A Pilot Efficacy and Tolerability Trial of Memantine for Essential Tremor

WHIGET and TETRAS Ratings of Action Tremor in Patients with Essential Tremor: Substantial Association and Agreement

Background

Evaluating tremor severity is a critical component of diagnosing and clinically managing patients with essential tremor (ET). We examined the comparability of tremor severity ratings derived from two frequently used tremor rating scales: the Washington Heights-Inwood Genetic Study of Essential Tremor (WHIGET) rating scale and the Tremor Research Group Essential Tremor Rating Scale (TETRAS).

Methods

A trained assistant administered and videotaped a neurological examination, including eight items assessing upper limb action tremor (arms outstretched, arms in the wingbeat position, finger-nose-finger maneuver, and drawing of Archimedes spirals). An experienced movement disorders neurologist reviewed the videos and assigned WHIGET and TETRAS ratings. We calculated associations between TETRAS and WHIGET ratings using Spearman rank order correlations. Subsequently, we collapsed these ratings into four tremor severity categories (absent, mild, moderate, severe) and then two broader tremor severity categories (absent/mild, moderate/severe). We calculated weighted Kappa coefficients to assess agreement between category assignments based on the TETRAS and the WHIGET.

Results

Spearman's r' s were significant for all items (p's ≤ 0.001, mean r = 0.89). Weighted Kappa's revealed substantial to near perfect agreement for all eight items (mean k = 0.86, range = 0.64 to 1.00).

Conclusion

Analyses revealed substantial strength of association and substantial to near perfect agreement between items rated with the WHIGET and TETRAS scales. These data indicated that ratings provided by each scale are highly comparable.

Current and Future Neuropharmacological Options for the Treatment of Essential Tremor

BACKGROUND

Essential Tremor (ET) is likely the most frequent movement disorder. In this review, we have summarized the current pharmacological options for the treatment of this disorder and discussed several future options derived from drugs tested in experimental models of ET or from neuropathological data.

METHODS

A literature search was performed on the pharmacology of essential tremors using PubMed Database from 1966 to July 31, 2019.

RESULTS

To date, the beta-blocker propranolol and the antiepileptic drug primidone are the drugs that have shown higher efficacy in the treatment of ET. Other drugs tested in ET patients have shown different degrees of efficacy or have not been useful.

CONCLUSION

Injections of botulinum toxin A could be useful in the treatment of some patients with ET refractory to pharmacotherapy. According to recent neurochemical data, drugs acting on the extrasynaptic GABAA receptors, the glutamatergic system or LINGO-1 could be interesting therapeutic options in the future.

Oral Memantine for the Treatment of Glaucoma: Design and Results of 2 Randomized, Placebo-Controlled, Phase 3 Studies

PURPOSE

To evaluate the effectiveness and safety of oral memantine as a potential neuroprotective agent in open-angle glaucoma (OAG) at risk for progression.

DESIGN

Two randomized, double-masked, placebo-controlled, parallel-group, multicenter, 48-month studies identically designed, initiated 1 year apart, and completed in 2006. Protocol amendments included a 1-year extension (first study) and change in primary endpoint and analysis (second study).

PARTICIPANTS

Patients (2298 total) with bilateral OAG; glaucomatous optic disc damage and visual field loss in 1 eye; glaucomatous optic disc damage and/or visual field loss in the contralateral eye (at screening), topically treated or untreated intraocular pressure (IOP) of 21 mmHg or less (at baseline); and at risk of glaucomatous progression (per prespecified criteria).

METHODS

Patients were randomized 3:2:2 to receive memantine 20 mg, memantine 10 mg, or placebo tablets daily. Glaucomatous progression was assessed in the intent-to-treat population by full-threshold standard automated perimetry (SAP), frequency doubling technology (FDT), and stereoscopic optic disc photographs, standardized by quality control assessment at centralized reading centers. Safety evaluations included adverse events (AEs), best-corrected visual acuity, biomicroscopy, IOP, and ophthalmoscopy. Efficacy data from each study were analyzed per protocol. Pooled analyses of efficacy and safety data were also performed.

