Orthostatic Tremor - How unique is it?

Research Fellow, Leah Jones reports on a project that she has been working on at Charring Cross Hospital, London with Dr Peter Bain.

The purpose of this study was to portray an accurate picture of Orthostatic Tremor (OT) by comparing it to other tremors. It has been classed as a separate, unique tremor (despite some common features), and we’ve shown that it is quite disabling; that treatments are unsatisfactory and much further work needs to be done in this field.

By characterising the tremor we have provided further information for patients living with this disabling condition and highlighted the need for increased awareness of OT among physicians.

OT is a rarely reported condition. Some will be very familiar with this condition, either living with it, or have cared for people with the tremor, and others will have diagnosed patients with the disorder, but for some this will be the first time you’ve come across OT. So to begin with I will briefly describe the background of OT.

The tremor is defined as a feeling of unsteadiness when standing that is relieved by sitting or walking. This obviously has a strong impact on the patient’s quality of life and tends to avoid situations where they have to stand still. But despite the fear of falling, falls and injuries are rare.

OT can be confirmed neurophysiologically by making electromyographic recording of the lower limbs, which show a much higher frequency than any other tremor. On standing the frequency can be between 13-18Hz, this is very high compared to, say, essential tremor which has an average frequency of 6-8Hz.

Unlike any other tremor the timing between tremor bursts in different muscles remain constant, so there’s high coherence. Also, despite lower limb predominance, the tremor is not confined to the legs. Postural tremor may also appear in the arms. And one case has even been reported to be present in the jaw.

The primary cause of OT is unclarified but a number of hypotheses have been sought. A FH is uncommon, although reports have been published that may suggest a genetic basis.

The most widely accepted concept is that there’s a primary CNS generator (due to the high frequency tremor that radiates bilaterally), unlike any other tremor. The location of the oscillator remains unknown, but PET studies have shown abnormal bilateral over activity in the cerebellum. Other reports indicate an impaired DA system, similar to other movement disorders, like Parkinson’s disease.

Literature constantly debates whether OT is a distinct condition, or a variant of ET. The argument for overlap comes from the presence of an UL tremor in OT and some FH of ET in OT patients. But we already know the frequencies differ, and that the pharmacological responses are different.

Study design

It is a very rare disorder, and the literature describing its demographics, clinical features and treatments are sparse and usually only describe a few cases. Therefore the purpose of the study was to characterise the clinical features of OT using the largest series of patients to date.

To achieve this current literature was reviewed and an audit study was performed.

The study comprised:

• 56 individuals all diagnosed with OT (based on clinical presentation and EMG frequencies (if available), that were seen at either Charing Cross Hospital or the Cromwell Hospital by Dr Bain in the last 18 years.
• Data was collected from their medical records, which mainly comprised of clinic letters, detailing the patient’s history and examination findings, investigation results and response to treatments.
• A database with 7 sub-sections, including demographics, clinical features and treatments, was constructed.
• Finally the findings were analysed and using Microsoft Excel simple statistics (including mean, ranges and percentages) tables were produced.

A vast amount of data was produced in the study and the most relevant results are:

• Analysis of 56 cases confirmed a female preponderance and a middle-age onset, with a 51.4 year mean age. But the distribution of age of onset only had a single peak, where ET is bimodal whereas ET presents at least a decade earlier. We also noted that the duration from onset to diagnosis of OT shows lack of awareness of the condition- the mean duration being over 9 years.
• Our data shows that OT has a weak hereditary component.
• Only 5.4% had relatives who also suffered from a similar clinical picture. The same percentage had a first degree relative with Parkinson’s disease.
• The strongest association was with ET, 10.7% having first degree relatives and 8.9% second degree relatives with ET.

We have further defined the parts of the body afflicted by OT:

• Symptomatic leg tremor was ubiquitous, but OT is not just a condition of the legs it seems, it affected the arms in about 50%, the head and trunk in a smaller percentage, and voice and tongue tremor was rare.
• Tremulous writing and spiral drawings were present in 26.8% and 58.9% respectively. Spirals can be used to diagnose the severity of a tremor, and graded 0-10, 10 being the most severe. 55.4% of the patients had undergone EMG studies, only 41.1% had exact information regarding the tremor frequency. Around half the patients were therefore diagnosed on clinical history and examination alone.
• A high frequency tremor is diagnostic of OT; the mean frequency recorded in this study was 15.6Hz with the majority distributed between 14-18Hz, and 2 atypical recordings at 10Hz and 20Hz.
• Unfortunately some patients who went for EMG recordings had normal results; due to the fact the physiologist didn’t stand the patient up, therefore failing to elicit the high frequency OT.

Cleary, in future this requirement needs to be specified to the physiologist by the referring physician.

This article is from the NTF News December 2009 issue. To get access to the rest of the NTF News please become a member.