Disease activity in multiple sclerosis (MS) is strongly linked to the formation of new lesions, which involves a complex sequence of inflammatory, degenerative, and reparative processes. Conventional magnetic resonance imaging (MRI) techniques, such as T2-weighted and gadolinium-enhanced T1-weighted sequences, are highly sensitive in demonstrating the spatial and temporal dissemination of demyelinating plaques in the brain and spinal cord. Hence, these techniques can provide quantitative assessment of disease activity in patients with MS, and they are commonly used in monitoring treatment efficacy in clinical trials and in individual cases. However, the correlation between conventional MRI measures of disease activity and the clinical manifestations of the disease, particularly irreversible disability, is weak. This has been explained by a process of exhaustion of both structural and functional redundancies that increasingly prevents repair and recovery, and by the fact that these imaging techniques do not suffice to explain the entire spectrum of the disease process and lesion development. Nonconventional MRI techniques, such as magnetization transfer imaging, diffusion-weighted imaging, and proton magnetic resonance spectroscopy, which can selectively measure the more destructive aspects of MS pathology and monitor the reparative mechanisms of this disease, are increasingly being used for serial analysis of new lesion formation and provide a better approximation of the pathological substrate of MS plaques. These nonconventional MRI-based measures better assess the serial changes in newly forming lesions and improve our understanding of the relationship between the damaging and reparative mechanisms that occur in MS.
diffusion-weighted imaging; lesion development; magnetic resonance imaging; magnetization transfer imaging; multiple sclerosis; proton magnetic resonance spectroscopy
The world of metabolic myopathies has been dramatically modified by the advent of enzyme replacement therapy (ERT), the first causative treatment for glycogenosis type II (GSDII) or Pompe disease, which has given new impetus to research into that disease and also other pathologies. This article reviews new advances in the treatment of GSDII, the consensus about ERT, and its limitations. In addition, the most recent knowledge regarding the pathophysiology, phenotype, and genotype of the disease is discussed. Pharmacological, immunotherapy, nutritional, and physical/rehabilitative treatments for late-onset Pompe disease and other metabolic myopathies are covered, including treatments for defects in glycogen metabolism, such as glycogenosis type V (McArdle disease), and glycogenosis type III (debrancher enzyme deficiency), and defects in lipid metabolism, such as carnitine palmitoyltransferase II deficiency and electron transferring flavoprotein dehydrogenase deficiency, or riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency.
Glycogenosis type II; McArdle disease; RR-MADD; glycogenosis type III; CPT2 deficiency
Pharmacological targeting of ion channels has long been recognized as an attractive strategy for the treatment of various diseases. Multiple sclerosis (MS) is an autoimmune disorder of the central nervous system with a prominent neurodegenerative component. A multitude of different cell types are involved in the complex pathophysiology of this disorder, including cells of the immune system (e.g. T and B lymphocytes and microglia), the neurovascular unit (e.g. endothelial cells and astrocytes) and the central nervous system (e.g. astrocytes and neurons). The pleiotropic expression and function of ion channels gives rise to the attractive opportunity of targeting different players and pathophysiological aspects of MS by the modulation of ion channel function in a cell-type and context-specific manner. We discuss the emerging knowledge about ion channels in the context of autoimmune neuroinflammation. While some pharmacological targets are at the edge of clinical translation, others have only recently been discovered and are still under investigation. Special focus is given to those candidates that could be attractive novel targets for future therapeutic approaches in neuroimmune autoinflammation.
channelopathy; ion channels; Kv1.3; multiple sclerosis; sodium channels
In the European Union, the high-concentration capsaicin patch is licensed for the management of neuropathic pain conditions in nondiabetic patients, including postherpetic neuralgia (PHN) and HIV-associated distal sensory polyneuropathy (HIV-DSP). However, in the USA, the Food and Drug Administration approved its use only in PHN patients. Capsaicin is a transient receptor potential vanilloid-1 agonist, which increases the intracellular calcium ion concentration. This triggers calcium-dependent protease enzymes causing cytoskeletal breakdown and leads to the loss of cellular integrity and ‘defunctionalization’ of nociceptor fibres. Efficacy and therapeutic effect has been shown in several clinical studies of PHN and HIV-DSP. The high-concentration capsaicin patch and its practical application are different from low-concentration creams; one application can help for up to 3 months. The process of setting up of a service to use the capsaicin 8% patch is also discussed.
