Olfactory dysfunction in patients with narcolepsy with and without REM sleep behaviour disorder
1 Department of Neurology, Center of Nervous Diseases, Philipps-University of Marburg Schwalmstadt-Treysa, Germany 2 Department of Neurology, Hephata-Klinik Schwalmstadt-Treysa, Germany
Correspondence to: Karin Stiasny-Kolster, MD, Department of Neurology, Center of Nervous Diseases, Rudolf-Bultmann-Strasse 8, D-35033 Marburg, Germany E-mail: stiasny{at}med.uni-marburg.de
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Patients with idiopathic rapid eye movement sleep behaviour disorder (RBD) frequently develop Parkinson's disease and the majority present with hyposmia, which is a potential preclinical non-motor sign of Parkinson's disease. Accordingly, it has been proposed that the clinical symptoms of hyposmia and RBD in combination have to be considered as very early symptoms of Parkinson's disease. Since not only patients with idiopathic RBD but also patients in whom RBD is associated with narcolepsy present with an olfactory dysfunction we investigated if hyposmia in RBD patients with concomitant narcolepsy is RBD specific or if narcolepsy per se is associated with olfactory dysfunction. We studied olfactory function in 20 narcoleptic patients each with RBD (9 male and 11 female; mean age 45.4 ± 14.0 years, range 20–75 years) and without associated RBD (8 male and 12 female; mean age 44.4 ± 13.40 years, range 20–70 years) and 40 age- and gender-matched healthy control subjects using standardized ‘Sniffin’ Sticks'. Both, narcoleptics with (Narc/+RBD) and without RBD (Narc/–RBD) had a significantly higher olfactory threshold (Narc/+RBD, P = 0.0001; Narc/–RBD, P = 0.0001), lower discrimination scores (P = 0.001; P = 0.014) and lower identification scores (P = 0.057; P = 0.003) than controls. There were no symptoms or signs for early parkinsonism in both patient groups. Our results show for the first time that narcolepsy per se is associated with olfactory dysfunction. In contrast to patients with idiopathic RBD, hyposmia in patients with RBD associated with narcolepsy is unlikely to be a predictor for developing parkinsonism.
Key Words:
narcolepsy; REM sleep behaviour disorder; Parkinson's disease; 
-synucleinopathy; olfactory dysfunction
Abbreviations: Hcrt, hypocretin; Hcrtr, hypocretin receptor; HLA, human leucocyte antigen; RBD, REM sleep behaviour disorder; REM, rapid eye movement; TDI, threshold-discrimination-identification; UPDRS, Unified Parkinson's Disease Rating Scale
Received August 10, 2006. Revised October 16, 2006. Accepted November 13, 2006.
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Introduction |
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Narcolepsy is characterized by excessive daytime sleepiness, disturbed nocturnal sleep and pathological manifestations of rapid eye movement (REM) sleep. REM abnormalities include sleep-onset-REM periods, and the dissociated REM sleep events of cataplexy, sleep paralysis and hypnagogic hallucinations. Narcolepsy has a prevalence of 0.02–0.07% (Ohayon et al., 2002
; Silber et al., 2002) with 95% of cases having sporadic narcolepsy (Mignot, 1998). The age of onset varies from early childhood to the fifties with a large peak around puberty and a smaller one at 36 years (Dauvilliers et al., 2001; Mayer et al., 2002). In up to 18% narcolepsy is associated with REM sleep behaviour disorder (RBD) (Schenck and Mahowald, 1992; Mayer et al., 2002), which is clinically characterized by the intermittent loss of normal skeletal muscle atonia during REM sleep with the appearance of elaborate motor activity associated with dreaming. Dramatic, often violent behaviours such as punching or kicking, usually accompanied by vivid dreams are the most common complaints of RBD patients (for review see Schenck and Mahowald, 2002). An increasing number of studies shows that RBD is associated with -synucleinopathies, e.g. Parkinson's disease. Sixty-five per cent of patients initially diagnosed with idiopathic RBD develop a parkinsonian neurodegenerative disorder 10–30 years after the onset of RBD. Furthermore, the majority of patients with idiopathic RBD present with hyposmia (Fantini et al., 2005; Stiasny-Kolster et al., 2005) which has been shown to be a