Tremor is one of the important motor phenotypes of dystonia and can be present in approximately 12–86.7% of patients. [1-3] Among focal dystonia, tremor is more frequently seen in cervical and upper limb dystonia (ULD). [1,2] A remarkable feature of ULD is its task specificity, meaning that dystonia manifests only during a specific motor task such as writing, typing, or playing a musical instrument. [4] Although over time task-specificity may be lost and ULD may become non-task-specific. More commonly ULD starts as non-task-specific manifesting even at rest or during different types of voluntary movements. Tremor has been more frequently reported in non-task-specific dystonia patients compared to task-specific dystonia. [5] There is a lacuna in the literature with respect to the presence of tremor in task-specific dystonia such as writer’s cramp (WC), which is one of the most common idiopathic task-specific upper limb dystonia seen in the clinic. [6] Most studies have included a relatively small number of patients and they have not looked into the different demographic and clinical factors associated with tremor in WC patients. [7-19] Also, as we gradually move to a digital world, writing has become less common these days, leading to an underreporting of WC, and difficulty in establishing the precise onset of dystonia and tremor.

Therefore, we undertook this retrospective study of our patients who presented with writing difficulty (with and without tremor) to delineate the demographic and clinical characteristics of tremor.

This is a detailed chart review of 105 patients of WC who attended our movement disorder clinic over the past 3 years (2017–2020). In addition to demographic details, notes were also made regarding the occupation. Patients were examined while writing with the dominant and non-dominant hands. During the act of writing, involuntary contraction of elbow and shoulder joints (suggestive of motor overflow) was noted. We systematically looked for the presence of mirror movements in the relaxed dominant hand while writing with the non-dominant hand. It was considered to be present when the mirror movements partially or completely reproduced a dystonic posture in the dominant hand (similar to that observed when writing with this hand).

WC was classified into simple (presence of dystonia only during writing) and complex (dystonia during other activities as well). Patients were further subdivided into WC with dystonic action tremor (WCT+) and without tremor (WCT-). Patients (n = 11) diagnosed to have primary writing tremor (defined as task-specific tremor which occurs while writing or attempting to write with lack of overt abnormal posture) during this period were not included for the review. [3] Patients were treated with oral pharmacological drugs (anticholinergics, benzodiazepines, and beta-blockers) and injection botulinum toxin. Data were entered into Microsoft Office Excel and analysis was performed with IBM SPSS software version 27. Values were expressed as means ± standard deviations and as percentages and ranges. The mean between the two groups was compared using the t-test, frequencies between the various groups were compared using the χ² test, and p values ≤ 0.05 were considered statistically significant.

Our study of a total of 105 patients with WC observed that 37.1% of the patients with WC had associated dystonic action tremor (WCT+). A review of the literature shows that there is a great deal of variation in the frequency of tremor in WC (5–65%) patients which may be due to several factors such as types of patients included and methods of assessment used (Table 2). [7-22]

We observed that men are affected more commonly than women in WC, which is consistent with the literature. [7,11,23,24] Interestingly, in our study, for all the 12 females with WC, writing was an essential part of their profession, with 5 (42%) being students, 3 (25%) working as nurses, 2 (17%) as teachers, one (8%) as a policewoman and one (8%) as a doctor. The majority of the students in our cohort did not have tremor, whereas all the three doctors had tremor. This finding is interesting as a significant amount of writing is done by both students and doctors. But, the students had fewer number of writing-years compared to the doctors who were in the medical profession for many years. However, no conclusion can be made considering the very small sample size for the doctor population.

We also observed that the onset of WCT-was earlier in females compared to males (p = 0.034), which corroborates the findings reported by other studies. [8,11] However, we did not find a similar association among patients with WCT+ (p = 0.086). One study with 957 cases of primary idiopathic dystonia also found a significantly earlier age of onset of WC in females compared to males, contrary to other forms of dystonia (blepharospasm, cervical dystonia) where males had an earlier age of onset. [24]

Our WCT + patients had a significantly longer duration of disease compared with WCT-patients (4.16 ± 4.48 years vs. 2.57 ± 2.81, p = 0.024). Another study from India also reported that WC patients with dystonic tremor or with associated dystonia in other body parts had a longer duration of symptoms, as well as a delayed onset at presentation. [11] The same study also observed that dystonic tremor was linked with greater disability at presentation compared to all other subtypes of WC. [11] Similar findings have also been observed in cervical dystonia patients where tremor has been observed with a long duration of disease. [25] Our finding lends more credence to the idea that dystonic action tremor may be one of the late manifestations of WC. Also, it is possible that the diagnosis of WCT-was delayed due to the absence of tremor and this hypothesis could be validated in future studies. Further, our findings suggesting more incidence of tremor in doctors compared to students are on similar lines suggesting that tremor is a late feature in WC patients and appears over some time. However, our study findings are limited by less number of patients.

While writing our WCT + patients had significantly more involvement of ipsilateral proximal joint musculature (elbow and shoulder) suggestive of overflow compared to patients with WCT- (p = 0.001). Thus, the presence of motor overflow may be an important marker for tremor in WC patients. Sitburana et al in a study of 30 patients reported that the extent of overflow was significantly associated with the severity of dystonia. [26] They speculated that the high frequencies of motor overflow were due to a loss of normal inhibitory mechanisms leading to widespread abnormalities of motor control. [26] Our findings may point to similar pathophysiology for the occurrence of tremor in WC and further studies are needed to explore this. Mirror dystonia was observed in 72.4% (n = 76) of our patients, which is much higher than in other studies. [8,11,14,16,26] However, there was no significant association between tremor and mirror dystonia in our patients.

In our study, over 80% of patients had simple WC and less than 20% of patients had complex WC. However, there was no association between the presence of complex WC and tremor. We did not find any association between complex WC and duration of disease, similar to the study by Jhunjhunwala et al. [8] They speculated that if complex WC were a progression of simple hand dystonia, then it would have been associated with a longer duration of disease. Since this was not the case, they postulated that complex WC might be a separate entity altogether. [8] On the other hand, Das et al reported that complex WC was associated with a longer duration of disease. [11] Thus, further studies are needed to get a more definitive answer about the pathophysiology of complex WC.

Our study findings have some important limitations. First, there may be some information bias inherent to any retrospective study. Second, our center is a tertiary care setting, the results are likely to be influenced by referral biases. Third, we have not provided any data regarding any lag in the diagnosis of our cohort or any misdiagnosis due to musculoskeletal or orthopaedic causes. Fourth, we have not provided any information regarding the effect of botulinum toxin and tremor. One study showed that the presence of tremor in WC was a predictor of poorer response to botulinum toxin. [14] Further studies are needed to explore how the presence of tremor affects the therapeutic response to various treatment modalities.