MAIN OUTCOME MEASURES

The predefined primary efficacy measure was glaucomatous visual field progression, as measured by SAP. Additional efficacy measures included glaucomatous progression of visual field (FDT) and optic nerve damage (stereoscopic optic disc photographs).

RESULTS

The proportion of patients who completed the studies was similar among groups (80%-83%). Compared with placebo, daily treatment with memantine 10 mg or 20 mg for 48 months did not delay glaucomatous progression significantly in the individual studies and pooled analyses. The pooled risk reduction ratio (95% confidence interval) assessed by SAP was -0.13 (-0.40, 0.09) and -0.17 (-0.46, 0.07) for memantine 10 mg and 20 mg, respectively. Results were similar per FDT and stereoscopic optic disc photographs. The most common AEs leading to treatment discontinuations were dizziness, headache, fatigue, and nausea.

CONCLUSIONS

With technologies available when the studies were conducted, daily treatment with memantine over 48 months was not shown to prevent glaucomatous progression in this patient population.

Essential Tremor: What We Can Learn from Current Pharmacotherapy

BACKGROUND

The pathophysiology of essential tremor, especially at the cellular level, is poorly understood. Although no drug has been specifically designed to treat essential tremor, several medications improve tremor, and others worsen it. Studying the mechanism of actions of these medications can help our understanding of tremor pathophysiology and contribute to future rational drug design.

METHODS

We reviewed literature, concentrating on mechanisms of action, of various medications that mitigate tremor.

RESULTS

Many medications have multiple mechanisms of actions, making simple correlations difficult. Medications that increase the duration of opening of gamma-aminobutyric acid (GABA)-A receptors are most consistently associated with tremor improvement. Interestingly, drugs that increase GABA availability have not been associated with improved tremor. Other mechanisms possibly associated with tremor improvement include antagonism of alpha-2 delta subunits associated with calcium channels, inhibition of carbonic anhydrase, and inhibition of the synaptic vesicle protein 2A. Drugs that block voltage-gaited sodium channels do not affect tremor. The ideal beta-adrenergic blocker requires B2 affinity (non-cardiac selective), has no sympathomimetic properties, does not require membrane stabilization properties, and may benefit from good central nervous system penetration.

DISCUSSION

To date, serendipitous observations have provided most of our understanding of tremor cellular physiology. Based on similarities to currently effective drugs or rational approximations and inferences, several currently available agents should be considered for tremor trials.

Pharmacotherapy of essential tremor

Essential tremor (ET) is a common movement disorder but its pathogenesis remains poorly understood. This has limited the development of effective pharmacotherapy. The current therapeutic armamentaria for ET represent the product of careful clinical observation rather than targeted molecular modeling. Here we review their pharmacokinetics, metabolism, dosing, and adverse effect profiles and propose a treatment algorithm. We also discuss the concept of medically refractory tremor, as therapeutic trials should be limited unless invasive therapy is contraindicated or not desired by patients.

The cognitive side of essential tremor: what are the therapeutic implications?

While essential tremor (ET) has traditionally been categorized as a pure motor disease, cross-sectional and longitudinal studies of cognition in ET have demonstrated that these patients may have cognitive dysfunction. Recent epidemiological studies demonstrate an association between ET (particularly with onset after age 65) and increased risk for cognitive impairment and dementia. Although existing studies have generally conceptualized cognitive changes in ET as consistent with a 'frontosubcortical' or 'corticocerebellar' profile, results from these same studies suggest that cognitive impairment in ET may in fact be heterogeneous. Furthermore, the underlying mechanisms remain uncertain. Cognitive changes could be a byproduct of the cerebellar dysfunction of ET itself; alternately, they may be a feature of concomitant neurodegenerative diseases that have been associated in several studies with ET, including Alzheimer's disease, Parkinson's disease or progressive supranuclear palsy. While the study of cognitive dysfunction in ET has received research attention in recent years, the results of these studies have not been translated into the clinical domain and clinical practice. This review first summarizes the current literature on the potential relationships between ET and cognitive change. We then suggest areas of further clinical evaluation and treatment; these suggestions are directed at physicians caring for ET patients who may demonstrate or complain of cognitive impairment. As we discuss, clinicians should ideally screen ET patients for possible signs or symptoms of cognitive impairment in addition to assessing for psychiatric comorbidity and quality of life. These recommendations are in contrast to most current clinical practice, which does not routinely include such assessment among ET patients. To our knowledge, there have been no pharmacotherapeutic trials to date of any agent for cognitive change associated with ET. We believe that studies for this indication are now called for. Future efforts in this direction will also need to take into account the pathobiology of cognitive changes in ET, which itself is an area that is ripe for future investigations.