preclinical non-motor sign of Parkinson's disease (Ponsen et al., 2004). Thus, it has been proposed that the clinical symptoms of hyposmia and RBD in combination have to be considered as early symptoms of Parkinson's disease (Stiasny-Kolster et al., 2005). Recently, we have shown that not only patients with idiopathic RBD but also patients in whom RBD is associated with narcolepsy present with a marked olfactory dysfunction characterized by an increased olfactory threshold and an impaired odour discrimination and identification (Stiasny-Kolster et al., 2005). To further clarify if hyposmia in RBD patients with concomitant narcolepsy is RBD specific or if narcolepsy per se is associated with an olfactory dysfunction, we investigated olfactory function in narcoleptic patients with and without associated RBD. |
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Patients and control subjects
Olfactory testing was performed in 20 patients with narcolepsy associated with RBD (Narc/+RBD) and in 20 narcolepsy patients who had no history or polysomnographic features of RBD (Narc/–RBD). Patients were referred to the sleep clinic for diagnostic work-up of narcoleptic symptoms. Narcolepsy was diagnosed according to standard criteria (Atlas Task Force of the American Sleep Disorders Association, 2005). The presence or absence (including subclinical forms) of RBD was assessed with polysomnography (PSG) as described previously (Stiasny-Kolster et al., 2005
).
Control subjects
Since olfactory function varies in the general population, differs between gender and declines with age (Doty et al., 1984; Deems et al., 1991), we investigated healthy control subjects in parallel although normative data are available for the ‘Sniffin’ Sticks' (see below). Olfactory testing was therefore performed in 40 age- and sex-matched healthy control subjects (20 controls for Narc/+RBD and Narc/–RBD each). All control subjects were systematically interviewed about their sleep history, daytime sleepiness and other symptoms such as cataplexy and excluded if there was any indication of the presence of narcolepsy and/or RBD. PSG was not performed in controls.
Patients or control subjects with cognitive impairment were excluded. To assess cognitive impairment the Mini-Mental State Examination (MMSE) was applied to both groups (Folstein et al., 1975). In addition, subjects with depression, asthma, allergic diseases or diseases which affect the olfactory capability, e.g. chronic nasal infection or chronic alcoholism, were excluded. Subjects with an infection of the upper airways at the time of investigation were also not allowed to participate. Since there is no clear relationship between smoking and olfactory function (Bramerson et al., 2004), smokers were not excluded from the study.
All patients and control subjects provided written informed consent. The study was performed in accordance to the Declaration of Helsinki and approved by the local ethical committee.
Neurological assessment
All patients and control subjects underwent a thorough neurological investigation including part II (Activities of Daily Living) and III (motor section) of the Unified Parkinson's Disease Rating Scale (UPDRS; 31 items; ratings range from 0 to 4, with 0 being normal and 4 reflecting maximal dysfunction) (Fahn et al., 1987).
Olfactory testing
Olfactory threshold, odour discrimination, and odour identification were investigated in three separate subtests using standardized ‘Sniffin’ Sticks' (Hummel et al., 1997). The ‘Sniffin’ Sticks' are commercially available felt-tip pens. The tampon is filled with liquid odourants or odourants dissolved in polypropylene glycol. For odour presentation the cap was removed for 3 s and the pen's tip was placed 2 cm in front of both nostrils. Patients were asked to sniff vigourously since increasing sniffing vigour is known to improve olfactory scores (Sobel et al., 2001). The interstimulus interval was at least 20 s to prevent olfactory desensitization. During the examination the patients were wearing blindfolds. The investigator instructed the patient only with respect to the sniffing test and had no influence on the scoring of the patient. Therefore, no blinding was performed with respect to the group assignment of the probands.
