NORMALIZATION OF MIDDLE LATENCY AUDITORY P1 POTENTIAL FOLLOWING POSTERIOR ANSA-PALLIDOTOMY IN IDIOPATHIC PARKINSONS-DISEASE

The P1 potential (50 msec) of middle latency auditory evoked potential was evaluated in seven patients with advanced idiopathic Parkinson's disease before and after contemporaneous bilateral posterior ansa pall idotomy. P1 potential was lacking in two patients preoperatively and w as prolonged in the remainder of the patients when examined in a best medicated 'on' state. All patients showed varying degrees of dementia preoperatively on clinical examination. Remarkable improvement in thei r Unified Parkinson's Disease Rating Scale (UPDRS) scores during 'on' shares from a mean of 45.85 to 14.28 post-operatively was achieved. Su bsidence of akinetic symptoms, abolishment of dyskinesia, improvement in gait freezing, rigidity, and tremors represented main areas of impr ovement. P1 latencies and amplitudes reverted back to normal values in six patients including the two patients who lacked P1 wave preoperati vely. One patient showed mild worsening of both Pa and P1 values posto peratively attributed to sleepiness during testing, a transient phenom enon commonly encountered following posterior ansa-pallidotomy. The po st-operative P1 changes were statistically significant for both latenc ies and amplitudes at p = 0.078 and p = 0.073, respectively, for all s even patients using one tailed paired t-test. The change in UPDRS post -operatively best correlated with the post-operative difference in P1; specifically, UPDRS subscore II with P1 amplitude (r = -0.068, p = 0. 09), and UPDRS subscore IV with Pa:P1 ratio (r = +0.77, p = 0.04). Ear lier clinical and experimental work support the tegmental pedunculopon tine nucleus (PPN) as the site of origin of P1 wave. In addition, its presence reflects the integrity of output cholinergic projection from PPN to both ascending reticular activating system ARAS and auditory pa thway. We believe this is the first report of restoring the integrity of P1 wave following posterior ansa-pallidotomy in patients with advan ced idiopathic Parkinson's disease. Disinhibition of the PPN by poster ior ansa-pallidotomy can explain improvements in motor symptoms in vie w of reemergence and normalization of P1 wave form.