A Case Series of Isolated Smiling Tremor: What Is the Phenomenology?

Abstract

Background

Five cases of tremor only upon smiling have been reported where no facial tremor is present at rest, when talking, or with full smile.

Cases

This report highlights four cases of tremor upon partial smiling, discusses the phenomenology of smiling tremor, and reviews the current literature. Four subjects with lower facial tremor present only upon smiling underwent movement disorders evaluation with video. Tremor frequencies were determined by parsing the video clips into 1‐second intervals and averaging the number of oscillations per interval and were determined to be high‐frequency 8 to 10 Hz irregular facial tremors with harmonic variations upon moderate effort in all cases. Slight or full‐effort smiling did not elicit facial muscle oscillations. Subjects had no other signs of tremor, dystonia, or parkinsonism on examination or in family history.

Conclusions

Tremor upon smiling only, or isolated smiling tremor, is a unique task‐ and position‐specific tremor of the facial musculature.

Tremor that occurs only upon smiling has been reported five times in the current literature. We present four cases of high‐frequency, irregular tremor that occurs upon moderate effort smiling only.

Four subjects (3 women, 1 man, ages 33–48 years) with lower facial tremor present only upon smiling (activation of risorius and levator anguli oris) underwent movement disorders evaluation with video. Tremor frequencies were determined by parsing the video clips into 1‐second intervals and averaging the number of oscillations per interval.

Tremor examination of the body, which is not shown in the attached subjects video, included evaluation for tremor at rest, distraction looking for rest tremor of the face, chin, hands and legs (with serial 7's or saying months of year backwards or while performing other exam maneuvers while the face was at rest), arms outstretched with palms down, followed by wing‐beating position, followed by arms outstretched with palms up, followed by arms outstretched with hands facing each other, and with action on finger‐nose‐finger. In addition to examining the body for tremor, all patients underwent a complete movement disorders examination, including evaluation of facial expression, speech, muscle tone, bradykinesia (finger tapping, hand opening/closing, rapid alternating movements, foot taps, and foot stomps), ataxia (eye movement examination, finger‐nose‐finger, finger chase, fast alternating hand movements), and gait examination (noting arm swing, base, speed, heel strike and foot lift, cadence, and stance). Postural stability was not assessed as none of the subjects had gait impairment on history or examination.

Case Series

Case 1

Patient 1 is a 33‐year‐old woman who developed tremulousness of the face upon partial smiling with half or moderate effort for over a year. Full effort smiling does not induce tremor. When the tremor occurs, she also had associated fatigue in her mouth and jaw. Video examination of patient 1 (Video 1) shows 8 to 10 Hz tremor involving the risorius, levator anguli oris, and the zygomaticus. Tremor severity in amplitude lessened over the course of 18 months from presentation with no intervention; however, tremor was still present with moderate effort smiling with otherwise unchanged examination.

Video 1.

Subject examination of smiling tremor. Three of the 4 subjects' video exams are depicted demonstrating a high frequency tremor that occurs with smiling or activation of muscles involved in smiling.

Case 2

Patient 2 is a 43‐year‐old woman who reports tremor upon smiling for 13 months. Tremor of her lower face does not occur at rest or upon a full effort smile. The tremor emerges with her natural, half‐smile and can sometimes come out when holding a position of the risorius or levator anguli oris. Electromyography/nerve conduction studies were negative for myokymia, myopathy, or generalized large fiber polyneuropathy. The Electromyography (EMG) report was notable for “8–9 Hz tremor on sustained activation (Fig. 1).” Follow‐up with the patient 14 months after the initial evaluation showed no change in tremor, emergence of tremor elsewhere, or new neurological symptoms or signs.

Figure 1.

Single channel EMG recording of the nasalis muscle during a partial smile from case 2. Note the semi‐regular, variable duration EMG bursts.