Odour thresholds
The olfactory threshold subtest consists of 16 ‘Sniffin’ Sticks' triplets with different concentrations of n-butanol. Three sticks were presented to the subject in a randomized order. Two contained only the solvent and the third the odourant at a particular dilution. The task of the subject was to identify the stick with the odourant. Presentation of the sticks was continued until the odourant had been successfully discriminated in two successive trials, which triggered a reversal of the staircase. Threshold was defined as the mean of at least four out of seven staircase reversal points. The sticks are numbered according to decreasing odour concentration. Thus, a low score indicates a high olfactory threshold. Normative value for 18- to 50-year-old controls: male 9.4 ± 0.9, female 9.5 ± 0.9; 51–80 years: male 7.1 ± 1.7, female 7.7 ± 2.6 (Hummel et al., 1997).
Odour discrimination
In the odour discrimination subtest 16 ‘Sniffin’ Sticks' triplets were presented in a randomized order. Two pens contained the same odourant and the third a different odourant. The task was to identify the stick that had a different smell. The higher the score (maximally possible score 16), the more the patient is able to discriminate different odours. Normative value for 18- to 50-year-old controls: male 12.1 ± 1.4, female 12.6 ± 1.6; 51–80 years: male 10.6 ± 1.8, female 10.6 ± 1.0 (Hummel et al., 1997).
Odour identification
The third subtest consisted of 16 single sticks and assessed the ability to identify an odour. Using a multiple choice task, identification of individual odourants was performed from a list of four descriptors. The sticks contained familiar fragrances such as orange, leather, cinnamon, peppermint, banana, lemon, liquorice, turpentine, garlic, coffee, apple, clove, pineapple, rose, aniseed and fish. Again, the subjects' scores ranged from 0 to 16 (the higher the score, the better the patient's ability to identify an odour). Normative value for 18- to 50-year-old controls: male 14.9 ± 1.2, female 14.5 ± 1.2; 51–80 years: male 14.2 ± 1.5, female 13.3 ± 1.5 (Hummel et al., 1997).
Based on an investigation of >1000 subjects (Kobal et al., 2000), olfactory function assessed by the ‘Sniffin’ Sticks' can be categorized into five stages using the bilateral test scores of threshold (T), discrimination (D) and identification (I) testing resulting in the threshold-discrimination-identification (TDI) sum score. The stages are defined as follows: anosmia (TDI score 
15), severe hyposmia (15 < TDI score 20), moderate hyposmia (20 < TDI score 25), mild hyposmia (25 < TDI score 30) and normosmia (30 < TDI score).
Statistics
Differences between narcolepsy patients (Narc/+RBD and Narc/–RBD) and control subjects were analysed by means of the Mann–Whitney U-Test. For correlation analysis, Spearman Rank Correlation Coefficients were calculated. Results are reported as mean ± standard deviation.
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Results |
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Patient characteristics
Twenty narcolepsy patients with RBD (9 male and 11 female; mean age 45.4 ± 14.0 years, range 20–75 years) and 20 narcolepsy patients without RBD (8 male and 12 female; mean age 44.4 ± 13.40 years, range 20–70 years) who fulfilled the diagnostic criteria of the ASDA (Atlas Task Force of the American Sleep Disorders Association, 2005
) participated. In the Narc/+RBD group narcolepsy was present for 19.4 ± 11.2 years (range 1–40 years) compared with a mean duration of 15.9 ± 17.3 years (range 1–61 years) in the Narc/–RBD group. In the Narc/+RBD group 19 patients suffered from cataplexy, and their MSLT revealed a mean sleep latency of 5.1 ± 3.6 (1.0–13.0) min with 2 or more SOREM episodes being present in 10 patients. A positive human leucocyte antigen (HLA) DQB1*0602 association was documented in 16 patients. In the Narc/–RBD group 19 patients suffered from cataplexy, and their MSLT revealed a mean sleep latency of 5.1 ± 3.0 (1.8–13.2) min with 2 or more SOREM episodes being present in 13 patients. A positive HLA DQB1*0602 association was documented in 11 patients of this group. The mean CSF hypocretin-1 concentration which was determined in six patients (Narc/+RBD, n = 2; Narc/–RBD, n = 4) who were negative for HLA DQB1*0602 was 339.9 ± 320.4 (151.7–962.5) pg/ml. In narcoleptic patients with RBD symptoms of RBD were present for an average duration of 21.2 ± 13.4 years (range 3–45 years). In all but two patients (90%) RBD symptoms preceded symptoms of narcolepsy for several years or RBD started concomitantly.
