Detection of floccular hypometabolism in downbeat nystagmus by fMRI

[Pharmacotherapy of central oculomotor disorders]

Nystagmus causes blurred vision due to oscillopsia, as well as impaired balance. Depending on etiology, additional cerebellar and brain stem signs may occur. We present the current pharmacotherapy of the most common forms of central nystagmus: downbeat nystagmus (DBN), upbeat nystagmus (UBN), acquired pendular nystagmus (APN), and congenital nystagmus (CGN). Recommended medical therapies are aminopyridines (4-AP) for DBN and UBN, gabapentin and memantine for CGN and APN, and baclofen for periodic alternating nystagmus (PAN).

Eye hyperdeviation in mouse cerebellar mutants is comparable to the gravity-dependent component of human downbeat nystagmus

Humans with cerebellar degeneration commonly exhibit downbeat nystagmus (DBN). DBN has gravity-independent and -dependent components, and the latter has been proposed to reflect hyperactive tilt maculo-ocular reflexes (tilt-MOR). Mice with genetically determined cerebellar ataxia do not exhibit DBN, but they do exhibit tonic hyperdeviation of the eyes, which we have proposed to be the DBN equivalent. As such, the tilt-MOR might be predicted to be hyperactive in these mutant mice. We measured the tilt-MOR in 10 normal C57BL/6 mice and in 6 tottering, a mutant exhibiting ataxia and ocular motor abnormalities due to mutation of the P/Q calcium channel. Awake mice were placed in body orientations spanning 360 degrees about the pitch axis. The absolute, equilibrium vertical angular deviations of one eye were measured using infrared videooculography. In both strains, eye elevation varied quasi-sinusoidally with tilt angle in the range of 90 degrees nose-up to 90 degrees nose-down. Beyond this range the eye returned to a neutral position. Deviation over +/-30 degrees of tilt was an approximately linear function of the projection of the gravity vector into the animal's horizontal plane, and can thus be summarized by its slope (sensitivity). Sensitivity measured 14.9 degrees/g for C57BL/6 and 20.3 degrees/g for tottering, a statistically significant difference. Thus the pitch otolithic reflex of the ataxic mutants is hyperactive relative to controls and could explain tonic hyperdeviation of the eyes, consistent with the idea that the tonic hyperdeviation is analogous to DBN.

Functional brain imaging of peripheral and central vestibular disorders

This review summarizes our current knowledge of multisensory vestibular structures and their functions in humans. Most of it derives from brain activation studies with PET and fMRI conducted over the last decade. The patterns of activations and deactivations during caloric and galvanic vestibular stimulations in healthy subjects have been compared with those in patients with acute and chronic peripheral and central vestibular disorders. Major findings are the following: (1) In patients with vestibular neuritis the central vestibular system exhibits a spontaneous visual-vestibular activation-deactivation pattern similar to that described in healthy volunteers during unilateral vestibular stimulation. In the acute stage of the disease regional cerebral glucose metabolism (rCGM) increases in the multisensory vestibular cortical and subcortical areas, but simultaneously it significantly decreases in the visual and somatosensory cortex areas. (2) In patients with bilateral vestibular failure the activation-deactivation pattern during vestibular caloric stimulation shows a decrease of activations and deactivations. (3) Patients with lesions of the vestibular nuclei due to Wallenberg's syndrome show no activation or significantly reduced activation in the contralateral hemisphere during caloric irrigation of the ear ipsilateral to the lesioned side, but the activation pattern in the ipsilateral hemisphere appears 'normal'. These findings indicate that there are bilateral ascending vestibular pathways from the vestibular nuclei to the vestibular cortex areas, and the contralateral tract crossing them is predominantly affected. (4) Patients with posterolateral thalamic infarctions exhibit significantly reduced activation of the multisensory vestibular cortex in the ipsilateral hemisphere, if the ear ipsilateral to the thalamic lesion is stimulated. Activation of similar areas in the contralateral hemisphere is also diminished but to a lesser extent. These data demonstrate the functional importance of the posterolateral thalamus as a vestibular gatekeeper. (5) In patients with vestibulocerebellar lesions due to a bilateral floccular deficiency, which causes downbeat nystagmus (DBN), PET scans reveal that rCGM is reduced in the region of the cerebellar tonsil and flocculus/paraflocculus bilaterally. Treatment with 4-aminopyridine lessens this hypometabolism and significantly improves DBN. These findings support the hypothesis that the (para-) flocculus and tonsil play a crucial role in DBN. Although we can now for the first time attribute particular activations and deactivations to functional deficits in distinct vestibular disorders, the complex puzzle of the various multisensory and sensorimotor functions of the phylogenetically ancient vestibular system is only slowly being unraveled.