Case 3

Patient 3 (Video 1) is a 48‐year‐old woman who reports an 8‐month history of tremor upon smiling. The tremor began around her mouth and progressed to involve the zygomaticus. Tremor is absent at rest and with a full smile but emerged with a half‐smile or natural smile. She was treated with low‐dose botulinum toxin therapy in the bilateral risorius and levator anguli oris. Initial treatment of botulinum toxin injection (diluted 50 units per 1 cc) 1.25 units in each muscle group bilaterally provided little benefit. With increase of botulinum toxin (diluted 50 units per 1 cc) dosing to 2.5 units bilaterally in each muscle group, tremor severity reduced by approximately 75%. Prior medication exposure includes sertraline, which was started around the time of tremor onset. Uptitration to sertraline 100 mg from starting dose of 50 mg did not worsen the tremor.

Case 4

Patient 4 is a 36‐year‐old man who presents with many years of tremoring of his lower face. Tremor occurs with his half smile and when holding a position of activation of the risorius and levator anguli oris, for example when saying “aayy” and contracting these muscles at the same time. His normal speech does not elicit tremor. Examination shows an 8 to 10 Hz tremor with notable harmonic overtones. He was lost to follow‐up clinically. In a phone conversation approximately a year after his presentation, his tremor was unchanged, and no new neurological symptoms arose.

In their aggregate, these four cases demonstrate a unique, isolated, irregular lip and face tremor of high frequency 8 to 10 Hz that is absent at rest, occurs with only partial smiling, and does not occur with other facial movements, such as talking, drinking, or eating. Further, the subjects have no family history of tremor, no tremor noted elsewhere in the body, and no signs of parkinsonism or dystonia. Except for testing for postural instability, the United Parkinson's disease Rating Scale was 0 for each case.

Discussion

We describe four cases of high‐frequency 8 to 10 Hz irregular tremor that occurs only upon smiling, which we are terming isolated smiling tremor. Four cases demonstrated rhythmic and oscillating movements of the lower face upon partial smiling. Tremor is defined as rhythmic and oscillating, however, considering other potential movement disorders in crucial. Besides tremor, myokymia and myoclonus could be considered as the possible presentation. EMG/nerve conduction studies (NCS) of one patient was negative for myokymia and myoclonus. The muscle movements seen in these four cases do result in large displacement of the lower facial muscles, which is not typically seen in myokymia. Action myoclonus can also be considered as this can be repetitive. The rhythmicity of the lower face movements in these cases, however, is most consistent with tremor rather than myoclonus. Additionally, the presence of oscillatory movements only with partial smiling and not full effort smiling is more indicative of tremor as one would consider myoclonus to be present with more forceful action movements of the face.

Smiling itself is a complex action that involves multiple facial muscles. “Risus tremulous” has previously been offered to name this tremor, however, our cohort has many other muscles involved, including the levator anguli oris, and in two cases, the zygomaticus, making the term risus tremor incomplete. None of our reported subjects have associated parkinsonism, tremor elsewhere in the body, or dystonia.

Orolingual tremors are defined as involuntary oscillatory and rhythmic movements of the jaw, tongue, pharynx, or lower face. Classification of orolingual tremors is complicated ¹ as tremor is generally defined as regular or irregular oscillatory movement around a joint and facial muscles are not connected via joints. In a limb, repetitive torque on a joint produces the oscillation we characterize as tremor. ² Further, the delineations of rest, posture, and kinetic are not easily applied to orolingual area. ¹ Regarding isolated smiling tremor, classification is even more elusive due to the rarity of the disorder.

In this study, we describe isolated smiling tremor phenomenologically and comparatively to previously reported cases. Apart from analyzing the facial tremor movements from video segments, physiological assessments were not performed. One subject had EMG/NCS that showed no presence of neuropathy, myokymia, or any neuromuscular condition that may result in tremor.

To our knowledge, smiling tremor has only been reported in four publications. ³ , ⁴ , ⁵ , ⁶ In these previously reported cases, a high‐frequency tremor is noted with a half or moderate smile. The first ever reported case of a 27‐year‐old had a pertinent family history of facial tremor. ³ Only one case associates an underlying disease of young‐onset Parkinson's disease. However, this subject's tremor was action‐specific and non‐responsive to dopaminergic therapy, ⁴ providing evidence against a parkinsonian phenomenon. Another case of a 32‐year‐old man was noted to have hypomimia, slight rigidity, and slight bradykinesia but a negative DaTscan. He was trialed on propranolol and primidone with no effect. ⁵ The most recent reported case of progressive worsening tremor upon smiling was that 16‐year‐old with onset of tremor onset in the early teens without family history of tremor. ⁶

