One and a half syndrome

Overview

One and a half syndrome is a rare neuro-ophthalmologic condition resulting from a specific brainstem lesion that affects horizontal eye movement. It is characterized by a combination of a conjugate horizontal gaze palsy on one side and an internuclear ophthalmoplegia (INO) on the other side, hence the term "one and a half", one full gaze palsy (conjugate gaze loss) plus a "half" (INO) on the other side.

First described in the 1960s, one and a half syndrome typically results from damage to two critical areas in the pons: the paramedian pontine reticular formation (PPRF) or abducens nucleus, and the medial longitudinal fasciculus (MLF). This lesion affects coordination between cranial nerves involved in horizontal gaze, especially cranial nerves III (oculomotor), VI (abducens), and their associated pathways. Most patients present with horizontal gaze abnormalities, but vertical movements and convergence are often preserved.

Causes

One and a half syndrome is caused by a lesion in the dorsal tegmentum of the pontine brainstem that affects both the PPRF or abducens nucleus and the ipsilateral MLF. This damage disrupts the normal coordination of eye movements.

Common Causes

• Ischemic stroke: The most common cause, usually due to small vessel occlusion affecting the pontine perforators.

• Demyelinating diseases: Such as multiple sclerosis, particularly in younger patients.

• Brainstem tumors: Including gliomas or metastatic lesions.

• Infections: Such as brainstem encephalitis.

• Trauma: Brainstem injury affecting the pons.

• Vascular malformations: Such as cavernous angiomas in the brainstem.

Symptoms

Patients with one and a half syndrome present with distinctive and characteristic eye movement abnormalities that help in clinical diagnosis.

Oculomotor Symptoms

• Conjugate horizontal gaze palsy: Inability to move both eyes toward the side of the lesion (complete gaze palsy in one direction).

• Internuclear ophthalmoplegia (INO): On attempted gaze to the opposite side, the ipsilateral eye does not adduct (move inward), while the contralateral eye abducts with nystagmus (jerking movement).

• Preserved convergence: Most patients can still converge both eyes (look inward toward the nose), indicating that the medial rectus muscles are intact.

Additional Features

• Diplopia: Double vision due to misalignment of the eyes.

• Nystagmus: In the abducting eye on attempted gaze opposite the lesion.

• Ptosis or facial weakness: May be present if adjacent cranial nerve nuclei are involved.

Diagnosis

Diagnosis of one and a half syndrome is primarily clinical and supported by neuroimaging to identify the underlying cause.

Clinical Examination

• Bedside neuro-ophthalmologic evaluation reveals:

  • Complete gaze palsy on one side (both eyes cannot look in one direction).

  • INO on the contralateral side (loss of adduction in the affected eye with nystagmus in the abducting eye).

• Vertical eye movements and convergence are typically preserved.

Neuroimaging

• Magnetic Resonance Imaging (MRI): The gold standard for detecting pontine lesions, strokes, demyelination, or tumors.

• Diffusion-weighted imaging (DWI): Helps identify acute infarction in stroke cases.

Additional Tests

• MR angiography or CT angiography: To evaluate for vascular causes such as stroke or dissection.

• CSF analysis: If infection or multiple sclerosis is suspected.

Treatment

Treatment of one and a half syndrome focuses on managing the underlying cause of the brainstem lesion. There is no specific cure for the syndrome itself, but rehabilitation and symptom management can improve quality of life.

Treatment by Underlying Cause

• Ischemic stroke: Antiplatelet therapy, blood pressure control, and stroke rehabilitation. Thrombolysis may be considered in acute cases.

• Multiple sclerosis: Corticosteroids for acute flares, disease-modifying therapies for long-term management.

• Tumors: May require surgical removal, radiation therapy, or chemotherapy depending on tumor type and location.

• Infections: Antibiotics or antiviral agents, supportive care, and sometimes corticosteroids.

Symptomatic and Supportive Treatment

• Prism glasses: May help reduce double vision in selected cases.

• Eye patching: For patients with severe diplopia.

• Neurological rehabilitation: To help adapt to eye movement limitations and improve overall functioning.

Prognosis

The prognosis of one and a half syndrome depends on the etiology and extent of the brainstem lesion. In stroke-related cases, some recovery of eye movement may occur over weeks to months, but full resolution is rare. In demyelinating diseases like multiple sclerosis, symptoms may improve significantly with treatment.

Permanent eye movement abnormalities may persist in many patients, but compensatory mechanisms often allow them to adapt. If the underlying condition is progressive or recurrent (e.g., MS or tumor), the prognosis is more guarded.

With prompt diagnosis and appropriate treatment of the underlying cause, complications can be minimized, and patients can often return to functional daily activities with adjustments.