An update on acquired nystagmus

Proper evaluation and treatment of acquired nystagmus requires accurate characterization of nystagmus type and visual effects. This review addresses important historical and examination features of nystagmus and current concepts of pathogenesis and treatment of gaze-evoked nystagmus, nystagmus due to vision loss, acquired pendular nystagmus, peripheral and central vestibular nystagmus, and periodic alternating nystagmus.

A model-based theory on the origin of downbeat nystagmus

The pathomechanism of downbeat nystagmus (DBN), an ocular motor sign typical for vestibulo-cerebellar lesions, remains unclear. Previous hypotheses conjectured various deficits such as an imbalance of central vertical vestibular or smooth pursuit pathways to be causative for the generation of spontaneous upward drift. However, none of the previous theories explains the full range of ocular motor deficits associated with DBN, i.e., impaired vertical smooth pursuit (SP), gaze evoked nystagmus, and gravity dependence of the upward drift. We propose a new hypothesis, which explains the ocular motor signs of DBN by damage of the inhibitory vertical gaze-velocity sensitive Purkinje cells (PCs) in the cerebellar flocculus (FL). These PCs show spontaneous activity and a physiological asymmetry in that most of them exhibit downward on-directions. Accordingly, a loss of vertical floccular PCs will lead to disinhibition of their brainstem target neurons and, consequently, to spontaneous upward drift, i.e., DBN. Since the FL is involved in generation and control of SP and gaze holding, a single lesion, e.g., damage to vertical floccular PCs, may also explain the associated ocular motor deficits. To test our hypothesis, we developed a computational model of vertical eye movements based on known ocular motor anatomy and physiology, which illustrates how cortical, cerebellar, and brainstem regions interact to generate the range of vertical eye movements seen in healthy subjects. Model simulation of the effect of extensive loss of floccular PCs resulted in ocular motor features typically associated with cerebellar DBN: (1) spontaneous upward drift due to decreased spontaneous PC activity, (2) gaze evoked nystagmus corresponding to failure of the cerebellar loop supporting neural integrator function, (3) asymmetric vertical SP deficit due to low gain and asymmetric attenuation of PC firing, and (4) gravity-dependence of DBN caused by an interaction of otolith-ocular pathways with impaired neural integrator function.

Ocular motor syndromes of the brainstem and cerebellum

PURPOSE OF REVIEW

The brainstem and cerebellum contain many neuronal types that play a critical role in eye movement control. In a physiological approach, understanding how these neuronal assemblies cooperate provides strong insight into general brain functions. Furthermore, eye movements provide an interesting model for understanding neural mechanisms of sensorimotor learning, and a knowledge of the mechanisms underlying oculomotor plasticity is essential for correctly diagnosing and effectively managing patients. Finally, knowledge of the ocular motor syndromes frequently helps localize the pathological abnormality.

RECENT FINDINGS

We review the recently published works dealing with the physiological organization and pathology of slow and rapid eye movements at a brainstem and cerebellar level.

SUMMARY

The main recent findings of great interest for clinical practice or research concern the physiopathology of head shaking nystagmus, downbeat nystagmus and oculopalatal tremor; the neural substrates of three-dimensional control of eye movements and of optokinetic nystagmus; the understanding of saccade generation and of its consequences on physiological and pathological eye oscillations; and, finally, the physiological basis of saccadic adaptation.

