The Mystery of the Inner Shakes

By Dr. Dietrich Haubenberger

As physicians interacting with essential  tremor patients and their families, there are several areas where medical research has not yet given us good explanations for, or strategies to manage symptoms that are so commonly reported to us – such as internal tremor. On a regular basis, patients with ET report feeling “internal tremor”, “inner tremor” or the “inner shakes”.  Often, it is even hard to separate which tremor is more bothersome, the “external” or visible tremor, or the internal tremor. Internal tremor is frequently described as a quivering movement sensation inside the trunk or inside the limbs, without actual visible movement. This sensation is typically described as being unpleasant, and it may even be the symptom causing the greatest discomfort. It is often difficult to explain, whether the sensation of inner tremor is a feeling in the absence of actual movement, or whether there is actual tremor-movement, which is just too fine to be visibly noticeable.

ET is defined as a tremor of the hands and arms, which can also affect other body areas such as the head, legs, voice, or trunk. In the medical literature, the description of ET – or any other tremor disorder – is focused on the visible shaking of one or more body parts. Therefore, there are many questions that need to be answered in order to understand internal tremor: What do patients with internal tremor experience? How common is internal tremor? Is internal tremor restricted to patients with already existing tremor disorders? Is internal tremor in patients with tremor disorder similar to what patients with anxiety disorders experience as “trembling” sensation, e.g., from fear?

Much more is unknown about internal tremor than what is known. The medical literature is indeed quite sparse when it comes to the research that has been done to find the potential cause(s) of internal tremor. The first and, so far, most detailed report about internal tremor was examining internal tremor in patients with Parkinson’s disease (PD). The goal of this study, which was published 20 years ago, was to answer the questions on how common internal tremor is in patients with PD, the characteristics distinguishing patients with from patients without internal tremor, as well as the features of internal tremor.  To answer this question, 100 patients with PD were asked to complete a questionnaire during one of their clinic visits at the Movement Disorders Center of the University of Miami. The results were compared to the questionnaires completed by 50 control subjects without PD. Just under half, 44%, of patients with PD experienced internal tremor, while only 6% in the control group. While slightly more women experienced internal tremor than man (52% vs. 35%), internal tremor was equally present in patients in mild or more severe stages of the disease. Patients with other abnormal sensory symptoms (aching, burning, tingling) were more likely to have internal tremor. While in the group of patients with internal tremor, resting tremor was present slightly more often than in patients without internal tremor (91% vs. 80%), the extent of visible tremor was not associated with a higher likelihood of internal tremor. Internal tremor was felt both in the limb, in the trunk (neck, chest, and/or abdomen), or both in limbs and trunk.

Regarding medication, patients with internal tremor were more likely to be taking anti-anxiety medication as well as the anti-PD drug levodopa. Episodes of internal tremor were usually short: less than five minutes in 28% of patients and less than 30 minutes in 61% of patients. While not 100%, still the majority of patients (64%) reported they noticed internal tremor when they were feeling anxious. And the internal tremor responded in 41% of patients to relaxation or anti-anxiety medication.

While this first research report focused on the symptoms of internal tremor in PD patients, another report studied ET patients as well as patients with tremor related to multiple sclerosis (MS). While the frequency of internal tremor in PD appeared to be lower than in the first study (33%), internal tremor was most common in the group of ET patients. Although the group of ET patients studied was small, six out of 11 ET patients in the study reported symptoms of internal tremor. Due to the small group of study participants, no further comparisons regarding characteristics and features of the symptom were made for ET. In the group of patients with PD and MS, however, internal tremor was associated with visible tremor, as well as anxiety.

What can we learn from these reports? Mainly that internal tremor is present in disorders beyond ET. It is a symptom that is common and therefore should be taken seriously. As a patient experiencing internal tremor, it is important to note these are common symptoms. One central point to take away from the reports on internal tremor is there are likely several causes for internal tremor. Internal tremor can be caused by actual tremor activity in muscles (e.g., of the limbs), which is subtle and not yet visible to the eye of the patient him/herself or the examiner. The association with anxiety in a majority of patients is furthermore an important sign to be recognized, especially as anxiety is more common in ET than in subjects without ET.  However, not all patients with internal tremor also suffer from anxiety, therefore internal tremor needs an individualized approach.

It is important that your physician recognize your internal tremor as a real symptom, which can have different potential causes. When physicians initiate therapy for the (visible) tremor, it is important to monitor the response on all presentations of tremor and distinguish between external and internal tremor.  If internal tremor is present and possibly also does not respond to tremor-therapy, the doctor may screen for symptoms of anxiety, as this would require a different management approach. Internal tremor may be the first symptom of an otherwise not yet recognized anxiety disorder.

In summary, the recognition of internal tremor as a symptom in tremor patients is a crucial first step for both physicians as well as patients to identify a potential underlying cause, and to eventually select a successful approach for treatment. More research needs to be conducted to identify the individual contributing factors for this clearly common and impairing symptom seen in patients with essential tremor.

Dietrich Haubenberger, MD – Director, Clinical Trials Unit, National Institute of Neurological Disorders and Stroke (NINDS), Intramural Research Program, National Institute of Health (NIH), Bethesda, MD, and member of the IETF’s Medical Advisory Board.