Capsaicin; high concentration patch; neuropathic pain; post herpetic neauralgia; HIV; efficacy
Efficacy and tolerability profiles of Treximet [sumatriptan/naproxen sodium combination tablet (SNC)] have been established in clinical trials but have to date been virtually unstudied in pragmatic research. The primary objective of this study was to compare the overall satisfaction of SNC to its monotherapy components, S/N [one 100 mg Imitrex tablet (S) and two Aleve (naproxen sodium) 220 mg tablets, total dose 440 mg (N)] administered concomitantly using the Patient Perception of Migraine Questionnaire –Revised (PPMQ-R).
Adults with migraine (n = 50) without ‘medication overuse headache’ were treated for up to 18 migraine attacks per 3-month study period with study medication; SNC during one study period and S/N during the other study period. For all endpoints, differences between treatments were compared with paired t tests.
The percentage of patients reporting satisfied/very satisfied for Overall Satisfaction of SNC versus S/N (primary endpoint) was 85% versus 72% respectively (p = 0.054). For Overall Effectiveness, the results were 82% for SNC versus 73% for S/N (p = 0.159); and for Overall Side Effects the results were 86% for SNC versus 69% for S/N (p = 0.005). Mean PPMQ-R scores reflect greater satisfaction with SNC than S/N for Total score and for each of four subscales. The difference between SNC and S/N was significant for the Ease of Use subscale (p = 0.004) and met the criterion of being clinically meaningful for both the Total score and Ease of Use. SNC did not differ from S/N with respect to pain-free response 2 h post dose, pain relief 2 h post dose, sustained 24 h pain-free response, or sustained 24 h pain relief.
Although the primary endpoint only just failed, the results of this pragmatic outcomes study demonstrate SNC to have benefits over its concomitantly administered components in the acute treatment of migraine.
headache; migraine; naproxen sodium; patient treatment satisfaction; sumatriptan
Essential tremor (ET), once considered a monosymptomatic disease, is now viewed as a syndrome characterized by action tremor as well as a variety of other neurological features. These features include deficits of both balance (i.e. the ability to maintain the body within its base of support) and gait. Balance and gait abnormalities in ET have been studied most extensively in the laboratory, with the disability and burden on life, as experienced by patients with ET, being a subject of relatively less scrutiny. In this paper, we review the data on balance and gait impairment in ET. These include data both from objective assessments (bedside examination or laboratory gait analysis) and subjective assessments (patients’ reports of imbalance). We discuss the clinical implications of the current data, and suggest a tiered approach to begin to address the still unanswered questions in this field. This tiered approach starts with an attempt to isolate a subpopulation of patients who are at greatest risk of difficulty with balance and gait, and then later focuses on this subgroup to assess the burden of any balance and gait impairments as well as the impact of potential therapeutic interventions.
balance; clinical; essential tremor; falls; gait
Central nervous system inflammatory demyelinating disease can affect patients across the life span. Consensus definitions and criteria of all of the different acquired demyelinating diseases that fall on this spectrum have magnetic resonance imaging criteria. The advances of both neuroimaging techniques and important discoveries in immunology have produced an improved understanding of these conditions and classification. Neuroimaging plays a central role in the accurate diagnosis, prognosis, disease monitoring and research efforts that are being undertaken in this disease. This review focuses on the imaging spectrum of acquired demyelinating disease.
demyelinating disease; multiple sclerosis; MRI; neuroimaging
This paper reviews the clinical implications of topiramate (TPM)-induced cognitive deficits in patients with epilepsy, migraine headache, obesity, and in normal populations, followed by reviews of the literature describing the reversal of such deficits upon medication discontinuation. It also discusses animal investigations of TPM’s role of neuroprotection in brain injury. TPM’s most intolerable adverse effects (AEs) are on verbal fluency and reaction time, resulting in high discontinuation rates in patients taking it for epilepsy and migraine headache. However, because TPM is so effective in the treatment of epilepsy and migraine headache, its use is expected to continue. There appears to be greater tolerance of TPM’s cognitive AEs when it is used in the treatment of obesity, perhaps because of the lower doses required. Research attempting to predict the populations most vulnerable to the cognitive effects caused by TPM is ongoing. Studies suggest that one such population may include patients with a past psychiatric history. Slow titration and administration of the lowest possible doses may decrease risk of cognitive deficits.
