Prevalence of ISP, FISP, and Panic Disorder
Two (1.5%) patients who endorsed SP episodes were excluded from the ISP groups due to preexisting medical conditions likely responsible for their SP experiences (one patient had narcolepsy and one had a seizure disorder). As is evident from , definitions of SP have an impact on reported prevalence: 29.3% of the sample reported having a lifetime experience of ISP using International Classification of Sleep Disorders criteria.
Isolated Sleep Paralysis, Fearful Isolated Sleep Paralysis, and Recurrent Fearful Isolated Sleep Paralysis Prevalence Rates and Percentages by Ethnicity
The majority of the ISP patients in this sample endorsed significant fear. Of the total sample, 20.3% met criteria for a lifetime FISP episode and 12.8% met recurrent FISP criteria. Of note, 12.8% of the total sample reported experiencing at least one FISP episode in the past month.
Prevalence rates using the three definitions of ISP as a function of ethnicity are presented in . Although small sample sizes for three of the five ethnicities precluded individual analyses for each group, non-Caucasians were more likely to have ISP regardless of definition (lifetime ISP χ2[1, N = 132] = 8.28, p < .005; lifetime FISP episode χ2[1, N = 132] = 14.69, p < .001; recurrent FISP χ2[1, N = 132] = 10.45, p < .004). Gender differences were found as well, with men being more likely to report lifetime ISP, χ2[1, N = 133] = 4.26, p < .04, but this pattern was not evident for lifetime FISP episode or recurrent FISP.
The majority of the sample met diagnostic criteria for panic disorder (95.5%). Further, 100% of the recurrent fearful ISP, 96.3% of the lifetime fearful ISP episode, and 92.3% of lifetime ISP episode groups met full panic disorder criteria.
Severity, Fear, and Interference
Mean lifetime ISP episode severity (which is actually a measure of frequency in AASM, 2001
) was 1.37 (SD
= 0.74; range = 0–3), indicating ISP occurred approximately once per month. Mean lifetime FISP episode fear severity was 5.53 (SD
= 1.04; range = 0–8), indicating moderate to severe levels of fear. The overall recurrent FISP severity was assessed using Structured Severity Scale scoring. Total mean score was 5.22 (SD
= 1.94; range = 0–9) with individual component means of 1.98 (SD
= 0.67) for frequency, 2.18 (SD
= 0.61) for fear/distress, and 1.06 (SD
= 0.95) for interference.
Duration, Frequency, and Sleeping Position of ISP and FISP Episodes
The average self-reported duration of FISP episodes was 6.9 minutes (SD = 10.0; range = 6 seconds–45 minutes), with 3.5 minutes being the median score. For those meeting criteria for recurrent FISP, the mean number of episodes for the past month was 4.1 (SD = 5.7; range = 0–22) with a median of 2.0. Past 6-month frequencies for the same group averaged 20.1 (SD = 31.0; range = 2–132) with a median frequency of 9.0. 58.3% of participants experienced lifetime FISP episodes while sleeping in a supine position, 8.3% on their chest, 16.7% on their side, and 16.7% reported more than one position.
Hallucinatory Content of FISP
Hypnagogic and hypnopompic hallucinations were frequent in individuals who met lifetime FISP episode criteria. Only 11.5% of these (and 11.7% of the recurrent FISP group) did not describe hallucinatory experiences. An average of 5.54 of 15 hallucinatory symptoms () were endorsed in the lifetime FISP episode group (SD = 4.08, range = 0–15).
Fearful Isolated Sleep Paralysis Hallucinatory Content—Percentage Endorsing Symptoms and Distress/Severity Means (Standard Deviation)
ISP’s and FISP’s Relationship to Mood and Anxiety Symptomatology
As a preliminary step toward better understanding its relation to other forms of psychopathology, we correlated the three definitions of ISP to several clinician-administered (Hamilton Anxiety and Depression Scales, Panic Disorder Severity Scale) and self-report (Anxiety Sensitivity Index, Brief Bodily Sensations Interpretation Questionnaire) measures. Contrary to expectation (see ), ISP and FISP were largely unrelated to interview measures of depression, anxiety, and panic with the exception of a small significant positive correlation between the Hamilton Depression Scale and recurrent FISP. With regard to self-report instruments, the Anxiety Sensitivity Index was significantly correlated with lifetime FISP episode and recurrent FISP, and the Brief Bodily Sensations Interpretation Questionnaire was significantly associated only with lifetime FISP episode. The presence of a lifetime ISP episode was not significantly associated with any symptom measures. Supplementary analyses () indicated a positive relation between each of the definitions of ISP and increasing numbers of nonpsychotic mood and anxiety disorder diagnoses.
Correlations Between Isolated Sleep Paralysis, Fearful Isolated Sleep Paralysis, Recurrent Fearful Isolated Sleep Paralysis, and Other Measures
ISP’s and FISP’s Relation to PTSD
Individuals with PTSD were significantly more likely to have recurrent FISP than those without, χ2(1, N = 128) = 11.94, p < .004. Although only 17 participants met PTSD criteria, 6 of these also met recurrent FISP criteria, and comprised 42.9% of the total recurrent FISP sample. Similar results were evidenced with the lifetime FISP episode group, χ2[1, N = 128] = 6.47, p < .02, but not the lifetime ISP episode group (p = .052).
ISP’s and FISP’s Relationship to IQ
Contrary to expectation, lower IQs are associated with all three definitions of ISP (see ).
ISP’s and FISP’s Relationship to SSRI Use
SSRI use was not found to be associated with any of the definitions of ISP (range of rs = −.07–−.14, all ps > .10).
ISP’s and FISP’s Relationship to BMI
As can be seen in , BMI was positively and consistently correlated with lifetime FISP episodes and recurrent FISP, but not lifetime ISP episodes.
Untangling the Relationship Between Race and FISP
As FISP, recurrent FISP, and ISP appear to be associated with ethnicity, it is important to assess whether these relationships were due to other variables (i.e., BMI, PTSD). When controlling for BMI and PTSD, the correlation between non-Caucasian status and lifetime FISP episode (r = .209; p = .029) remains significant, but not for recurrent FISP (r = .125; p = .196). Further, the correlation between PTSD and recurrent FISP remains significant (r = .208; p = .030) when BMI and non-Caucasian status are partialed out, but the correlation for lifetime FISP episode (r = .107; p = .269) does not. Due to the relatively small and nonsignificant differences between correlations as well as the relatively small Ns involved in these analyses, caution should be exercised in their interpretation.