Of 96 Parkinson’s disease (PD) patients at the University of Florida Movement Disorders Center, one (1%) met diagnostic criteria for binge eating disorder (BED). Eight (8.3%) exhibited subthreshold BED. Psychometric criteria classified problem gambling in 17.8%, hoarding in 8.3%, buying in 11.5%, hypersexuality in 1.0%, and mania in 1.0% of patients. More overeaters met psychometric criteria for at least one additional impulse control disorder (67% vs. 29%). No more overeaters than non-overeaters were taking a dopamine agonist (44% vs. 41%). More overeaters had a history of subthalamic DBS (44% vs. 14%). History of DBS was the only independent predictor of overeating.
Parkinson’s disease; binge eating; impulse control disorders
It is essential to recognize and treat psychosis in Parkinson disease (PD) for several reasons. Multiple studies have shown that psychosis in PD patients is a strong risk factor for nursing home placement. Psychosis may be the single greatest stress for caretakers of PD patients, it is often persistent, and its presence markedly increases the risk of mortality.
Treatment of psychotic symptoms should occur only after potential medical and environmental causes of delirium have been eliminated or addressed. Initial pharmacologic changes should include limiting the patient’s anti-PD medications to those that are necessary to preserve motor function. Should that fail, an atypical antipsychotic agent is presently the treatment of choice. An emerging treatment option is the use of acetylcholinesterase inhibitors. This article reviews what is currently known about the course, prognosis and treatment strategies in PD psychosis.
Psychotic symptoms in Parkinson’s disease (PD) are relatively common, and in addition to being a disturbance to patients’ daily lives, they have consistently been shown to be associated with poor outcome. Our understanding of the pathophysiology of psychosis in PD has expanded dramatically over the past fifteen years, from an initial interpretation of symptoms as dopaminergic drug side effects to the current view of a complex interplay of extrinsic and disease-related factors. The present article reviews the unique clinical features of psychosis as expressed in PD, associated risk factors, and current theories behind its pathogenesis, including medications, visual processing deficits, sleep disturbances, genetics, and neurochemical and structural abnormalities. Finally, we review both traditional and emergent management strategies for PD psychosis, including antipsychotic agents, cholinesterase inhibitors, electroconvulsive therapy (ECT), and other pharmacological and psychological interventions.
This study investigates the prevalence and demographic characteristics of hypersexuality in Parkinson’s disease (PD). Impulse control disorders in PD patients have been associated with dopamine agonist therapy. Moreover, hypersexuality and pathological gambling have been associated with males, while females may be inherently thought to be more likely to participate in compulsive shopping and binge-eating behaviors. In this study, a screening mail-in survey was sent to all PD patients at a single Movement Disorders Center. One hundred forty one of 400 (35.3%) research packets were returned completed. Fifteen of 141 patients met initial screening criteria for hypersexual behavior. After detailed interview, only 6/141 (4.3%) of PD patients met criteria for pathologic hypersexual behavior. These behaviors included: compulsive masturbation, prostitution, and paraphilias. Patients with a younger age of PD onset were more likely to exhibit hypersexual behavior. Unlike previous report, no significant association was found between hypersexuality and gender or dopamine agonist use. Rather, this study suggests that physicians should be vigilant for hypersexual behavior in all PD patients, regardless of gender and PD medication regimen. Ultimately, given the innate sensitivity of the topic and survey limitations, it is very likely that hypersexual behavior in our cohort, as it is in the general PD population, has been under-reported.
Parkinson’s disease; hypersexuality; impulsive behavior; dopamine agonists
Rasagiline is a second generation, selective, irreversible monoamine oxidase type B (MAO-B) inhibitor. It has demonstrated efficacy in monotherapy for early Parkinson’s disease (PD) patients in one large randomized, placebo-controlled trial (TVP-1012 in Early Monotherapy for Parkinson’s Disease Outpatients), and has shown ability to reduce off time in more advanced PD patients with motor fluctuations in two large placebo-controlled trials (Parkinson’s Rasagiline: Efficacy and Safety in the Treatment of “Off”, and Lasting Effect in Adjunct Therapy With Rasagiline Given Once Daily). Preclinical data abound to suggest potential for neuroprotection by this compound against a variety of neurotoxic insults in cell cultures and in animals. The lack of amphetamine metabolites provides an advantage over the first generation MAO-B inhibitor selegiline. One large trial has investigated the potential for disease modification in PD patients (Attenuation of Disease progression with Azilect Given Once-daily) and preliminary results maintain some possible advantage to earlier initiation of the 1 mg/day dose. The clinical significance of the difference detected remains a consideration.