cognition; epilepsy; migraine; obesity; psychiatric; topiramate
Multiple sclerosis (MS) is universally found to be more prevalent in women than men. This has led to extensive studies of differences in the immune system or nervous system between women and men, which might be caused by the effects of gonadal hormones, genetic differences, and different environmental exposures and modern lifestyle in men and women. We review the effects of sex and gender from a genetic, immunological and clinical point of view. We discuss the effects of sex on the clinical expression of MS and responses to therapy, as well as issues concerning pregnancy.
Multiple sclerosis; sex; female; pregnancy; genes; immunology
Fingolimod, the first oral disease-modifying therapy (DMT) approved for the treatment of multiple sclerosis (MS) and the only sphingosine 1-phosphate receptor modulator approved for any disease state, represents an important addition to the expanding DMT options for patients with MS. In three large phase III clinical trials, fingolimod 0.5 mg reduced relapses by approximately half compared with either placebo or weekly intramuscular interferon β1a. The risks associated with the use of fingolimod include first-dose bradycardia, macular edema, and elevation of liver enzymes; fingolimod may increase the risk of infections, some serious in nature, and potentially cause fetal harm. Breakthrough disease or intolerance of injectable medications may be factors that influence the initiation of fingolimod. Identification of potential patients suitable for fingolimod treatment requires a thorough understanding of the potential risks and the particular fingolimod indication of the national authority. To minimize risk, recommended baseline assessments that should be made prior to fingolimod initiation include complete blood count, liver transaminase levels, total bilirubin levels, electrocardiogram (ECG), ophthalmologic examination, varicella zoster infection status, and for women, childbearing potential. First-dose observation is required for all patients for at least 6 h, with hourly pulse and blood pressure measurements and ECG before and 6 h after the first dose. In the European Union, continuous telemetry monitoring is recommended. Healthcare providers should be aware of the potential for symptomatic bradycardia and the need for continuous overnight ECG monitoring for those at higher risk for bradycardia. With experience in over 63,000 patients and over 73,000 patient-years of exposure in clinical trials and postmarketing use, the benefits and full safety profile of fingolimod continue to become better elucidated. This information will enable healthcare providers to initiate fingolimod in appropriately selected and screened patients with MS.
fingolimod; multiple sclerosis; patient selection; risk; safety
Pediatric neuromuscular disorders comprise a large variety of disorders that can be classified based on their neuroanatomical localization, patterns of weakness, and laboratory test results. Over the last decade, the field of translational research has been active with many ongoing clinical trials. This is particularly so in two common pediatric neuromuscular disorders: Duchenne muscular dystrophy and spinal muscular atrophy. Although no definitive therapy has yet been found, numerous active areas of research raise the potential for novel therapies in these two disorders, offering hope for improved quality of life and life expectancy for affected individuals.
Duchenne muscular dystrophy; spinal muscular atrophy; therapy
Recent years have substantially broadened our view on the pathogenesis of multiple sclerosis (MS). While earlier concepts focused predominantly on T lymphocytes as the key cell type to mediate inflammatory damage within central nervous system (CNS) lesions, emerging evidence suggests that B lymphocytes may play a comparably important role both as precursors of antibody-secreting plasma cells and as antigen-presenting cells (APCs) for the activation of T cells. With greater appreciation of this pathogenic B-cell function in MS, B-cell-directed therapies, and in particular B-cell-depleting monoclonal antibodies targeting the CD20 molecule, have gained enormous interest over recent years. Clinical trials demonstrated that anti-CD20 treatment, which depletes immature and mature B cells but spares CD20 negative plasma cells, rapidly reduces formation of new inflammatory CNS lesions. While these findings clearly corroborate a pathogenic contribution of B cells, recent experimental but also clinical findings indicate that not all B cells contribute in an equally pathogenic manner and that certain subsets may in contrast mediate anti-inflammatory effects. In this review, we summarize current findings in support of pathogenic B-cell function in MS, including the encouraging clinical data which derived from anti-CD20 MS trials. Further, we review novel findings suggestive of regulatory properties of B-cell subsets which may be collaterally abolished by pan-CD20 depletion. In conclusion, we aim to provide an outlook on how this currently differentiating concept of pro- and anti-inflammatory B-cell function could be harnessed to further improve safety and effectiveness of B-cell-directed therapeutic approaches in MS.