The lower facial tremors seen in our four cases and prior reported cases are all focal and induced only on partial smiling. The action‐specific nature of this tremor without rest components suggests that parkinsonism is an unlikely etiology. Interestingly, rabbit syndrome is a low‐frequency rest tremor of the perioral region associated with Parkinson's disease and drug‐induced parkinsonism with no similarities to smiling tremor. Conditions with action tremors, for example isometric, enhanced physiologic, essential, dystonic, and isolated task and position‐specific tremors are possible etiologic considerations.

Isometric tremors worsen with increasing load. While facial movements are not isometric, smiling tremor may be similar to isometric tremor as it worsens with increased smiling effort. However, unlike the phenomenology of isometric tremor, isolated smiling tremor abates with a full effort smile.

Enhanced physiologic mechanisms may play a role in isolated smiling tremor. Physiologic tremor is thought to be due to sensitization of muscle spindles which in turn amplifies rhythmic oscillation and synchronization. ⁷ Facial muscles, particularly around the mouth, are devoid of muscle spindle fibers, ⁸ arguing against smiling tremor being modulated by typical enhanced physiologic mechanisms.

Isolated task‐ and position‐specific tremors are produced during certain tasks or held positions. These tremors can be associated with essential tremor or dystonic tremor. ⁹ The semi load‐specific nature with clinical “null” point of no tremor on maximum smile makes a purely essential tremor phenomenon incongruous with the nature of smiling tremor. Additionally, isolated lip tremor is not consistent with classical essential tremor given the absence of tremor in any other body region. ⁹

While dystonia was not present in the lower face or other body regions, isolated smiling tremor shares similar properties to focal dystonic tremor and might be classified as a task‐related dystonic tremor. Smiling may be considered a task that uses muscles to maintain a facial position, and smiling tremor exhibited a “null” point with no tremor on maximum smile, similar to dystonic tremor. Also, like dystonic tremor, the type of tremor exhibited in isolated smiling tremor is irregular. Further, isolated smiling tremor shares features with embouchure dystonia, ¹⁰ which is tremulous with rapid semi‐rhythmic lip movements. The similarities of high frequency rhythmic shaking that is seen in both entities does raise interesting questions as to the tremor generation in isolated smiling tremor.

As detailed above, isolated smiling tremor is difficult to categorize but appears acquired, focal, isolated task‐ and position‐specific. ⁹ Phenomenologically, isolated smiling tremor best fits as an isolated task‐and position‐specific orolingual tremor as it exclusively only occurs during the period of moderate or half smiling. For this reason, it could be classified as a dystonic tremor. More reported examples are necessary to further characterize this tremor. Although one prior reported case is described in the setting of verified Parkinson's disease, based on our subject cohort and other reported cases, smiling tremor appears to be an isolated, discrete phenomenon.

Author Roles

(1) Research Project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript: A. Writing of the first draft, B. Review and Critique.

S. R.: 1A, 1B, 1C, 3A, 3B.

C. J. A.: 3B.

S. L. P.: 1B, 1C, 3A, 3B.

L. H. W.: 1C, 3B.

Disclosures

Ethical Compliance Statement: The authors confirm that the approval of an institutional review board was not required for this work. Informed patient consent was obtained. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines.

Funding Sources and Conflicts of Interest: Consultancies: Best In Class MD (S.R.), Roche (L.H.W.). Advisory Boards: Dystonia Medical Research Foundation: Musicians with Dystonia, Scientific Advisory Board (S.L.P.); Tremor and Other Hyperkinetic Movement Disorders, Editorial Board (S.L.P.), National Medical Advisory Board for the Charcot–Marie‐Tooth Association (L.H.W.), Current Neurology and Neuroscience Reports (L.H.W.). Employment: Albany Medical Center (S.R.); Columbia University Irving Medical Center (A.J.C., S.L.P.). The authors report no conflicts of interest related to the research of this manuscript.

Financial Disclosures for the Previous 12 Months: None.