rasagiline; Parkinson’s disease; neuroprotection; selegiline
We performed an analysis of prospectively-acquired cross sectional data on 106 Parkinson disease (PD) patients who underwent comprehensive neuropsychological testing and the Unified Parkinson Disease Rating Scale (UPDRS) motor scale. A significant correlation between the UPDRS motor and neuropsychological tests in all cognitive domains except for general intelligence and visuo-spatial function was seen. In this study, cognitive decline within this PD cohort correlated with motor impairment but not disease duration. Our findings suggest that overall cognitive impairment (except visuospatial dysfunction) may track motor progression in PD more than duration of disease. Longitudinal studies are needed to confirm our results.
Parkinson; dementia; cognition; visual-spatial dysfunction
Several studies have related pathological gambling in PD to dopamine agonist therapy. A mail-in survey was sent to PD patients seen at the University of Florida Movement Disorders Center to determine gambling frequency and behavior, and any lifestyle or environmental factors associated with compulsive gambling in PD. 462 surveys were sent and 127 completed surveys were returned, of which ten were from patients who met criteria for compulsive gambling. All ten were taking dopamine agonists coincident with the compulsive gambling. Compulsive gamblers were younger, and psychological distress measures revealed that compulsive gamblers exhibited higher levels of anxiety, anger, and confusion. Thus in this cohort, we have uncovered the several characteristics of the most likely PD compulsive gambler, namely: (young) age, “angry”, “anxious”, and using a (dopamine) agonist.
Parkinson; gambling; compulsive behavior; dopamine agonist; anxiety
This review addresses the literature surrounding Parkinson’s disease (PD) and mild cognitive impairment (MCI). It discusses the neuropsychological, pharmaceutical, and pathological overlap, the socioeconomic impact of PD and MCI, and the value of recognizing, understanding, and treating MCI in PD. It is concluded from this review that MCI in PD does exist and should be considered in clinical and research investigations. Due to the lack of accepted clinical criteria, an inclusive operating definition of MCI in PD is proposed. Research guidelines for studying the presence of MCI in PD and evaluating the efficacy of pharmaceutical interventions are also suggested.
Parkinson’s, dementia; mild cognitive impairment; therapy; cognition
Fatigue is a common and disabling nonmotor symptom seen in Parkinson's disease (PD). While deep brain stimulation surgery (DBS) improves motor symptoms, it has also been associated with non-motor side effects. To date no study has utilized standardized instruments to evaluate fatigue following DBS surgery. Our objective was to determine the prevalence of fatigue following DBS surgery in PD its impact on quality of life and explore predictive factors. We recruited 44 PD subjects. At least one year following DBS placement, we administered the Fatigue Severity Scale (FSS), the Parkinson's Disease Questionnaire (PDQ-39), the Beck Depression Inventory, the Beck Anxiety Inventory, the UPDRS, and a neuropsychological battery. Fifty-eight percent of subjects had moderate to severe fatigue. Fatigue was significantly associated with quality of life, depression, and anxiety. Depression preoperatively was the only predictive factor of fatigue. Fatigue is common following DBS surgery and significantly impacts quality of life.
McLeod syndrome (MLS) is a rare, X-linked, late-onset, disease involving hematological, brain, and neuromuscular systems, caused by mutations in XK that result in either defective XK or complete loss of XK protein. Acanthocytosis of erythrocytes is a typical feature. We report novel mutations in two patients who exhibited typical clinical characteristics of MLS. The coding and flanking intronic regions of XK were amplified by PCR, sequenced, and compared with the normal XK sequence. XK protein, and its complexed partner protein, Kell, were assessed by Western blot analysis. Patient 1 was found to have a single base insertion, 605insA at 175Ile creating a frame shift within the coding sequence of XK. Patient 2 had a single base substitution in the 3′ splice sequence of intron 2 (IVS2–2a>g). In both cases mutations resulted in the absence of XK protein.