Diagnosis and treatment of vertigo and dizziness

INTRODUCTION

Vertigo is not a separate disease process, but a multisensory and sensorimotor syndrome with various etiologies and pathogeneses. It is among the commonest symptoms presented to doctors, with a lifetime prevalence of around 20% to 30%. Patients have often consulted multiple physicians before a diagnosis is made and therapy initiated.

METHODS

Selective literature research and review of the guidelines of the German Neurological Society.

RESULTS

A careful history remains the cornerstone of diagnosis. Once the correct diagnosis is made, specific and effective treatments are available for most peripheral, central, and psychogenic forms of dizziness. Treatment may include medication, physiotherapy, and psychotherapy; a few limited cases may require surgical treatment. The treatment of choice for acute vestibular neuritis is the administration of corticosteroids. Menière's disease is treated with high-dose, long-term betahistine. A new approach to the management of downbeat and upbeat nystagmus, and of episodic ataxia type 2, involves the use of aminopyridines as potassium-channel blockers. Close multidisciplinary cooperation is essential in dizziness, and further multicenter studies are needed.

Decreased cortical inhibition and yet cerebellar pathology in ‘familial cortical myoclonic tremor with epilepsy’

Cortical hyperexcitability is a feature of "familial cortical myoclonic tremor with epilepsy" (FCMTE). However, neuropathological investigations in a single FCMTE patient showed isolated cerebellar pathology. Pathological investigations in a second FCMTE patient, reported here, confirmed cerebellar Purkinje cell degeneration and a normal sensorimotor cortex. Subsequently, we sought to explore the nature of cerebellar and motor system pathophysiology in FCMTE. Eye movement recordings and transcranial magnetic stimulation performed in six related FCMTE patients showed impaired saccades and smooth pursuit and downbeat nystagmus upon hyperventilation, as in patients with spinocerebellar ataxia type 6. In FCMTE patients short-interval intracortical inhibition (SICI) was significantly reduced. Resting motor threshold, recruitment curve, silent period, and intracortical facilitation were normal. The neuropathological and ocular motor abnormalities indicate cerebellar involvement in FCMTE patients. Decreased SICI is compatible with intracortical GABA(A)-ergic dysfunction. Cerebellar and intracortical functional changes could result from a common mechanism such as a channelopathy. Alternatively, decreased cortical inhibition may be caused by dysfunction of the cerebello-thalamo-cortical loop as a result of primary cerebellar pathology.

Central compensation of deviated subjective visual vertical in Wallenberg's syndrome

The central compensation of vestibular tonus imbalance due to unilateral peripheral vestibular lesions has been repeatedly documented. Little is known, however, about the central compensation of vestibular tonus imbalance due to central lesions. Dorsolateral medullary infarctions (Wallenberg's syndrome) typically cause a central vestibular tonus imbalance in the roll plane with deviations of perceived verticality and ipsiversive body lateropulsion. The course of normalisation of the tilts of subjective visual vertical (SVV) in 50 patients who had acute Wallenberg's syndrome were retrospectively compared with that in 50 patients with acute vestibular neuritis. The initial displacement of SVV was 9.8 degrees in Wallenberg's syndrome and 7 degrees in vestibular neuritis. The deviation of SVV significantly decreased over time within days to weeks in both groups. This finding shows that the time courses of the central compensation for dorsolateral medullary infarctions and peripheral vestibular lesions are similar.

Functional brain imaging: a window into the visuo-vestibular systems

PURPOSE OF REVIEW

Advances have been made in identifying how areas involved in processing vestibular, ocular motor, and visual information are represented in the human cortex as well as the cortical interaction between these systems in healthy subjects.

RECENT FINDINGS

While we know how some vestibular and ocular motor disorders modify visuo-vestibular interaction by changing the 'normal' cortical activation-deactivation patterns, it is still early days in functional magnetic resonance imaging studies of patients with specific disorders. Findings from current brain imaging studies of several vestibular, ocular motor, and cerebellar disorders are presented.

SUMMARY

The promise of more insights into the complex neuronal networks of the human cortex is great.