With this combination, the affected eye barely moves horizontally, and only abduction of the healthy eye is possible. During abduction of the healthy eye, monocular nystagmus is observed, and adduction via convergence is usually preserved. In primary position, the healthy eye is always exotropic, which is called paralytic pontine exotropia.
This syndrome was first described and named by Charles Miller Fisher in 1967 as a pattern of ocular muscle palsy in patients with pontine lesions. 1)
Isolated OHS is rare and usually accompanied by other cranial nerve palsies, hemiparesis, or hemisensory disturbance. In a study of 20 cases by Wall & Wray (1983), contralateral hemiparesis was found in 30% and hemisensory disturbance in 35%. 1)
“1” corresponds to conjugate horizontal gaze palsy toward the affected side (complete loss of both eyes moving toward the affected side), and “0.5” corresponds to ipsilateral internuclear ophthalmoplegia (INO). In total, “1.5” of horizontal eye movement is lost, hence the name.
QDoes the one-and-a-half syndrome cause problems with vertical eye movements?
A
Vertical gaze is usually preserved. This is because the pathways for vertical eye movements are controlled at the midbrain level, which is different from the MLF and pontine PPRF. However, if the lesion extends to the midbrain, vertical movements may also be impaired.
MRI (±MRA): First choice for detection and localization of brainstem lesions. For imaging of the pontine tegmentum, request coronal sections in addition to axial views.
Note on diffusion-weighted imaging (DWI): Cerebral infarction may not show high signal on DWI immediately after onset, and repeat imaging may be necessary.
CT: Pontine hemorrhage and pontine tumors can be diagnosed relatively easily. Infarcts in the inferior pontine tegmental branches are rarely identified.
Multiple sclerosis: Characteristic imaging findings around the lateral ventricles. However, pontine lesions are often not visualized.
Wernicke encephalopathy: Characteristic imaging findings in the mammillary bodies and periaqueductal region. However, pontine lesions are often not visualized.
Angiography: Indicated when a vascular etiology is suspected.
It is important to differentiate from diseases that present with eye movement abnormalities similar to OHS.
Pseudo-OHS (myasthenia gravis): Myasthenia gravis (MG) can mimic OHS. In MG, adduction deficit does not improve with convergence. 1) It can also be differentiated by the absence of diurnal variation and no improvement with the Tensilon test.
Fisher syndrome: Differentiated by the absence of bilateral symmetric eye movement disorder and no unsteadiness due to truncal ataxia.
Differentiation from oculomotor nerve palsy: Distinguished by the absence of upward and downward gaze limitation, no ptosis, and no internal ophthalmoplegia such as mydriasis or diminished light reflex.
Differentiation from 8.5 syndrome: OHS is distinguished by the absence of facial nerve palsy. 1)
Thyroid eye disease: If thyroid eye disease is suspected, thyroid function tests should be performed.
QHow is it differentiated from myasthenia gravis?
A
Myasthenia gravis (MG) can present as pseudo-OHS mimicking 1.5 syndrome. In OHS, adduction is preserved with convergence, whereas in MG, adduction deficit does not improve with convergence. The absence of diurnal variation and no improvement with the Tensilon test also support differentiation.
Treatment is primarily directed at the underlying disease. There is no direct treatment for OHS itself; improvement of eye movement abnormalities is achieved by treating the causative brainstem lesion.
Treatment of the Underlying Disease
Cerebral infarction (pontine tegmentum infarction): If the infarct is confirmed on diffusion-weighted imaging within 4.5 hours of onset, intravenous t-PA (alteplase: Activacin) 0.6 mg/kg is possible. If recanalization is not achieved after t-PA, endovascular treatment with a stent retriever device may be considered.
Acute phase drug therapy: Intravenous Radicut (edaravone) is also an option within 24 hours of onset. However, these ultra-acute treatments are rarely performed solely for eye movement abnormalities.
Routine drug therapy: Often, observation is done with 3 tablets of Mecobalamin (500 μg) + 3 tablets of Kallikrein (10 units) divided into three doses (both are off-label use).
Pontine hemorrhage/pontine tumor: Neurosurgery is primarily involved.
Multiple sclerosis: Steroid pulse therapy. If ineffective, plasmapheresis (in collaboration with neurology).
Wernicke encephalopathy: Vitamin B₁ therapy (in cooperation with neurology).
Symptomatic treatment (diplopia management)
Fresnel membrane prism prescription: Selected when diplopia persists in primary gaze.
Eye patch / monocular occlusion: Useful for immediate management of diplopia.
