A lesion in the medial longitudinal fasciculus (MLF) results in the MLF syndrome or internuclear ophthalmoplegia (INO), which is classically defined by the triad of lack of adduction of the ipsilateral eye, dissociated nystagmus in the contralateral eye during abduction, and preserved vergence. However, the clinical manifestations of lesions in the MLF may be more complex and variable, because not only fibers concerned with horizontal eye movements, but also those concerned with vertical eye movements, such as vertical VOR or smooth pursuit, pass through the MLF. Accordingly, in addition to the above triad, a skew deviation, that is, dissociated vertical-torsional nystagmus, with the eye beating down on the side of the lesion may also occur. The most common causes of this syndrome are brainstem infarction and multiple sclerosis (MS). The adduction weakness can be explained by the interruption caused by lesions of the abducens internuclear neurons, which project via the MLF to the contralateral medial rectus subgroups of the oculomotor nucleus. However, the mechanism of the abduction nystagmus in the contralateral eye is still unknown. Although several hypotheses have been put forward, the most plausible one appears to be an adaptive mechanism: In order to overcome the weakness of the adducting eye, there is an increasing change in the innervation to the adducting eye, and according to Hering's law of equal innervation, it must be accompanied by compensatory changes in reinforcing innervation to the abducting eye. This could cause abnormal overshooting saccades followed by backward postsaccadic drift, manifesting clinically as abduction nystagmus. Presumably, extension of the lesion into structures near but outside the MLF, or the involvement of cell bodies intermixed with MLF fibers, may also be involved in the pathogenesis of abduction nystagmus and occasional slowing of abducting saccades. Another potential mechanism of the abduction nystagmus is the presence of a superimposed gaze-evoked nystagmus, coincidentally involving extensive lesions of structures near but outside the MLF, such as the medial vestibular nuclei, prepositus hypoglossal nuclei or the cerebellar flocculus/paraflocculus.

　One-and-a-half syndrome refers to horizontal gaze palsy in one eye and adduction palsy (MLF syndrome) in the contralateral eye. This occurs due to lesions in the dorsal pontine tegmentum that affects the ipsilateral paramedian pontine reticular formation (PPRF) or abducens nucleus and ipsilateral MLF.

　Occasionally, vertical one-and-a-half syndrome is also encountered, which consists of vertical gaze palsy in one eye and upward or downward palsy in the contralateral eye. This syndrome is mainly caused by paramedian thalamo-mesencephalic lesions.

　This review provides an overview of the history, etiology, clinical features, and pathophysiology of both MLF syndrome and one-and-a-half syndrome.

　There are differences in the recording methods for cVEMP used across institutions. However, the impact of head and body positions on the results of cVEMP testing is yet to be determined in patients with dysequilibrium. We assessed the effects of three body positions used for cVEMP recordings on the results, and demonstrated the advantages and disadvantages of each position. Three body positions were used for the cVEMP recordings; A. supine/head rotated; B. semi-recumbent/head rotated and elevated; and C. supine/head elevated. The subjects of the present study were 139 patients (9-91 years old) with dysequilibrium of various causation, 41 patients underwent the recordings in two positions, and a total of 360 ears were included. The results of the cVEMP testing were classified as normal, abnormal, and indeterminate. The rates of normal, abnormal, and indeterminate results were 50.6%, 35.0%, and 14.4%, respectively. The rate of abnormal results of the test increased with increasing age. The rate of indeterminate results was 13.2%, 11.3%, and 16.7% for positions A, B, and C, respectively. The mean background sternocleidomastoid muscle (SCM) electrical activity (SCM activity) was smaller in the recordings obtained in position A than in those obtained in the other positions. A positive correlation was identified between the age and SCM activity and the raw p13-n23 complex amplitude (amplitude) was larger in the recordings obtained in position B than in other positions. A negative correlation was identified between the age and amplitude of the potentials recorded in position C. These results suggest that positions A and C might be less burdensome to the patients, but the amplitudes of the potentials were a smaller, and there appeared to be a higher likelihood of the p13-n23 complexes being overlooked. On the other hand, while position B was more burdensome for the patients, the detection rate of the p13-n23 complexes was higher. As each body position has different characteristics, selection of the measurement position appropriate for individual patients is recommended. VEMP recordings may be useful to detect bilateral otolith dysfunction that may cause chronic dizziness in the elderly.