McLeod syndrome; Neuroacanthocytosis; XK protein; Kell protein; Acanthocytosis; McLeod blood group; Kx erythrocyte antigen; Kell blood group antigens
Parkinson's disease (PD) management has traditionally focused largely on motor symptoms. Deep brain stimulation (DBS) of the subthalamic nucleus (STN) and globus pallidus internus (GPi) are effective treatments for motor symptoms. Nonmotor symptoms (NMSs) may also profoundly affect the quality of life. The purpose of this pilot study was to evaluate NMS changes pre- and post-DBS utilizing two recently developed questionnaires. Methods. NMS-Q (questionnaire) and NMS-S (scale) were administered to PD patients before/after unilateral DBS (STN/GPi targets). Results. Ten PD patients (9 STN implants, 1 GPi implant) were included. The three most frequent NMS symptoms identified utilizing NMS-Q in pre-surgical patients were gastrointestinal (100%), sleep (100%), and urinary (90%). NMS sleep subscore significantly decreased (−1.6 points ± 1.8, P = 0.03). The three most frequent NMS symptoms identified in pre-surgical patients using NMS-S were gastrointestinal (90%), mood (80%), and cardiovascular (80%). The largest mean decrease of NMS scores was seen in miscellaneous symptoms (pain, anosmia, weight change, and sweating) (−7 points ± 8.7), and cardiovascular/falls (−1.9, P = 0.02). Conclusion. Non-motor symptoms improved on two separate questionnaires following unilateral DBS for PD. Future studies are needed to confirm these findings and determine their clinical significance as well as to examine the strengths/weaknesses of each questionnaire/scale.
X-linked dystonia-parkinsonism (XDP; DYT3; Lubag) is an adult-onset hereditary progressive dystonia/parkinsonism which is typically minimally responsive to pharmacological treatment.
We report a 63- year-old man with a diagnosis of XDP who underwent bilateral globus pallidus internus deep brain stimulator (GPi-DBS) placement. His course initially began with right hand tremor and dystonia at age 57 and progressed to also include bradykinesia and rigidity. The patient tolerated the procedure without significant complications. GPi-DBS improved his right hand dystonia, but did not significantly improve his parkinsonism.
DBS may be a therapeutic option for select cases of XDP, but its specificindications must be carefully discussed, as the available cases have had mixed responses. Whether other targets may be more effective is not known.
X-linked dystonia-parkinsonism; Globus pallidus internus; Cognitive impairment; Medication-resistant parkinsonism
A substantial number of individuals with Parkinson’s disease who display impaired postural stability experience accelerated cognitive decline and an increased prevalence of dementia. To date, studies suggest that this relationship, believed to be due to involvement of nondopaminergic circuitry, occurs later in the disease process. Research has yet to adequately investigate this cognitive-posturomotor relationship especially when examining earlier disease states. To gain greater understanding of the relationship between postural stability and cognitive function/dysfunction we evaluated a more stringent, objective measure of postural stability (center of pressure displacement), and also more specific measures of cognition in twenty-two patients with early to moderate stage Parkinson’s disease. The magnitude of the center of pressure displacement in this cohort was negatively correlated with performance on tests known to activate dorsolateral frontal regions. Additionally, the postural stability item of the UPDRS exhibited poor correlation with the more objective measure of center of pressure displacement and all specific measures of cognition. These results may serve as rationale for a more thorough evaluation of postural stability and cognition especially in individuals with mild Parkinson’s disease. Greater understanding of the relationship between motor and cognitive processes in Parkinson’s disease will be critical for understanding the disease process and its potential therapeutic possibilities.
neurodegenerative disease; dementia; balance
Background. Nonmotor symptoms (NMS) of Parkinson's disease (PD) may be more debilitating than motor symptoms. The purpose of this study was to determine the frequency and corecognition of NMS among our advanced PD cohort (patients considered for deep brain stimulation (DBS)) and caregivers.
Methods. NMS-Questionnaire (NMS-Q), a self-administered screening questionnaire, and NMS Assessment-Scale (NMS-S), a clinician-administered scale, were administered to PD patients and caregivers. Results. We enrolled 33 PD patients (23 males, 10 females) and caregivers. The most frequent NMS among patients using NMS-Q were gastrointestinal (87.9%), sleep (84.9%), and urinary (72.7%), while the most frequent symptoms using NMS-S were sleep (90.9%), gastrointestinal (75.8%), and mood (75.8%). Patient/caregiver scoring correlations for NMS-Q and NMS-S were 0.670 (P < 0.0001) and 0.527 (P = 0.0016), respectively. Conclusion The frequency of NMS among advanced PD patients and correlation between patients and caregivers varied with the instrument used. The overall correlation between patient and caregiver was greater with NMS-Q than NMS-S.
