Pathologic findings in prospectively ascertained essential tremor subjects

Essential tremor (ET) is the most common movement disorder and is one of the more common neurologic conditions. Epidemiologic studies have found a prevalence rate as high as 12.6% in individuals aged 70 to 79 years.¹ Although ET is often thought to be “benign,” significant disability is reported by nearly all patients with ET.² However, despite this high prevalence and associated impact on society, little is known about the etiology and pathogenesis of ET. Routine brain imaging is generally normal, although magnetic resonance spectroscopy and blood flow imaging have supported abnormalities in cerebellar circuitry.^3–5 Genetic studies have linked ET to certain chromosomal loci in this often hereditary condition^6,7; however, pinning down the genetic abnormality has proved elusive. There have been few neuropathologic studies of patients with ET. Most studies have emphasized the lack of consistent brain pathology in ET. The largest study conducted was in 20 patients with ET followed up in a movement disorder clinic in Saskatchewan, Canada.⁸ Brain histopathology techniques were not detailed; however, the cerebellum and the substantia nigra were included in the analysis. No consistent abnormalities were found, unless the patient had additional features of Parkinson disease (PD; 6/20). The second largest study was conducted in 11 patients as part of the Honolulu-Asia Aging Study.⁹ Again, researchers found no consistent pathology. Furthermore, there may be the confound that the subjects were not specifically diagnosed as having ET, but rather the diagnosis of ET was presumed by the presence of an action tremor on one assessment using a PD rating scale. The reliability of this type of assessment in ET is not understood. The most recent study of 10 patients with ET proposed that nonspecific cerebellar degeneration and brainstem Lewy bodies (LBs) were seen more frequently compared with controls.¹⁰ We extend the neuropathologic evaluation of ET presenting 24 cases and compare findings to a control sample.

METHODS

The Sun Health Research Institute (SHRI) Brain and Body Donation Program actively recruits and prospectively assesses individuals longitudinally who have pledged to donate their organs for research purposes. All subjects signed informed consent approved by the SHRI Institutional Review Board. Although the program has focused on neurodegenerative diseases such as Alzheimer disease and PD, healthy controls and those with neurologic disease such as restless leg syndrome and ET are also encouraged to participate.

Subjects are assessed annually with a general neurologic examination, a movement disorder evaluation, and neuropsychological testing. The movement disorder evaluation consists of the Unified Parkinson's Disease Rating Scale, tremor rating scale,¹¹ restless leg syndrome rating scale (if applicable), and assessment of other involuntary movements (dystonia, myoclonus). Subjects are diagnosed with ET if they had previously been diagnosed by an outside physician and the examination was consistent with it. Additional subjects were diagnosed with ET if they had the presence of a Grade 2 postural or kinetic tremor of the hands or forearms without identifiable secondary cause or other exclusion criteria (e.g., prominent unilateral tremor, rigidity, or bradykinesia).^12,13 Subjects with a tremor score between 0.5 and 2 were also considered to have ET if the research evaluations found the tremor was present for at least 3 years with similar exclusion criteria. Additionally, the presence of isolated head tremor without dystonia was also included as ET. Subjects with PD or other movement disorders were excluded, as were those with dementia. Subjects meeting Petersen's criteria for mild cognitive impairment (MCI) on neuropsychological testing were included in the study.¹⁴

Brain pathology is evaluated according to standard protocols with descriptive assessment of cortex, basal ganglia, brainstem, and cerebellum. Routine hematoxylin and eosin staining is performed, along with Gallyas silver stain (neurofibrils, glial inclusions), Campbell–Switzer silver stain (amyloid plaques), thioflavine S (amyloid plaques and neurofibrillary changes), LB509 antibody (alpha-synuclein), and AT8 antibody (tau) staining. Quantitative values are given for age and brain weight. Semiquantitative assessment is performed for substantia nigra depigmentation, plaque total, plaque density (Consortium to Establish a Registry for Alzheimer's Disease), tangle score, Braak Alzheimer stage, LB score, and LB stage according to the Dementia with Lewy Bodies Consortium criteria. Brainstem assessment included the dorsal motor nucleus of the vagus in the medulla. Two subjects were excluded from quantitative comparison because they died before routine quantification of pathology. These subjects are still included in the descriptive table.

For statistical purposes, an age-matched control population of 21 subjects from the same donor population was used as a comparison group. These subjects had no evidence for a movement disorder on serial examinations and were not demented at time of death. Statistical significance between the two groups was calculated by using the two-sample t test or the Fisher exact test.

