extraocular muscles has been reported to provide some symptomatic relief in certain cases, specifically those with oscillopsia due to ataxic disconjugate nystagmus. While not a cure, the transient therapeutic effect of the botulinum toxin makes it a suitable option for the management of complex ocular motility problems during the rehabilitation period .
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