Mitochondrial dysfunction and oxidative stress are pathophysiologic mechanisms implicated in experimental models and genetic forms of Parkinson’s disease (PD). Certain pesticides may affect these mechanisms, but no pesticide has been definitively associated with PD in humans.
Our goal was to determine whether pesticides that cause mitochondrial dysfunction or oxidative stress are associated with PD or clinical features of parkinsonism in humans.
We assessed lifetime use of pesticides selected by mechanism in a case–control study nested in the Agricultural Health Study (AHS). PD was diagnosed by movement disorders specialists. Controls were a stratified random sample of all AHS participants frequency-matched to cases by age, sex, and state at approximately three controls: one case.
In 110 PD cases and 358 controls, PD was associated with use of a group of pesticides that inhibit mitochondrial complex I [odds ratio (OR) = 1.7; 95% confidence interval (CI), 1.0–2.8] including rotenone (OR = 2.5; 95% CI, 1.3–4.7) and with use of a group of pesticides that cause oxidative stress (OR = 2.0; 95% CI, 1.2–3.6), including paraquat (OR = 2.5; 95% CI, 1.4–4.7).
PD was positively associated with two groups of pesticides defined by mechanisms implicated experimentally—those that impair mitochondrial function and those that increase oxidative stress—supporting a role for these mechanisms in PD pathophysiology.
aging; agricultural epidemiology; environmental epidemiology; epidemiology; fungicides; herbicides; insecticides; persistent organic pollutants; pesticides
Psychosis is a leading reason for nursing home placement of patients with Parkinson’s disease (PD). It may also be the single greatest stressor for caregivers of PD patients, it is generally persistent, and its presence markedly increases the risk of mortality. For these reasons, it is essential to recognize and appropriately treat psychosis in PD. Treatment of psychotic symptoms should be initiated after potential medical and environmental causes of delirium (eg, infection) have been eliminated or addressed. Initial pharmacologic changes should include limiting the patient’s anti-PD medications to those that are necessary to preserve motor function (ie, eliminating adjunctive agents). Should that fail, an atypical antipsychotic agent is the treatment of choice. Clozapine is presently the gold standard, and quetiapine represents another option because of its ease of use and good tolerability profile. Emerging treatment options include the use of acetylcholinesterase inhibitors, antidepressants, and cognitive behavioral therapy. This article reviews what is currently known about treatment strategies in PD psychosis.
Apathy is a unique, multidimensional syndrome commonly encountered in patients with Parkinson disease (PD). Recently, the Lille Apathy Rating Scale (LARS), a semistructured interview yielding a global score, and composite subscores for different domains of apathy (i.e., cognitive, behavioral, affective, self awareness), was developed and given to a sample of patients with PD in France. This study is the first outside of its original developers to examine the English language version of the LARS in PD. We found the LARS to be a coherent instrument demonstrating both convergent and divergent validity, as compared to the Apathy Scale (AS) and Beck Depression Inventory (BDI-II). Using a receiver operating characteristic (ROC) analysis comparing the LARS to the AS, a validated and widely-used measure, we identified a cut-off score (sensitivity = 64%, specificity = 92%, PPV = 88%, NPV = 75%) that was higher than that proposed by the original authors, who derived their cutoff by comparing LARS global scores to clinical judgments of apathy. Although the present study does not compare the LARS to a diagnostic gold standard or promote its utility for diagnosing apathy, it provides further support for the LARS as a promising instrument to examine apathy in PD.