　Bow hunter's syndrome (BHS) is a clinical condition caused by circulatory insufficiency in the vertebral basilar artery region during head rotation. We report a case of BHS diagnosed after treatment for vestibular neuritis.

　A 79-year-old woman visited our hospital complaining of vertigo upon head rotation to the left. Examination revealed right horizontal nystagmus in the supine position, additional torsional nystagmus in the lower right head position. Initially, left acute peripheral vestibular syndrome was suspected, and the patient received conservative treatment, including vestibular rehabilitation, but the dizziness associated with the cervical torsion persisted even after a month. Duplex ultrasound revealed severe stenosis of the right vertebral artery during head rotation to the right, indicating circulatory insufficiency in the vertebral artery region as the cause of the dizziness. Vertebral angiography revealed significant meandering of the right vertebral artery at C3/C4 and occlusion during right cervical hyper-rotation. Therefore, the patient was diagnosed as having BHS and treated with antiplatelet drugs and provided lifestyle guidance to prevent rightward cervical hyper-torsion. Thereafter, the patient remained asymptomatic for at least 1 year.

　BHS is a rare condition and can be confused with peripheral dizziness when accompanied by vestibular disorder. In cases presenting with dizziness during head rotation, we believe that BHS should be included in the differential diagnosis.

　Purpose: Persistent postural-perceptual dizziness (PPPD) is often precipitated by vestibular disorders, and there are cases in which peripheral vestibular dysfunction persists at the time of diagnosis of PPPD. The purpose of this study was to determine whether PPPD cases with canal palsy (CP) on caloric testing were more severe than those without CP.

　Methods: Of the patients diagnosed as having PPPD between 2019 and 2021, 75 with preceding vestibular disorder who underwent caloric testing were included in this study. Age, sex, duration, preceding vestibular disorder, and the scores on the DHI (Dizziness Handicap Inventory), HADS (Hospital Anxiety and Depression Scale), NPQ (Niigata PPPD questionnaire), and VSS-sf (Vestibular Symptom Scale-Short Form) were compared between the groups with and without CP.

　Results: There was no significant difference in the age, sex, duration, preceding vestibular disorder, DHI score, or HADS score between the two groups. Significantly higher scores in the movement and visual stimulation subscales of the NPQ and the autonomic symptom subscale of the VSS-sf (p<0.05) were observed in the group without CP.

　Conclusion: None of the scores was higher in the group with CP. Autonomic symptoms were more frequent, and vestibular symptoms were more likely to be exacerbated by movement or visual stimulation in the group without CP.

　A 68-year-old woman visited a neighbor hospital with a 2-month history of floating dizziness. Brain MRI revealed a large tumor occupying the entire left cerebellar hemisphere, and the patient was transferred to the department of neurosurgery at our hospital. The patient was diagnosed as having a diffuse large B-cell lymphoma primarily originating from the cerebellum by stereotactic needle biopsy study and positive results of immunohistochemistry for the surface markers CD19 and CD20. The patient recovered completely after 4 cycles of combined chemotherapy with high-dose methotrexate and Ara-C. However, she visited our department again complaining of persistent floating dizziness. Neurological examination revealed no significant findings other than the upward and leftward gaze-evoked nystagmus (GEN) . ENG showed GEN during upward and leftward gaze in the light. Both horizontal and vertical pursuits were disturbed and the saccadic pursuit was marked. The horizontal optokinetic nystagmus (OKN) was poorly induced, while caloric nystagmus was well induced by cold water bilaterally. GEN is attributed to a deficit in the velocity-to-position integrator, suggestive of brain stem or cerebellar lesions. GEN is also known to be closely correlated with deficits of smooth pursuit eye movements and slow phase of OKN, as in our case. The MRI in our case revealed brainstem lesions (medial vestibular and prepositus hypoglossi nuclei) and cerebellar lesions, involving the flocculus/paraflocculus, vermis, cerebellar hemisphere VI, VII and uvula, which could explain the above-mentioned abnormal eye movements. The reduced slow phase of OKN and well induced caloric nystagmus seemed to be apparently contradictory results from the point of view of the function of the velocity storage integrator (VSI). However, the impaired OKN was presumably ascribed to insufficient visual inputs conveyed to the VSI from the vestibulo-cerebellum. Therefore, it was assumed that the function of the VSI was still intact, while the velocity-to-position integrator seemed to show functional deficit.