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A 20-year-old woman with abnormal eye movements
Digital Journal of Ophthalmology 2021
Volume 27, Number 1
January 4, 2021
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Vivian Paraskevi Douglas, MD, DVM, MBA | Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts
Homer H. Chiang, MD | Department of Ophthalmology, UT Health San Antonio, San Antonio, Texas
Konstantinos A. A. Douglas, MD, DVM, MBA | Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts
Tavé Van Zyl, MD | Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts
Nurhan Torun, MD | Division of Ophthalmology, Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts
Differential Diagnosis
One-and-a-half syndrome is most often localized to a unilateral lesion of the medial longitudinal fasciculus (MLF) and ipsilateral paramedian pontine reticular formation (PPRF) or the abducens nerve. The MLF forms a connection between the contralateral abducens nerve nucleus and the ipsilateral oculomotor nerve nucleus. Lesions of this tract result in internuclear ophthalmoplegia, with an ipsilateral adduction palsy and abducting nystagmus in the contralateral eye. The additional involvement of the PPRF or abducens nerve nucleus results in a conjugate gaze palsy to the ipsilateral side. Together, a lesion of the PPRF– abducens nerve nucleus (one) and MLF (half) combine to form the clinical entity known as one-and-a-half syndrome, as demonstrated in this patient.

The syndrome can be expanded based on involvement of additional brainstem structures. Additional facial nerve deficit may be present as fibers course around the abducens nerve nucleus before exiting the brainstem. When this occurs, the condition is termed eight-and-a-half syndrome, as was present in our patient.(2) Another example is the addition of ipsilateral trigeminal nerve involvement that produces a thirteen-and-a-half syndrome variant.(3)

These syndromes, as in our patient, result from a unilateral lesion in the dorsal pontine tegmentum. The differential diagnosis includes ischemia (eg, pontine lacunar infarction), compressive lesion (eg tumor), demyelinating disease, and infection.3 Thorough medical history and physical examination along with neuroimaging are key to early diagnosis and treatment of these syndromes, because the prognosis is variable and the severity of the disorder is determined largely by the specifics of the underlying pathology.

On imaging, these lesions can resemble gliomas, hemorrhagic telangiectasias, such as those seen in Osler-Weber-Rendu disease, and even brain metastasis.(4)
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