Speech intelligibility deteriorated 1 year after STN-DBS in 78% of patients in contrast with the marked 50.7% improvement in parkinsonian motor symptoms. This percentage is higher than most clinical series in the literature. Nevertheless, most series have focused on the motor benefit and speech has mostly been assessed by item 18 of the UPDRS, which shows poor sensitivity to detecting speech problems22
and indeed identified only 12 patients (38%) with speech deterioration in our sample. Disease progression in the medical group accounted for only a 3.6% deterioration of speech intelligibility off-medication and 4.5% on-medication over 1 year, as reported before.23
The majority of the speech deterioration in the surgical group occurred between 6 months and 1 year. This was not alleviated by switching the stimulation off. Also, voltage was not significantly increased between 6 months and 1 year. At 3 years, 53% (95% CI 25 to 81) of patients showed speech deterioration in the off-medication/on-stimulation condition and 73% (95% CI 42 to 92) of patients in the on-medication/on-stimulation condition.
Of the clinical factors studied, only the preoperative on-medication UPDRS-III motor score was associated with speech change at 1 year of STN-DBS. This is consistent with the studies on predictive factors for movement improvement after STN-DBS.10,24
The fact that the severity of the residual parkinsonian motor score in the on-medication condition was predictive of a poor speech postoperative outcome is probably explained by the presence of nondopaminergic pathology25
and it is corroborated by the limited effect of levodopa on speech.26,27
Speech response was not improved by administration of levodopa before or after STN-DBS. Indeed, for some patients, speech was worse on-medication/on-stimulation, as reported earlier.8,12
The nature of this worsening was not linked to increased dyskinesias and would require further investigation. Postoperatively, the amount of reduction of levodopa was not associated with speech deterioration either.
Speech outcome was not linked to either age or disease duration, unlike motor outcome.24,28,29
Indeed, speech problems may arise at any stage of the disease process and are not necessarily related to the degree of motor disability.30
Interestingly, speech intelligibility before surgery, off- or on-medication, was not a predictive factor of speech intelligibility at 1 year after surgery. Higher LTAS means in both reading and monologue when on-medication was a predictive factor of good speech outcome. Dromey19
examined the use of a number of acoustical variables to describe PD speech and concluded that lower LTAS means was the variable that differentiated PD speech most from that of normal controls. However, the relationship between these acoustic measures and perceptual judgments is still not clear.31
Systematic evaluation of the anatomic location of the electrode contact and its effects on speech showed that electrodes placed medial to the left STN were worse for speech intelligibility than electrodes inside the STN, confirming results from other studies.32–34
Equally, information on the particular STN segment in which the active contact is located and speech outcome is scarce. In our study, stimulation in the left superior segment of the STN improved speech by 6.6% over a year compared to a deterioration of 31% from stimulation in the left posterolateral segment (). Despite the different methodology of electrode localization, stimulation of this same superior segment (sometimes referred to as “dorsal STN” in the literature) is reported to be more effective for limb motor control.35,36
Improvement in both speech and motor control from stimulation of this segment compared to improvement predominantly in motor control with stimulation of the posterolateral segment may have implications for surgeons when targeting the STN. However, the number of electrodes in each of the 5 segments inside the STN was too limited to make firm conclusions about their effects on speech. Higher voltage on the left STN at 1 year was also associated with speech deterioration. The worsening effect of higher voltage has been described before.12,17
Some studies have attributed this deterioration to the spread of current to the internal capsule.12,13
In our study, the strong association of a medial contact and higher voltage with poor speech outcome suggests that speech deterioration may be due to current spread to the cerebellothalamic tract.17
It also points to the preponderance of good contact localization for both speech and motor outcome: medially placed contacts needed higher voltage for control of movement, which in turn affects speech negatively. The stronger association of the left STN contact with speech response also conforms to the findings from other studies.32
There was a discrepancy between deterioration in speech intelligibility and improvement in loudness; this is contrary to other studies in PD dysarthria, where increased loudness is associated with increased speech intelligibility.37
Our finding supports similar results from the limb motor literature on the effects of STN-DBS, which show an increase in force production but deterioration on more complex movement.38
So far, evidence of impaired performance following STN-DBS has been limited to selected cognitive tasks and complex manual tasks.39,40
Speech is a uniquely human complex ability requiring fast and precise movement under constantly changing circumstances.