Multiple sclerosis; B cell; anti-CD20; experimental autoimmune encephalomyelitis; antibody; cellular subsets; intrathecal; immune phenotyping; therapy
Multiple sclerosis (MS)-related spasticity is associated with disability and impairment in quality of life. We report on a patient with secondary progressive MS and spastic tetraparesis (Expanded Disability Status Scale score 8.5). The right arm exhibited flexor spasticity resulting in functional disability despite multimodal symptomatic treatment. Intrathecal baclofen led to side effects despite decreasing efficacy. Low-dose nabiximols improved spasticity and function with recovery of daily-life activities and spasticity-related symptoms. Reduction of intrathecal baclofen ameliorated adverse drug reactions. Add-on cannabinoid therapy was effective in therapy-refractory spasticity with supra-additive effect in combining intrathecal baclofen and nabiximols, hypothetically explained by mutually complementing mechanisms of action.
cannabinoids; CB1 receptor; multiple sclerosis; nabiximols; secondary progressive multiple sclerosis; spastic tetraparesis; symptomatic treatment
Impulse control disorders are a psychiatric condition characterized by the failure to resist an impulsive act or behavior that may be harmful to self or others. In movement disorders, impulse control disorders are associated with dopaminergic treatment, notably dopamine agonists (DAs). Impulse control disorders have been studied extensively in Parkinson’s disease, but are also recognized in restless leg syndrome and atypical Parkinsonian syndromes. Epidemiological studies suggest younger age, male sex, greater novelty seeking, impulsivity, depression and premorbid impulse control disorders as the most consistent risk factors. Such patients may warrant special monitoring after starting treatment with a DA. Various individual screening tools are available for people without Parkinson’s disease. The Questionnaire for Impulsive-Compulsive Disorders in Parkinson’s Disease has been developed specifically for Parkinson’s disease. The best treatment for impulse control disorders is prevention. However, after the development of impulse control disorders, the mainstay intervention is to reduce or discontinue the offending anti-Parkinsonian medication. In refractory cases, other pharmacological interventions are available, including neuroleptics, antiepileptics, amantadine, antiandrogens, lithium and opioid antagonists. Unfortunately, their use is only supported by case reports, small case series or open-label clinical studies. Prospective, controlled studies are warranted. Ongoing investigations include naltrexone and nicotine.
Impulse control disorders; Parkinson’s disease; restless leg syndrome; parkinsonism; dopamine agonist; non-motor complication; neurobehavioural
Rufinamide is a novel anticonvulsant medication approved by the US Food and Drug Administration (FDA) in 2008 for the treatment of seizures associated with Lennox–Gastaut syndrome in patients 4 years of age and older, based upon clinical trials demonstrating clinical efficacy and tolerability. Rufinamide is especially effective for tonic–atonic seizures in Lennox–Gastaut syndrome, but is subsequently proving to be safe and effective in clinical practice for a broad patient population with refractory epilepsy. Although further research and clinical experience is needed, rufinamide holds the promise to positively impact the care of children with epilepsy. In this review, we review the use of rufinamide in pediatric epilepsy, with a focus on efficacy and safety.
anticonvulsants; epilepsy; Lennox–Gastaut syndrome; rufinamide
Menstrual migraine (MM) is a highly prevalent condition associated with considerable disability. Migraine attacks occur exclusively around the menstrual period in approximately 10% of women with migraine, that is, pure menstrual migraine, while at least 50% of them also experience migraine at other times of the month, that is, menstrually related migraine (MRM). The therapeutic approach to patients with MRM is based on treatment of the attack, or prophylactic strategies. Triptans are recommended as first-line treatments for moderate to severe migraine attacks, including MM. Frovatriptan is one of the newest triptans. Its high affinity for 5-HT1B/1D receptors and long half-life contribute to its distinctive clinical effect, characterized by a more sustained and prolonged effect than other triptans. Indeed, frovatriptan proved to be effective in treating the acute attack, but was particularly effective in the short-term preventive therapy of MM. In addition, frovatriptan is one of the safest triptans, with the lowest risk of treatment-emergent adverse events. Following extensive evidence from randomized pharmacological trials, frovatriptan has now gained a grade A recommendation from the guidelines for short-term prophylaxis of MM. Recent post-hoc analyses of direct comparative trials also suggest that frovatriptan might have an important role in the acute treatment of MRM. In these studies, frovatriptan showed pain relief and pain-free rates similar to those of zolmitriptan, rizatriptan, and almotriptan, but with significantly lower recurrence rates. More well-designed, randomized, prospective studies, specifically enrolling women with MM, will be needed in the near future to confirm the efficacy of frovatriptan in this migraine subtype.
