In recent years, there has been a resurgence in the use of neurosurgical treatments to alleviate motor disturbances in Parkinson's Disease (PD). Despite the increased use of neurosurgical treatments, minimal research into the effects of these procedures on motor speech function in PD has been reported. Previously, it was assumed that motor speech function may improve following post-operative improvements in limb motor symptoms. The primary aim of the current thesis was to examine the effects of pallidotomy, thalamotomy, and deep brain stimulation (DBS) on perceptual, acoustic, and physiological speech parameters in a group of individuals with PD.
Twenty-two individuals with idiopathic PD who were treated with stereotactic neurosurgical procedures (surgical PD participants) participated in the study. The speech function of the 22 surgical PD (SPD) participants was assessed pre-surgery and three months post-surgery. In addition, the SPD participants' results were compared to a group of 25 normal control participants (NC group) matched for age and sex. The current thesis also collected data from a comparison group of 16 'non-surgical PD' participants (NSPD participants) who had not undergone surgical intervention. The NSPD group was included to control for the influence of disease progression on the pre-post surgical measures in the SPD participants.
The speech function of the SPD, NSPD, and NC participants was assessed using a perceptual speech analysis (FitzGerald, Murdoch, & Chenery, 1987), the Frenchay Dysarthria Assessment (Enderby, 1983), and the Assessment of Intelligibility of Dysarthric Speech (ASSIDS; Yorkston & Beukelman, 1984). Although the SPD group's general motor ratings revealed a significant post-operative improvement in overall motor performance, perceptual speech findings indicated that neurosurgical intervention did not systematically change the SPD participants' speech production.
To determine the effects of pallidotomy on motor speech function, the respiratory, laryngeal, velopharyngeal, and articulatory subsystems were subsequently evaluated in a series of three studies examining speech function pre- surgery and 3 months post-surgery. The assessment of respiratory function, utilising kinematic and spirometric techniques revealed that the participants who underwent pallidotomy (pallidotomy PD group) had essentially the same breath patterning for speech as the control participants. Furthermore, there was no significant change observed in the pallidotomy PD (PPD) group's speech breathing patterns and respiratory-phonatory control following pallidotomy.
The results of the electroglottographic and aerodynamic assessment of laryngeal function failed to identify a significant change in the PPD group's phonatory function following surgery. Similarly, acoustic parameters of sound pressure level and voice quality did not reveal significant change following surgery. In addition, the PPD group's velopharyngeal function was assessed using the Nasometer (Kay Elemetrics) while articulatory function was assessed using lip and tongue pressure transduction systems. The PPD group's nasalance and lip function parameters revealed that there were no significant changes in these two aspects of speech production following pallidotomy. Furthermore, the physiological tongue measures generally recorded no significant changes, although there was a post-operative improvement in tongue fine force control that occurred within the normal range.
The current thesis also examined the motor speech profile of 4 PD participants who underwent left ventrolateral thalamotomy. The results demonstrated that left thalamotomy had a minimal impact on the thalamotomy PD (TPD) participants' functional speech production, while one TPD participant recorded a post-operative decline in lingual function. An investigation of another 4 PD participants who had undergone DBS comprised the final investigation of this thesis. Results suggested that DBS in these four cases failed to effect functional change in the DBS participants' speech production despite improvements in limb motor symptoms.
The current research revealed that neurosurgical treatments resulted in differential effects on limb and speech motor impairments in a cohort of PD participants with relatively mild pre-operative dysarthria. The findings were discussed with respect to the heterogeneous nature of PD and differences between speech and limb motor function. Future directions for research include the examination of larger cohorts of moderate to severely dysarthric individuals with PD and further exploration of the somatotopic organization of the basal ganglia circuitry with respect to speech function