Table 1

Sample sizeTarget (all studies with stimulation on versus stimulation off)Outcome measureResultsR

STN ( ), cZi ( ); pre-op and 12 months post-opPhonatory control: alternations between voicing and voiceless states in a reading taskProgressive deterioration of phonatory control but unaffected by DBS[58]

STNPerceptual ratings and acoustic analysisDeterioration of overall speech performance (perceptual ratings); mixed results concerning single speech parameters (acoustic analysis)[59]

STN ( ), cZi ( ); pre-op and 12 months post-opMean intensity during reading, intensity decay during syllable repetitionSTN: increase of intensity;
cZi: slight reduction of intensity during reading; no significant change of intensity decay
[60]

STN ( ), cZi ( )Articulatory capacity, accuracy of plosive consonantsSTN: increased articulatory rate;
cZi: deterioration of articulatory rate and quality
[61]

STNAcoustic analysis of different speech parametersMixed results with positive and negative effects on single speech parameters[62]

STNAcoustic analysis of syllable repetition precisionDeterioration of syllable repetition capacity[63]

(I) with both electrodes within STN,
(II) with only one electrode within STN,
(III) with no electrode within STN
Perceptual rating according to UPDRS speech item(I) 50% improvement, 36% deterioration of speech;
(II) 69% improvement, 25% deterioration
(III) 100% deterioration
[64]

STN and GPi; pre-op and 6 months post-opPeak velocities of jaw movementsSTN: deterioration of jaw movement velocity
GPi: no deterioration
[65]

STN; stimulation frequencies with 60 Hz and 130 HzPerceptual rating, acoustic analysis, and aerodynamic measurementsAmelioration of outcome measures under 60 Hz stimulation[66]

STNCase reports: description of speech performanceReoccurrence of severe stuttering[67]

STNAcoustic analysis of different speech tasksNo changes in speech performance[68]

STNArticulatory accuracy measured by electropalatography Deterioration in one patient, amelioration in the other patient[69]

STN; pre-op, 1 month, 6 months, and 12 months post-opPerceptual analysis based upon validated rating scaleAmelioration of speech in ; deterioration in the other patients by an average of off-medication; deterioration more often with high voltage and medially located electrodes within the left STN[70]

STNAerodynamic measuresIncreased intraoral pressure in ; increased velopharyngeal closure in [71]

STNPerceptual and acoustic analysisAmelioration of voice, no influence on fluency[72]

STNAerodynamic measuresIncreased inspiratory driving pressure ( ); increased vocal fold closure ( ); more benefit from low-frequency stimulation[73]

STN; 4 V (high) and 2 V (low) voltagePerceptual analysis based upon validated rating scaleDeterioration during high-amplitude stimulation in , in patients with electrodes in medial and/or posterior position[74]

within and above STN; 4 V (high) and 2 V (low) voltagePerceptual analysis based upon validated rating scaleDeterioration of speech during high-amplitude stimulation independently from side of stimulation[75]

STNAcoustic analysis of sustained vowel phonationImprovement of single measures of voice [76]

STNPerceptual ratings by patient, physician, and professional speech therapist, additional acoustic analysisDeterioration of overall speech performance (perceptual ratings); amelioration of single speech/voice parameters (acoustic analysis)[77]

STNPerceptual rating and acoustic analysisNo deterioration; amelioration of glottal stability and vocal tremor [78]

STNAcoustic analysis of articulatory and phonatory functionMixed results with improvement or deterioration of articulation and phonation in different patients[79]

STN, different parameter settingsPerceptual analysis of different speech tasksDeterioration in patients under “normal” stimulation parameters; further deterioration during high-amplitude and/or frequency stimulation[80]

STNQualitative description and acoustic analysisMixed results with improvement and deterioration of speech in the different patients[81]

STNPerceptual rating according to UPDRS speech item, PET study Improvement of overall speech performance[82]

STN; pre-op and 3 months post-opPerceptual dysarthria assessment and rating according to UPDRS speech itemModest improvement of lip movements, loudness, and pitch; slight reduction of intelligibility[83]

STNAcoustic analysis and force measurements of articulatory musclesDecrease of reaction and movement time of articulatory organs; increase of maximal strength and precision; improvement of respiratory and phonatory function[84]

STN; follow up at several years post-opPerceptual ratings, measurement of articulatory force (lip and tongue force)Improvement of articulatory force; deterioration of overall speech performance (perceptually rated) in a subgroup of patients[85]

STNCase report: descriptive Emergence of dysfluencies under STN-DBS[86]

STNAcoustic analysisImprovement of distinctive speech parameters[87]

STN; pre-op and 3 months and 6 months post-opAcoustic analysisMild improvement of sound pressure level and pitch variability[88]

STN ( ), Vim ( )Measurement of articulatory force (lip and tongue force)Vim: deterioration of static and dynamic control of articulatory organs
STN: improvement
[89]

STN, pre-op and 2 years post-opDescriptiveImprovement of oral control and intelligibility[90]

STNPerceptual rating according to UPDRS speech item, measurement of articulatory force (lip and tongue force)Improvement of static and dynamic control of articulatory organs; improvement of reaction time; improvement of overall speech performance[91]

Vim (patients with tremor)Perceptual ratingDevelopment of dysarthria in [52]

STN: Subthalamic nucleus, GPi: Globus pallidus internus, cZi: Caudal zona incerta, R: Reference.