Strabismus surgery (extraocular muscle recession + adjustable suture): Performed to improve binocular vision, head posture, and cosmesis.
Botulinum toxin injection: Particularly effective for oscillopsia due to ataxic dissociated nystagmus. Although temporary, it is suitable for management during rehabilitation.
QWhat are the treatment options if diplopia persists?
A
If diplopia persists in primary gaze, Fresnel membrane prism prescription is selected. For more active intervention, strabismus surgery with extraocular muscle recession and adjustable suture is performed to improve binocular vision, head posture, and cosmesis. When oscillopsia is problematic, botulinum toxin injection is also an effective option.
PPRF (paramedian pontine reticular formation): The horizontal gaze center in the brainstem. Receives input from the cerebral hemispheres, superior colliculus, vestibular nuclei, and cerebellum.
Abducens nucleus: Receives signals from the PPRF and controls the ipsilateral lateral rectus muscle. Also controls the contralateral medial rectus muscle via the contralateral MLF.
MLF (medial longitudinal fasciculus): A pathway of interneurons that decussate from the abducens nucleus to the contralateral oculomotor nucleus medial rectus subnucleus. It is a long fiber bundle extending from the pons to the midbrain.
OHS results from simultaneous damage to the ipsilateral PPRF/abducens nucleus and the ipsilateral MLF. The four possible lesion patterns are shown below.
Damage to both the ipsilateral abducens nucleus and PPRF
Damage to the ipsilateral abducens nucleus alone
Damage to the ipsilateral PPRF alone
Damage to the ipsilateral abducens nerve root fibers and contralateral MLF due to two separate lesions
Differences based on the site of PPRF lesion:
Lesion rostral to the abducens nucleus: Saccades and pursuit are impaired, but vestibulo-ocular reflex (VOR) horizontal eye movements are preserved.
Lesion at the level of the abducens nucleus: Both voluntary and vestibulo-ocular reflex movements are lost.
Damage to the abducens nucleus: All ipsilateral horizontal eye movements, including voluntary and reflex, are stopped.
Damage to internuclear neurons of the MLF → adduction palsy of the ipsilateral eye on contralateral gaze + horizontal jerk nystagmus of the contralateral abducting eye. The nystagmus observed on abduction is dissociated rhythmic nystagmus, considered an adaptive phenomenon to the adduction deficit of the affected eye. The degree of adduction impairment varies, and even after improvement, a reduction in adduction velocity often persists even if movement restriction disappears.
Convergence, pupillary light reflex, and vertical eye movements do not pass through the MLF, so they are usually preserved (unless the lesion extends to the midbrain).
Nathan et al. (2024) reported a case of a 42-year-old woman with untreated hypertension who was brought to the emergency room with right hemiplegia, dizziness, and projectile vomiting 1). Her blood pressure on arrival was 170/110 mmHg. Complete left horizontal gaze palsy and nystagmus on right eye abduction were observed, and CT confirmed hemorrhage in the left midbrain and upper pons. She was managed with antihypertensive treatment and discharged after one month of hospitalization. At a 2.5-year follow-up, eye movements had improved, and she was able to perform daily activities.
OHS due to cerebrovascular disease: Prognosis is relatively good. Mild infarcts without detectable lesions on imaging may heal within a few days.
Wernicke encephalopathy: With early treatment, eye movement abnormalities disappear within 1–2 weeks.
Multiple sclerosis: Complete resolution is rare, and slight limitations often remain. However, the prognosis of MS itself when presenting with eye movement abnormalities is good.
OHS due to pontine hemorrhage: Complete recovery within 6 months has been reported. 1)
MS, cerebrovascular disease, brainstem lacunar infarction: Most recover without sequelae. 1)
In recent years, a system of numbered syndromes centered on OHS has been organized. The establishment of a naming system according to the extent of the lesion, such as syndromes 8.5, 9, 13.5, 15.5, and 16.5, is progressing. 1)
Nathan B, Rajendran A, G E. One-and-a-Half Syndrome in a Case of Brainstem Bleed. Cureus. 2024.
Cao S, Feng Y, Xu W, Xia M. Vertical one-and-a-half syndrome overlapping with pupillary abnormality. Acta Neurol Belg. 2023;123(3):1149-1151. PMID: 36640253.
Marrodan M, Castiglione JI, Wainberg F, Rivero AD. Pseudo One-And-A-Half Syndrome in a Myasthenia Gravis Patient. Neurol India. 2022;70(5):2308. PMID: 36352691.
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