Parkinson’s disease; apathy; rating scales
Conflicting research suggests that deep brain stimulation surgery, an effective treatment for medication-refractory Parkinson’s disease (PD), may lead to selective cognitive declines. We compared cognitive performance of 22 PD patients who underwent unilateral DBS to the GPi or STN to that of 19 PD controls at baseline and 12 months. We hypothesized that compared to PD controls, DBS patients would decline on tasks involving dorsolateral prefrontal cortex circuitry (letter fluency, semantic fluency, and Digit Span Backward) but not on other tasks (Vocabulary, Boston Naming Test), and that a greater proportion of DBS patients would fall below Reliable Change Indexes (RCIs). Compared to controls, DBS patients declined only on the fluency tasks. Analyses classified 50% of DBS patients as decliners, compared to 11% of controls. Decliners experienced less motor improvement than non-decliners. The present study adds to the literature through its hypothesis-driven method of task selection, inclusion of a disease control group, longer-term follow-up and use of Reliable Change. Our findings provide evidence that unilateral DBS surgery is associated with verbal fluency declines and indicate that while these changes may not be systematically related to age, cognitive or depression status at baseline, semantic fluency declines may be more common after left-sided surgery. Finally, use of Reliable Change highlights the impact of individual variability and indicates that fluency declines likely reflect significant changes in a subset of patients who demonstrate a poorer surgical outcome overall.
Parkinson’s disease; Deep brain stimulation; Reliable Change
Current practice often assesses apathy with a single item from the Unified Parkinson’s Disease Rating Scale (UPDRS, item 4). Yet, the relationship between the UPDRS item 4 and the validated Apathy Scale (AS) is unknown. The purpose of this study was to evaluate the operating characteristics of UPDRS item 4 in relation to the AS. Three hundred and one patients with PD were administered the AS and the UPDRS. We compared the UPDRS item 4 to the standard AS classification of ≥14 as apathetic. A receiver operating characteristics (ROC) curve was obtained, and sensitivity, specificity, positive, and negative predictive power were calculated. The ROC curve showed area under the curve as 0.75. A cut-off of 1 had good sensitivity (81%) but poor specificity (53%; high false positive rate). A cut-off point of 2 had acceptable specificity (87%) but poor sensitivity (52%, high false negative rate). Continuing to increasing the cut-off point (e.g., 3, 4) continues to increase specificity at the expense of dramatically reducing sensitivity. These findings suggest the use of caution when screening for apathy with item 4 due to its poor sensitivity in relation to the AS.
Parkinson’s disease; apathy; mood disorders; UPDRS; assessment
Pathological behaviors such as problem gambling or shopping are characterized by compulsive choice despite alternative options and negative costs. Reinforcement learning algorithms allow a computation of prediction error, a comparison of actual and expected outcomes, which updates our predictions and influence our subsequent choices. Using a reinforcement learning model, we show data consistent with the idea that dopamine agonists in susceptible individuals with Parkinson's disease increase the rate of learning from gain outcomes. Dopamine agonists also increase striatal prediction error activity thus signifying a “better than expected” outcome. Thus, our findings are consistent with a model whereby a distorted estimation of the gain cue underpins a choice bias towards gains.
There is a paucity of level-one evidence comparing STN and GPi DBS. Our aim in this prospective blinded randomized trial was to compare the cognitive and mood effects of unilateral subthalamic nucleus (STN) vs. unilateral globus pallidus interna (GPi) deep brain stimulation (DBS) in patients with Parkinson disease (PD).
Fifty-two subjects with moderate-to-advanced PD were randomized to either unilateral STN or GPi DBS. Right or alternatively left sided stimulation was chosen to address the side of the body with the most bothersome symptoms. The co-primary outcome measures were the change in the 8 subscales of the Visual Analog Mood Scale (VAMS), and the change in the 2 versions of verbal fluency (i.e. semantic and letter), at 7 months post-DBS in the optimal setting compared to the pre-DBS state. In addition, at 7 months post-DBS, after subjects underwent initial evaluation off medications and on optimized DBS therapy, they were tested in four randomized and counterbalanced conditions (optimal DBS, ventral DBS, dorsal DBS, and off DBS) while remaining off medication. Secondary outcome measures then compared the differences in the VAMS items and verbal fluency subscales within the 4 DBS conditions at 7 months, and the change in the VAMS items and verbal fluency subscales from the pre-DBS state to the other 3 DBS conditions (ventral, dorsal and off ) at 7 months.