RESULTS

To date, 324 subjects enrolled in the Brain and Body Donation Program had tremor at their most recent movement disorder examination. Of these, 24 met criteria for ET before death and have come to autopsy. The median autopsy postmortem interval in this donor population is 2.7 hours. The mean age at death was 86.2 years, with a mean duration of tremor of 11.1 years (table). Only 11 of the 24 autopsied subjects had been formally diagnosed with ET during their lifetime; the remainders were diagnosed during their research assessments. Three subjects received a diagnosis of MCI by neuropsychological assessment; the remainder were cognitively normal. The 21 control subjects had a mean age of 86.9 years, and 4 had MCI.

The table details the descriptive pathologic findings in this ET population. Four subjects had moderate to marked depletion of locus ceruleus pigmented neurons. Two subjects had neocortical LBs; one had brainstem LBs limited to the locus ceruleus. Eleven subjects had at least some tangles seen in the substantia nigra or locus ceruleus. Seven subjects had cerebellar pathology, three with proliferation of Bergmann glia. Four subjects had incidental tauopathies; three were nonspecific cerebral tauopathies, and one met pathologic criteria for progressive supranuclear palsy (PSP). The subject with PSP had no evidence for parkinsonism or oculomotor dysmotility. Of the three subjects with MCI, one had an incidental cerebral tauopathy, one had cortical LBs, and the final had brainstem LBs in addition to cerebellar pathology.

In the control population, only one subject had at least moderate depletion of the locus ceruleus. There were two incidental LB cases, both cortical stage. Five subjects had tangles in the substantia nigra or locus ceruleus. Eight subjects had cerebellar pathology, with none having proliferation of Bergmann glia. There were two control subjects who were clinically normal but met pathologic criteria for PSP, and another two had an incidental cerebral tauopathy.

No significant differences were seen between cases and controls in brain weight, plaque total, plaque density, tangle total, Braak Alzheimer's stage, substantia nigra depigmentation, white matter rarefaction scores, or the presence of argyrophilic grains.

DISCUSSION

This study reveals the heterogeneous neuropathologic findings in subjects with ET. The most common finding was cerebellar degeneration. Our assessment of pathologic change in the cerebellum was limited to what was found at gross and with routine stains. We did not specifically quantify neurons or Bergmann gliosis. However, we did find greater gliosis in ET subjects than in controls in our population. Given previous pathologic evidence suggesting this,¹⁰ in addition to functional imaging studies implicating the cerebellum, it is likely that the cerebellum is involved in many cases of ET.

The relationship between ET and PD has been debated for years. Most recently, in a neuropathologic study, Louis et al.¹⁰ suggested that ET may sometimes represent a restricted form of LB disease because four of the six cases with LBs had more prominent involvement in the locus ceruleus. Our data do not support these findings. Although three of our ET cases were found to have LBs, only one had preferential involvement in the locus ceruleus. Furthermore, two of our control subjects had LBs, resulting in an overall insignificant difference. All of the LB cases may be considered incidental LBs because none showed clinical signs of PD. The reason for the different findings in our study may have to do with the way subjects were recruited for assessment and studied clinically. Many of the subjects in our brain donor population come into the program as neurologic controls; in fact, 13 of our subjects were not labeled as ET before being seen by us. Specifically recruiting ET may bias toward more advanced or escalating tremor patients who may indeed be developing PD clinically.

In addition to the cerebellar changes, locus ceruleus pathology was also frequently found in our ET subjects. Although not significant, depletion of locus ceruleus pigmented neurons and the presence of tangles was greater than in controls. This is despite notable similar pathology seen in the substantia nigra. If this were early PD or normal aging changes, one would suspect similar changes in both areas. Rajput et al.¹⁵ looked at the locus ceruleus biochemically in three ET patients and found higher levels of norepinephrine than in controls, the opposite of what is found in PD. Further quantitative assessment and additional biochemical study of this area is indicated.

Cognitive changes and tremor in the elderly may be associated with parkinsonian subtypes pathologically. Although this was not the focus of our study, the three cases included with MCI had LBs or tau changes pathologically. We specifically excluded subjects with ET and PD as well as ET with dementia; however, it would be worth further investigation of the pathologic substrate of action tremor associated with parkinsonism or cognitive changes. It is possible that some of the “tremor” seen in these elderly patients is cortical myoclonus/tremor. Clinical electrophysiology may help in defining this better.