PSG findings revealed an elevated tonic or phasic muscle tone of mentalis and tibialis muscles during REM sleep stages in all Narc/+RBD patients and a lack of increased skeletal muscle activity in all Narc/–RBD patients.
Neurological assessment
In the neurological examination signs of parkinsonism were found in none of the patients. In the Narc/+RBD group the mean UPDRS II score was 1.8 ± 1.8 (range 0–6) points and the UPDRS III score 0.4 ± 1.1 (0–5) points. In the Narc/–RBD group the mean UPDRS II score was 1.3 ± 1.1 (0–5) points, and the mean UPDRS III score 0 points. Increased UPDRS II or III scores were due to non-parkinsonian symptoms such as falls during cataplexy or gait disturbances due to concomitant diseases, e.g. arthrosis. None of the patients revealed cognitive impairment by means of the MMSE (Narc/+RBD: mean 29.1 ± 0.91, range 27–30; Narc/–RBD: mean 29.05 ± 0.82, range 28–30). In the Narc/+RBD group 13 patients were smokers compared with 6 in the Narc/–RBD group. Patients' clinical characteristics are presented in Table 1.
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Characteristics of control subjects
The Narc/+RBD control group consisted of 9 males and 11 females (mean age 45.5 ± 14.3 years, range 20–73 years) with a normal neurological examination and MMSE score (mean 29.9 ± 0.30, range 29–30). Six of them were smokers. The Narc/–RBD control group consisted of 8 males and 12 females (mean age 44.3 ± 13.3 years, range 20–69 years) with a normal neurological examination and MMSE score (mean 29.7 ± 0.66, range 28–30). Five of them were smokers. Symptoms for clinical narcolepsy, RBD or other sleep and movement disorders could not be found.
Olfactory testing
The mean olfactory threshold score of the Narc/+RBD control subjects was 8.93 ± 1.43, the discrimination score 13.05 ± 1.35 and the identification score 13.70 ± 1.17.
The mean olfactory threshold score of the Narc/–RBD control subjects was 8.91 ± 1.03, the discrimination score 13.05 ± 1.19 and the identification score 13.30 ± 1.34 (Fig. 1).
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One subject of the Narc/–RBD control group reported an impaired sense of smell.
The scores of the control subjects were within the range of the published normative data (Hummel et al., 1997).
The mean olfactory threshold score of the Narc/+RBD group was 5.86 ± 2.31, the discrimination score 11.20 ± 1.58 and the identification score 12.95 ± 1.61. Comparison of Narc/+RBD patients and sex- and age-matched control subjects showed a significantly higher mean olfactory threshold score (P = 0.0001), lower mean discrimination score (P = 0.001) and lower mean identification score (P = 0.057) in the Narc/+RBD patients (Fig. 1). The TDI score was significantly lower (P = 0.0001) in Narc/+RBD patients (30.03 ± 3.3) compared with controls (35.67 ± 2.14) (Table 1). In Narc/+RBD patients mild hyposmia as defined by the TDI score was present in 30% (n = 6), moderate hyposmia in 10% (n = 2) and severe hyposmia or anosmia in none. Two patients were aware of an olfactory deficit before testing.
