Introduction

Neurological movement disorders are known to be significant in middle to elderly age groups, ranging from 50 to 89 years old, with Parkinson’s Disease and essential tremors being the most prevalent.⁷ Subsequently, dystonia follows as the third most common movement disorder, characterised by involuntary muscle contractions that result in pain and eventually into abnormal postures and movements - these can progress to different areas of the body, which is known as primary focal dystonia.⁴ 

Understanding cervical dystonia

Cervical Dystonia (CD), also recognised as spasmodic torticollis or wry neck, is the most common form of primary focal dystonia affecting muscles residing in the neck and shoulder area.⁴ 

CD severely reduces the quality of life due to significant restrictions in daily activities and social participation as a consequence of the following symptoms:^2,4,5

• Pain
  • Pain from spasms of neck muscles
• Involuntary neck muscle contractions 
• Postures 
  • Torticollis     - neck rotation (most common)
  • Laterocollis - head tilting sideways
  • Retrocollis   -head extended backwards
  • Anterocollis - head flexed backwards
• Shoulder elevation
• Dystonic tremor 
  • Head tremors are the most common and occur in 30-60% of patients²
• Muscle hypertrophy

Symptoms tend to exacerbate over the first 5 years of diagnosis and can be further triggered by stress and fatigue.² However, these conditions generally decrease with relaxation and can be resolved during sleep.⁴ Patients have also discovered sensory tricks to help temporarily relieve or reduce dystonic muscle spasms, such as touching the back of the head, cheek, temple, or chin.²

Current traditional treatments for CD include:^1,3,4,5

• Botulinum toxin injections - primary treatment
• Oral medication
  • Analgesics
  • Muscle relaxants
  • Dopaminergic therapy
• Physical therapy
  • Rehabilitation
  • Occupational therapy 
• Selective peripheral denervation

Despite the various treatment options, they are limited by side effects and unsatisfactory, forcing patients to seek surgical intervention like Deep Brain Stimulation, especially in refractory cases of CD, in which a small proportion of patients are unable to respond well to conventional medical treatment.

What is deep brain stimulation?

Deep Brain Stimulation (DBS) is a surgical procedure that involves the implantation of leads that are connected to neurostimulators implanted in the patient’s right subclavicular pocket.⁵ This sends electrical signals to the target areas to regulate abnormal brain activity, causing the involuntary muscle movements. 

The DBS leads target the regions of the brain responsible for motor functions

• Globus Pallidus pars Interna (GPi)
• Subthalamic Nucleus (STN)
• Thalamus

DBS is typically intended for CD patients who have ‘medically intractable dystonia’ where they experience major adverse effects and resistance against conventional treatments.^3,5 Patients’ condition must be severe and substantially disabled by their dystonia to be considered as DBS candidates.³

How effective is DBS for cervical dystonia?

DBS is not a definite cure but has been significantly shown to be effective in managing symptoms and declining refractory cases not only in CD but in other movement disorders like Parkinson’s disease as a last resort treatment. DBS offers significant benefits, including improvement of the quality of life in patients for a long duration of time.

Long-term follow-up studies have proven that DBS targeting all three regions mentioned has positive comparable effects and is safe up to 15 years when treating dystonia^1,3; however, according to several studies, different forms of dystonia (focal, segmented, etc.) respond differently to different stimulation sites. 

Stimulation of the ventral oral posterior nucleus within the thalamus appears to produce the best results in CD patients.⁶ However, STN DBS has shown immediate symptomatic improvement post-procedure, most likely because of its involvement in the cortico-basal ganglia-cerebellar network, which, when impaired, is the hypothesised cause of CD.^3,5 

Safety of DBS

Refractory CD patients generally tolerate the DBS procedure quite well, as no serious adverse events such as device infections or intracranial haemorrhages have yet to be reported. However, occasionally, a handful of patients will report complications with DBS hardware, like an uncomfortable sensation surrounding the extension wire or mild pain in the subclavicular pocket where the neurostimulator is situated. 

Stimulation-associated adverse effects can also arise, including dyskinesia (involuntary movement), mild balance disturbances, and hand weaknesses. Nevertheless, studies have found that reducing the stimulation amplitude during the phase where symptoms are stable will not exacerbate CD but rather prevent stimulation-related effects as well as save neurostimulator power.⁵

Looking ahead: research and future directions

DBS is clearly a very effective alternative treatment not only for CD but also Parkinson’s disease and essential tremors, so there may potentially be hope for its efficacy in treating other types of dystonia and movement disorders. It is important to note that the majority of studies on DBS and CD are still limited by relatively small sample sizes; hence, there is a need for further research to improve our understanding.

Summary

To summarise, adult-onset neurological movement disorders are very prevalent in elderly individuals, Cervical Dystonia in particular, following as the third most common. CD is marked by involuntary contractions of the neck muscles, consequently resulting in pain that progresses into abnormal neck postures and movements, as well as tremors, all of which significantly affect the quality of life for those affected. It is thought that the cause of these involuntary muscle contractions is due to an impaired cortico-basal ganglia-cerebellar network responsible for motor functions. 

Conventional treatments such as botulinum toxin injections, oral medication, and physical therapies are often resorted to in common cases; however, there is a small proportion of CD patients who experience refractory isolated CD, where they show major adverse effects and resistance to these traditional treatments. 

As an alternative, these patients must be appointed to surgical intervention, where Deep Brain Stimulation comes in. DBS is very effective in targeting and regulating major motor regions and their abnormalities to improve the management of CD symptoms. 

While DBS is not a cure, it shows significant long-term efficacy in reducing pain and involuntary contractions in refractory cases. Emerging research on DBS indicates that it can be a promising treatment not only for dystonia but also for other movement disorders.