frovatriptan; menstrual migraine; menstrually related migraine
The treatment of familial amyloid polyneuropathy (FAP) requires a multidisciplinary approach, mainly neurological and cardiological. It includes specific treatments to stop the progression of systemic amyloidogenesis, the symptomatic treatment of the peripheral and autonomic neuropathy and the treatment of organs severely involved by amyloidosis (heart, eyes, kidneys). First-line specific treatment of met30 transthyretin (TTR) FAP is liver transplantation, which allows suppression of the main source of mutant TTR, to stop the progression of the neuropathy in 70% of cases in the long term and to double the median survival. In cases of severe renal or cardiac insufficiency, a combined kidney–liver or heart–liver transplantation can be discussed. Tafamidis (Vyndaqel) is a novel specific stabilizer of TTR which, in the very early stages of met30 TTR FAP, slows the progress of peripheral neuropathy. This drug should be proposed in cases of stage 1 symptomatic polyneuropathy. Other innovative medicines have been developed by biopharmaceutical companies to block the hepatic production of mutant and wild type TTR which are harmful in late-onset FAP (> 50 years old), including RNA interference therapeutics and antisense oligonucleotides, and to remove the amyloid deposits (monoclonal antibody antiserum amyloid P). Clinical trials should first assess patients with late onset FAP or non-met30 TTR FAP who are less responsive to liver transplantation or in case of significant progression of the neuropathy with Vyndaqel. Initial cardiac assessment and periodic cardiac investigations are important for patients with FAP because of the frequency of cardiac impairment, which is responsible for the high rate of mortality. Prophylactic pacemaker treatment should be discussed. Symptomatic treatments are required to improve patients’ quality of life. Familial screening of people with TTR mutation and regular follow up are essential. Appropriate clinical examination and complementary investigations are vital for the early detection of disease onset and to start specific therapy as soon as possible.
familial amyloidosis with polyneuropathy; gene therapy; liver transplantation; tafamidis; treatment
Natalizumab is approved for the treatment of patients with relapsing–remitting multiple sclerosis who have failed first-line treatment with traditional disease-modifying therapies or who have highly active disease. The drug has proved highly effective, both in a clinical trial setting and in clinical practice, with marked reductions in the rate of clinical relapses and slowed disease progression. These clinical outcomes are mirrored by a marked reduction in disease activity as evidenced by magnetic resonance imaging of the brain. However, natalizumab treatment has been associated with a risk of progressive multifocal leukoencephalopathy (PML), a potentially fatal condition caused by JC virus (JCV) activation. When this condition was detected in a clinical trial shortly after approval, the drug was immediately and voluntarily withdrawn from the market. As a condition of its reinstatement, stringent pharmacovigilance measures and a risk management plan were enforced. The recent availability of a two-step enzyme-linked immunosorbent assay (ELISA) test for the presence of anti-JCV antibodies (free testing is available in a central laboratory for registered centers), along with an ever-improving understanding of other risk factors such as prior immunosuppressant use and duration of treatment, allow an increasingly refined stratification of the risk of PML. This improved stratification of risk can help guide decisions about treatment. This review will also deal with other topics of relevance to clinical practice such as the development of antinatalizumab antibodies and their negative implications in terms of hypersensitivity reactions and loss of efficacy, withdrawal of treatment, and compassionate pediatric use.
multiple sclerosis; natalizumab; progressive multifocal leukoencephalopathy; risk stratification
The management of patients with chorea, in particular Huntington’s disease, is a complex task requiring skills in a number of areas. This paper reviews new knowledge on this topic and places it in the context of established procedures. It is focused on Huntington’s disease, since this is the disorder, for which most publications on management have been published in the past few years. Management starts with appropriate diagnosis and differential diagnosis, with the aim of finding disorders with chorea amenable to causative treatment. The place of genetic testing and the importance of genetic counselling is stressed, as well as the importance of precise observation in the course of the disorder to tailor appropriate therapies. Pharmacological treatment is based on poor evidence but to a large extent on expertise from centres devoted to the care of patients with Huntington’s disease. It is focused mainly on motor and psychiatric aspects of the phenotype. Nonpharmacological treatment is important and is best offered in a multidisciplinary care setting.