The mean olfactory threshold score of the Narc/–RBD patients was 6.5 ± 1.60, the discrimination score 11.25 ± 2.61 and the identification score 12.35 ± 0.99. Comparison of Narc/–RBD patients and controls showed a significantly higher mean olfactory threshold score (P = 0.0001), lower mean discrimination score (P = 0.014) and lower mean identification score (P = 0.003) in the Narc/–RBD patients (Fig. 1). (Table 1). The TDI score was significantly lower (P = 0.0001) in Narc/–RBD patients (30.10 ± 3.93) compared with controls (35.26 ± 3.07) (Table 1). In Narc/–RBD patients mild hyposmia was present in 30% (n = 6), moderate hyposmia in 15% (n = 3) and severe hyposmia or anosmia in none. One patient was aware of an olfactory deficit.
Subgroup analysis of the patients with CSF-hypocretin measurements showed a mean olfactory threshold score of 7.50 ± 2.32, a discrimination score of 12.50 ± 1.05 and an identification score 13.17 ± 1.50.
In both patient groups the duration of narcolepsy was not significantly correlated with olfactory dysfunction. However, in the Narc/+RBD group the duration of RBD significantly correlated with an impaired olfactory threshold (P = 0.008). CSF hypocretin-1 did not significantly correlate with olfactory impairment. In neither group smoking habits correlated with olfactory dysfunction.
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Discussion |
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This study shows for the first time that narcoleptic patients both with and without RBD have an impairment of olfactory function when compared with healthy controls. Both patient groups had a significantly increased olfactory threshold and a pronounced impairment of odour discrimination and identification. Motor symptoms or signs for early parkinsonism were not identified in any narcoleptic patient. Our results show that narcolepsy per se is associated with olfactory dysfunction. In contrast to patients with idiopathic RBD, hyposmia in patients with RBD associated with narcolepsy is unlikely to have a clinical relevance for developing parkinsonism. Since there is a well established strong pathophysiological link between hypocretin deficiency and narcolepsy, it is possible that hyposmia results from an impaired hypocretinergic neurotransmitter system.
Narcolepsy and hypocretin
The pathophysiology of narcolepsy is still unknown (for review see Dauvilliers et al., 2003). Ninety-five per cent of cases of human narcolepsy are sporadic and there is a relatively low rate of concordance (up to 32%) for narcolepsy in monocygotic twin studies (Mignot, 1998). This suggests that non-genetic factors play a major role. Besides less frequent HLA associations the DQB1*0602 allele is present in 
90% of patients suggesting an immunogenetic pathophysiology. However, the mechanisms why certain HLA associations predispose to narcolepsy remain unknown. Almost all attempts to prove that narcolepsy is an autoimmune disease have been negative. A loss of hypocretin (Hcrt)-containing neurons and a lack or low level of Hcrt-1 in the CSF are pathognomonic of narcolepsy (Nishino et al., 2000; Ripley et al., 2001; Mignot et al., 2002). Specific mutations at the preproHcrt, hypocretin receptor (Hcrtr)-1 or Hcrtr-2 loci as observed in animal models are rare in human narcolepsy (Peyron et al., 2000; Hungs et al., 2001; Olafsdottir et al., 2001; Chabas et al., 2003). It has been demonstrated that narcoleptics have an 85–95% reduction in the number of Hcrt neurons (Peyron et al., 2000; Thannickal et al., 2000) which are exclusively located in the hypothalamus. The residual gliosis in the Hcrt cell region (Thannickal et al., 2000) and the association of narcolepsy with the HLA system suggest the possibility of an autoimmune process leading to a selective loss of hypocretin neurons.