Harmaline tremor: underlying mechanisms in a potential animal model of essential tremor

BACKGROUND

Harmaline and harmine are tremorigenic β-carbolines that, on administration to experimental animals, induce an acute postural and kinetic tremor of axial and truncal musculature. This drug-induced action tremor has been proposed as a model of essential tremor. Here we review what is known about harmaline tremor.

METHODS

Using the terms harmaline and harmine on PubMed, we searched for papers describing the effects of these β-carbolines on mammalian tissue, animals, or humans.

RESULTS

Investigations over four decades have shown that harmaline induces rhythmic burst-firing activity in the medial and dorsal accessory inferior olivary nuclei that is transmitted via climbing fibers to Purkinje cells and to the deep cerebellar nuclei, then to brainstem and spinal cord motoneurons. The critical structures required for tremor expression are the inferior olive, climbing fibers, and the deep cerebellar nuclei; Purkinje cells are not required. Enhanced synaptic norepinephrine or blockade of ionic glutamate receptors suppresses tremor, whereas enhanced synaptic serotonin exacerbates tremor. Benzodiazepines and muscimol suppress tremor. Alcohol suppresses harmaline tremor but exacerbates harmaline-associated neural damage. Recent investigations on the mechanism of harmaline tremor have focused on the T-type calcium channel.

DISCUSSION

Like essential tremor, harmaline tremor involves the cerebellum, and classic medications for essential tremor have been found to suppress harmaline tremor, leading to utilization of the harmaline model for preclinical testing of antitremor drugs. Limitations are that the model is acute, unlike essential tremor, and only approximately half of the drugs reported to suppress harmaline tremor are subsequently found to suppress tremor in clinical trials.

The effect of memantine in harmaline-induced tremor and neurodegeneration

Essential tremor (ET) is one of the most common and most disabling movement disorders among adults. The drug treatment of ET remains unsatisfactory. Additional therapies are required for patients with inadequate response or intolerable side effects. The current study aims to investigate the anti-tremogenic and neuroprotective effects of memantine (NMDA receptor antagonist) on the harmaline model of transient action tremor. The effects of memantine were further compared with ethanol. Three separate groups of male Wistar rats were injected either with saline, ethanol (1.5 gr/kg), or memantine (5 mg/kg) 15 min prior to a single intraperitoneal injection of harmaline (20 mg/kg). Tremor and locomotion were evaluated by a custom-built tremor and locomotion analysis system. After 24 h of harmaline injection, cellular viability, and apoptosis were assessed using crystal violet staining, and caspase-3 immunostaining, respectively. Harmaline caused neuronal cell loss and caspase-3 mediated apoptosis in cerebellar granular and purkinje cells as well as the inferior olivary neurons. Despite a reduction in tremor intensity and duration with ethanol, this compound resulted in cell loss in cerebellum and olivary nucleus. Memantine exhibited neuroprotective efficacy on cerebellar and inferior olivary neurons albeit weaker anti-tremor effect compared to ethanol. In conclusion, anti-tremogenic and neuroprotective effects do not necessarily overlap. Memantine is a potential treatment for ET particularly given its neuroprotective efficacy.