Chorea; Huntington’s disease; diagnosis; treatment; management
The contribution of vitamin D insufficiency to the pathogenesis of multiple sclerosis (MS) is reviewed. Among the multiple recently discovered actions of vitamin D, an immunomodulatory role has been documented in experimental autoimmune encephalomyelitis and in humans. This action in the peripheral immune system is currently the main known mechanism through which vitamin D might influence MS, but other types of actions could be involved within the central nervous system. Furthermore, vitamin D insufficiency is widespread in temperate countries and in patients with MS at the earliest stages of the disease, suggesting that the deleterious effects related to vitamin D insufficiency may be exerted in these patients. In fact, many genetic and environmental risk factors appear to interact and contribute to MS. In genetics, several human leukocyte antigen (HLA) alleles (more particularly HLA-DRB1*1501) could favour the disease whereas some others could be protective. Some of the genes involved in vitamin D metabolism (e.g. CYP27B1) also play a significant role. Furthermore, three environmental risk factors have been identified: past Epstein–Barr virus infection, vitamin D insufficiency and cigarette smoking. Interactions between genetic and environmental risk or protective factors may occur during the mother’s pregnancy and could continue during childhood and adolescence and until the disease is triggered in adulthood, therefore possibly modulating the MS risk throughout the first decades of life. Furthermore, some clinical findings already strongly suggest that vitamin D status influences the relapse rate and radiological lesions in patients with MS, although the results of adequately powered randomized clinical trials using vitamin D supplementation have not yet been reported. While awaiting these incontrovertible results, which might be long in coming, patients with MS who are currently in vitamin D insufficiency should be supplemented, at least for their general health status, using moderate doses of the vitamin.
Epstein–Barr virus; genetics; multiple sclerosis; smoking; vitamin D
Alzheimer’s dementia (AD) is increasingly being recognized as one of the most important medical and social problems in older people in industrialized and non-industrialized nations. To date, only symptomatic treatments exist for this disease, all trying to counterbalance the neurotransmitter disturbance. Three cholinesterase inhibitors (CIs) are currently available and have been approved for the treatment of mild to moderate AD. A further therapeutic option available for moderate to severe AD is memantine, an N-methyl-D-aspartate receptor noncompetitive antagonist. Treatments capable of stopping or at least effectively modifying the course of AD, referred to as ‘disease-modifying’ drugs, are still under extensive research. To block the progression of the disease they have to interfere with the pathogenic steps responsible for the clinical symptoms, including the deposition of extracellular amyloid β plaques and intracellular neurofibrillary tangle formation, inflammation, oxidative damage, iron deregulation and cholesterol metabolism. In this review we discuss current symptomatic treatments and new potential disease-modifying therapies for AD that are currently being studied in phase I–III trials.
Alzheimer’s disease; amyloid; disease-modifying drugs; inflammation; tau protein; therapeutic targets
Autonomic dysfunction is a characteristic of migraine attacks, rarely, even cardiac repolarization abnormalities have been associated with migraine. We report a case of documented ventricular tachycardia during basilar-type migraine attack. The therapeutic implications of such a co-occurrence as well as a possible relationship between ventricular tachycardia and the underlying biology of basilar-type migraine are discussed.
autonomic dysfunction; basilar-type migraine; cardiac arrhythmia; ventricular tachycardia
Bilateral vestibular loss (BVL) may present with or without vertigo and hearing loss. Amongst the causes of BVL are vestibulotoxic antibiotics, autoimmune ear diseases, Menière’s disease and meningitis. Clinical diagnosis of BVL is based on the result of three simple bedside tests: a positive head impulse test, reduced dynamic visual acuity and a positive Romberg test on foam rubber. With these signs, diagnosis of severe BVL is usually straightforward to establish.
bilateral vestibular loss; dynamic visual acuity; head impulse test; vertigo