Hypocretin-1 and -2 (also called orexin-A and -B) are recently discovered neuropeptides. Due to the observation that lesions in the hypothalamus markedly reduce food intake (Bernardis and Bellinger, 1996) and that preprohypocretin transcripts were upregulated by fasting while central administration of these peptides stimulate feeding, hypocretins were initially associated with the regulation of food intake (Sakurai et al., 1998). The subsequent finding that hypocretin-containing cell bodies have widespread projections to the complete neuroaxis suggested a more complex function (Peyron et al., 1998). In particular, dense projections to all monoaminergic cell groups (locus coeruleus, raphe nucleus, substantia nigra, ventral tegmental area and tuberomamillary nucleus) exist (Peyron et al., 1998). Two receptors, Hcrtr1 and Hcrtr2 are known. Hcrtr1 has a significantly higher affinity for hypocretin-1 while Hcrtr2 binds hypocretin-1 and -2 with similar affinity (Marcus et al., 2001). Discoveries in animal models demonstrated that canine narcolepsy is caused by mutations in the Hcrtr2 gene (Lin et al., 1999) and that hypocretin knock-out mice have sleep and behavioural abnormalities similar to narcolepsy (Chemelli et al., 1999). These findings and the wake-promoting effects of centrally administered hypocretins (Hagan et al., 1999) further shifted the role of hypocretins to sleep regulation.
RBD, narcolepsy and hyposmia
An increasing number of studies shows that RBD is associated with -synucleinopathies, e.g. Parkinson's disease or multiple system atrophy. Longitudinal studies from Schenck et al. identified 38% of 29 patients with RBD who subsequently developed a parkinsonian disorder after 12.7 years. The follow-up study demonstrated that 65% developed parkinsonism and/or dementia. IPT-SPECT studies also revealed a reduced dopamine transporter binding in patients with ‘idiopathic’ RBD in terms of presymptomatic parkinsonism (Eisensehr et al., 2001). The neuropathological staging of Parkinson's disease proposed by Braak et al. (2003) using -synuclein immunohistochemistry revealed a striking overlap between the presumed brainstem areas involved in RBD [e.g. nucleus reticularis magnocellularis (NRMC), pontine areas, ventral mesopontine junction] and Parkinson's disease pathophysiology. According to the Braak classification Lewy bodies (containing aggregated -synuclein) are found initially in the IX/X motor nucleus and the olfactory nucleus/bulb (stage 1). In stage 2 additional lesions are found in the medulla and the pontine tegmentum (NRMC, coeruleus and subcoeruleus region). Only in stage 3 the pedunculo-pontine nucleus and the substantia nigra are affected. -Synuclein pathology finally ascends to more rostral and cortical structures (stages 4–6). This proposed temporal sequence of the spread of synucleinopathy could explain why RBD precedes Parkinson's disease (stage 3) and dementia (stages 4–6). In line with the Braak staging (stage 1) we found that 97% of patients with RBD have hyposmia (Stiasny-Kolster et al., 2005). Accordingly, the clinical symptoms hyposmia and RBD have to be considered early symptoms of Parkinson's disease. Interestingly, not only patients with idiopathic RBD (Fantini et al., 2005) but also patients with RBD associated with narcolepsy were found to have hyposmia (Stiasny-Kolster et al., 2005). Our present results replicate these findings but furthermore show that hyposmia is associated with narcolepsy independently of concomitant RBD. Thus, it appears likely that hyposmia points at an underlying -synucleinopathy in ‘idiopathic’ RBD patients only. On the one hand, the lack of parkinsonian signs in all investigated narcoleptic patients and the present level of knowledge that narcoleptics are not known to have an increased risk for developing parkinsonism argue for different pathophysiological mechanisms explaining hyposmia in these patients. On the other hand, no Lewy bodies are found in the brain of human narcoleptics, and neurodegeneration is probably restricted to the Hcrt neurons of the hypothalamus (Peyron et al., 2000; Thannickal et al., 2000). Accordingly, none of the 1500 narcolepsy patients in our sleep disorders centre has ever been diagnosed with Parkinson's disease. Nevertheless, our findings should prompt an epidemiological investigation on the prevalence of parkinsonism in narcolepsy patients (and vice versa).
