Troubleshooting Gait and Voice Problems After DBS for Parkinson’s Disease

Disease-Related Vocal Abnormalities

When a patient presents with speech problems, the neurologist must determine whether they result from the disease or from stimulation. Symptoms that have responded insufficiently to DBS are likely related to the disease, as are symptoms consistent with disease progression, such as gradually progressive dysarthria. These symptoms may respond to more aggressive stimulation. A patient with worsening hypokinetic dysarthria, however, may not improve, and could worsen, with more aggressive stimulation.

No clear criteria can help a neurologist determine whether to consider abnormal speech nonresponsive to stimulation. This determination relies on clinical judgment and should be communicated clearly to the patient, said Dr. Klassen. At that point, the neurologist and patient may consider speech therapy.

Stimulation-Related Vocal Abnormalities

DBS implantation itself sometimes causes dysarthria that may improve over the course of weeks or months. Implantation also may worsen pre-existing dysarthria. “That [side effect] does not necessarily have to limit what or how you are stimulating for tremor control,” said Dr. Klassen. If the symptom results from stimulation, it will improve when stimulation is stopped. It may take as little as a few seconds or as long as several weeks for vocal abnormalities to improve, but tremor worsens while stimulation is turned off.

A neurologist should locate the source of any stimulation-dependent vocal abnormality so that he or she can focus the stimulation field on that source. Although the left lead tends to be implicated in vocal abnormalities more often than the right lead, the neurologist needs to determine the leads’ contributions empirically by turning the leads off individually. “Depending on the washout [period], that may take more time than you would like,” said Dr. Klassen.

A review of the initial monopolar thresholds can indicate which regions along the electrode tend to affect speech the most. Postoperative imaging may help in this determination. If the patient has a prolonged washout period, the neurologist can give him or her “homework,” said Dr. Klassen. To do this, the neurologist sets the DBS device to run several programs and asks the patient to record his or her experiences in a notebook.

Optimizing the Stimulation Settings

Vocal abnormalities that arise after surgery may indicate that the stimulation parameters need to be modified. First, neurologists must choose the optimal lead location along the electrode. Eccentric steering or multiple-source current steering may reduce vocal tremor by better defining the distribution of current.

To reduce the volume of tissue activated, the neurologist can increase the pulse width, reduce the amplitude, or switch to a bipolar configuration. If a particular setting causes side effects, reducing the voltage may increase tolerability, albeit at the expense of efficacy. Switching from a high frequency to a low frequency also may reduce vocal tremor. If it is impossible to control limb tremor and vocal abnormalities optimally with a single setting, the patient may choose the setting that provides the most acceptable overall control.

Another option is to allow the patient to switch as necessary between a program optimized for tremor control and one optimized for speech. A patient may also choose to turn stimulation on and off as needed. Finally, adjunctive speech therapy can reduce vocal tremor, said Dr. Klassen.

—Erik Greb