Forty-five subjects (23 GPi and 22 STN) completed the protocol. The study revealed no significant difference between STN and GPi DBS in the change of co-primary mood and cognitive outcomes from pre- to post-DBS in the optimal setting (Hotelling's T2 test: p=0.16 and 0.08 respectively). When comparing the 4 DBS conditions at 7 months, subjects in both targets were less “happy”, less “energetic” and more “confused” when stimulated ventrally to the optimal stimulation site. When comparing the other 3 DBS conditions (ventral, dorsal and off DBS) to the pre-DBS state, the STN group showed a larger deterioration of letter verbal fluency scores than the GPi group, especially in the off DBS state. A 12-point mean improvement in the UPDRS motor subscale was seen post DBS, but there was no significant difference between targets.
There were no significant differences in in the co-primary outcome measures of mood and cognition between STN and GPi in the optimal DBS state.. However, adverse mood effects were noted when stimulating ventrally to the optimal site in both targets. Furthermore, a worsening for letter verbal fluency was noted in the 3 non-optimal post-DBS states in the STN target only. The persistence of deterioration in verbal fluency in the off DBS state at 7 months is, suggestive of a surgical rather than a stimulation-induced effect at the STN target. STN and GPi DBS resulted in similar motor improvement.
GPi; STN; DBS; Mood; Cognition; Side Effects; verbal fluency; motor; UPDRS
To test whether the synucleinopathies Parkinson’s disease and multiple system atrophy (MSA) share a common genetic etiology, we performed a candidate single nucleotide polymorphism (SNP) association study of the 384 most associated SNPs in a genome-wide association study of Parkinson’s disease in 413 MSA cases and 3,974 control subjects. The 10 most significant SNPs were then replicated in additional 108 MSA cases and 537 controls. SNPs at the SNCA locus were significantly associated with risk for increased risk for the development of MSA (combined p = 5.5 × 1012; odds ratio 6.2).
While deep brain stimulation (DBS) surgery is a well-accepted treatment for Parkinson disease (PD) that improves overall quality of life (QoL), its effects across different domains of QoL are unclear. The study reported here directly compared the effects of unilateral DBS in subthalamic nucleus (STN) or globus pallidus (GPi) on QoL in 42 non-demented patients with medication-refractory PD. Patients were enrolled in the COMPARE trial, a randomized clinical trial of cognitive and mood effects of STN versus GPi DBS conducted at the University of Florida Movement Disorders Center. Patients underwent motor, mood, verbal fluency and QoL (Parkinson disease questionnaire: PDQ-39) measures before and 6 months following surgery. Groups experienced motor and mood improvements that did not differ by target. Patients with STN DBS evidenced a slight decrement on letter fluency. On average, all patients endorsed better overall QoL after surgery. However, despite similar motor and mood improvements, GPi patients improved more than STN patients (38 vs. 14%, respectively; P = 0.03). Patients reported better QoL on subscales of mobility, activities of daily living (ADLs), emotional well-being, stigma, cognition and discomfort, but not on those of social support and communication. Improvements on the mobility, ADLs, stigma and social support subscales were greater amongst GPi patients. In regression analyses, only depression changes independently predicted changes in overall QoL as well as emotional well-being and social support changes. Within the STN group only, declining category fluency scores correlated with poorer QoL on the communication subscale. Unilateral DBS in both STN and GPi improved QoL overall and in disparate domains 6 months after surgery. Patients receiving GPi DBS reported greater improvements that cannot be explained by differential mood or motor effects; however, verbal fluency changes may have partially contributed to lesser QoL improvements amongst STN patients.
Cognition; Deep brain stimulation; Depression; Parkinson disease; Quality of life
We performed a genome-wide association study (GWAS) in 1,713 Caucasian patients with Parkinson’s disease (PD) and 3,978 controls. After replication in 3,361 cases and 4,573 controls, two strong association signals were observed: in the α-synuclein gene(SNCA) (rs2736990, OR=1.23, p=2.24×10−16) and at the MAPT locus (rs393152, OR=0.77, p=1.95×10−16). We exchanged data with colleagues performing a GWAS in Asian PD cases. Association at SNCA was replicated in the Asian GWAS1, confirming this as a major risk locus across populations. We were able to replicate the effect of a novel locus detected in the Asian cohort (PARK16, rs823128, OR=0.66, p=7.29×10−8) and provide evidence supporting the role of common variability around LRRK2 in modulating risk for PD (rs1491923, OR=1.14, p=1.55×10−5). These data demonstrate an unequivocal role for common genetic variability in the etiology of typical PD and suggest population specific genetic heterogeneity in this disease.