As mentioned above, narcolepsy can be considered a neurodegenerative disease with a selective loss of Hcrt-containing neurons (Peyron et al., 2000; Thannickal et al., 2000). Although these cell bodies are restricted exclusively to the hypothalamus, they have widespread projections throughout the brain (Peyron et al., 1998). Besides the olfactory bulb (Peyron et al., 1998) hypocretin fibres were also detected in the anterior olfactory nuclei, piriform cortex and amygdala (Caillol et al., 2003). In addition, Caillol and co-workers showed for the first time the presence of both hypocretins and their receptors in the nasal mucosa and that these peptides are locally synthesized in the olfactory mucosa (Caillol et al., 2003). Thus, hypocretin, their receptors and hypocretin fibres are present from the peripheral sensory level to central regions of integration. The neuroanatomical localization of hypocretins and their receptors at very specific areas of the olfactory system suggests that these neuropeptides might play a role in the modulation of the olfactory message. It has also been hypothesized that the hypocretins may increase the efficacy of olfactory perception during periods of fasting and wakefulness (Caillol et al., 2003). Our results of an olfactory impairment in the hypocretin deficiency disorder narcolepsy are consistent with these recently published animal data. The lack of correlation between hypocretin-1 levels and hyposmia is not surprising since hypocretin-1 was only determined in those with negative HLA association, cases which generally have normal hypocretin-1 levels. In addition, mean olfactory test scores in this subgroup of patients were higher for all subtests.
Pathophysiology of RBD in narcolepsy
Although both Parkinson's disease and narcolepsy can be associated with RBD and hyposmia, it appears probable that—in analogy to the olfactory dysfunction—RBD in Parkinson's disease and RBD in narcolepsy feature a different pathophysiology. It has been hypothesized that RBD in general is due to lesions to certain areas in the brainstem rather than to a specific type of lesion (Mayer and Blanke, 2004; Unger et al., 2006). As explained above, RBD in Parkinson's disease is probably caused by Lewy body pathology in such areas as the NRMC. The pathophysiology of RBD in narcolepsy, however, is still unknown. A recent study reported that up to 36% of narcolepsy patients have symptoms suggestive of RBD (Nightingale et al., 2005). Previous estimates ranged from 7 to 18% (Schenck and Mahowald, 1992; Mayer et al., 2002). One possible explanation for the occurrence of RBD in narcolepsy patients is that the lack of hypocretin reduces the suppression of muscle tone during REM sleep by the pontine inhibitory area, which is in close proximity to the locus coeruleus (Kiyashchenko et al., 2001). The locus coeruleus in turn has been implicated in the pathogenesis of cataplexy indicating that the degeneration of the hypocretinergic projections to these pontine areas may be involved in the comorbidity of RBD and cataplexy in narcolepsy patients (Mileykovskiy et al., 2005).
It is still under discussion if the severity of narcoleptic symptoms depends on the amount of destroyed hypocretinergic neurons. If this hypothesis was correct, one would expect RBD to manifest rather early in narcolepsy. Thus the duration of symptoms in the Narc/+RBD group supports this hypothesis, whereas the finding of normal CSF-hypocretin levels in HLA DQB1*0602 negative patients does not. With respect to olfactory dysfunction one would not expect the loss of hypocretinergic neurons in the nasal mucosa to be reflected by a change of hypocretin in the CSF. As we do not have any histological proof of early loss of these neurons in the nasal mucosa, we can at this point only speculate that the decrease of olfactory function may be an early symptom of neurodegeneration in narcolepsy. Since RBD in most cases precedes narcolepsy (Mayer and Meier-Ewert, 1993) degeneration of hypocretinergic cells in the nasal mucosa and thus olfactory impairment may play a role as early indicators for narcolepsy.
Conclusions
Olfactory dysfunction is an early non-motor sign of some neurodegenerative disorders including narcolepsy and RBD. The majority of narcoleptic patients present RBD as an early symptom as well; however, no degenerative process within the bulbus olfactorius or the brainstem has been shown so far. The neurodegenerative process seems to be different in the two disorders. Our results may provide an incentive for future research into the pathophysiology of narcolepsy, in which olfactory dysfunction may be an early predictor of local degeneration of hypocretinergic mucosa cells.
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