Are Nonmotor Features of Essential Tremor Primary or Secondary Manifestations?
Essential tremor (ET) is one of the most prevalent movement disorders in older adults, presenting with both motor and nonmotor features. It has been widely discussed whether the nonmotor features are caused by widespread neurodegeneration or are secondary to impaired motor functions as a result of the disorder. In recent review of the literature, Ketan Jhunjhunwala, MD, and Pramod K Pal, MD, department of neurology, National Institute of Mental Health & Neurosciences, Karnataka, India, analyzed retrospective and prospective clinical studies to find a correlation between nonmotor feature severities and ET. The review was published in August in Tremor and Other Hyperkinetic Movements.
The authors state, “The basic question related to ET is whether it is a primary motor disease with secondary nonmotor features or a neurodegenerative disease that simultaneously affects various neural domains.” To answer this question, the authors reviewed articles related to ET and its features through May 1, 2014. The studies had to contain the following subject matter: ET being evaluated, nonmotor features discussed, patients being compared with control groups, and rated tremor and nonmotor features. Studies that lacked a control group and appropriate rating scores were excluded from the review.
From their review of post-mortem neuropathological studies conducted by the Essential Tremor Centralized Brain Repository, the authors found two different types of patients with ET: the first has degenerative cerebellar changes, and the second has brainstem Lewy bodies with a generally preserved cerebellum. “These post-mortem studies have highlighted the pathological heterogeneity of ET and its degenerative nature. This heterogeneity may account for the varied motor and nonmotor features that are currently being explored,” the authors noted. ET neuropathological studies show that it has a varied array of manifestations that may be owed to its degenerative process rather than a secondary circumstance, similar to that of Parkinson’s disease.
Upon further review, nonmotor deficits often found in ET patients in executive function, attention, concentration, and memory appear to be a primary manifestation of the disease. It remains to be seen whether other nonmotor features such as anxiety, depression, and sleep disturbances are primary or secondary to motor manifestations of ET.
The authors conceded that most studies are cross-sectional, and that follow-up studies are imperative for determining a new occurrence of nonmotor features or progression of existing features in ET patients, in addition to the existence of ET in “healthy” controls. Whether treatment of ET’s motor symptoms improves the nonmotor features is something that has yet to be determined. If proven, it would indicate the nonmotor features are secondary in ET. The authors suggested that further population-based studies be conducted to help verify this information.
– Elizabeth Ellis
