Assessment of vigilance using pupillometry in epilepsy, idiopathic hypersomnia and narcolepsy

Fatigue, impaired vigilance and hypersomnolence are common but poorly
understood symptoms in epilepsy and sleep disorders. These symptoms are
incapacitating, impact the quality of life but simple assessments are still
lacking. As a result these measures are often not included in treatment
studies. This protocol aims to improve the assessment of these common
complaints.

Type 1 narcolepsy is a chronic neurological disorder, caused by the loss of
hypocretin producing neurons in the lateral hypothalamus, leading to cataplexy,
decreased vigilance and excessive daytime sleepiness. Hypocretin neurons show
activity during wakefulness and stimulate target neurons in the cortex, basal
forebrain, hypothalamus and brain stem, which maintain this state of
wakefulness after prolonged activity. Patients with type 2 narcolepsy similarly
suffer from excessive daytime sleepiness and decreased vigilance, but do not
experience cataplexy or a hypocretin deficiency (Scammel, 2015). In idiopathic
hypersomnia, patients are excessively sleepy with decreased vigilance, but no
cause for their symptoms can be found (Van Schie et al., 2012).
Patients with epilepsy may too exhibit impaired vigilance. In their
case, however, no structural deficits in the hypocretin-pathways are found. It
is assumed that their vigilance is either impaired as a result of nocturnal
epileptic seizures causing sleep deprivation (Altena et al., 2008) or due to
side effects of antiepileptic drugs (Loring&Meador, 2001).
All abovementioned neurological conditions may serve as a model to
improve our understanding of the biological substrate of vigilance.

The circadian modulation of vigilance is driven by the biological clock located
in the suprachiasmatic nucleus (SCN) of the hypothalamus, which is synchronized
to the 24-hour light/dark cycle (Moore et al., 1995). This photo-entrainment is
orchestrated by intrinsically photosensitive retinal ganglion cells (ipRGCs)
expressing the photopigment melanopsin (peak sensitivity ~ 480nm), which
directly transduce information on environmental light levels from the retina to
the biological clock. Furthermore, these cells contribute to the dynamics of
pupillary light reflex (Lucas et al., 2001).
The functionality of the intrinsic melanopsin-based phototransduction
circuitry can be assessed by the measurement of the sustained pupil
constriction after blue light, which is a feature of the pupillary light reflex
that is specific to the melanopsin-signaling pathway. This phenomenon has been
dubbed *Post-Illumination Pupil Response* (PIPR) (Gamlin et al., 2007).
Individual differences in the functionality of this melanopsin-based
photo transduction circuitry are associated with individual differences in
circadian phase (Van der Meijden et al., 2016), which led to the hypothesis
that the altered circadian modulation of vigilance in narcolepsy may arise from
differences in the functionality of the melanopsin-driven photo transduction
circuitry.

When activated by light, through expression of melanopsin, ipRGCs regulate
pupil diameter by activating the olivary pretectal nucleus (OPN) (Hattar et
al., 2006). Hypocretin is also hypothesized to affect vigilance by their
presence in ipRGCs (Liu et al., 2011; Savaskan et al., 2004). Interconnection
between the hypocretin and melanopsin pathways was confirmed in 2011, when Liu
and colleagues found hypocretin in numerous ipRGCs containing melanopsin and in
2015, when Liu and colleagues found potentiation of ipRGCs through hypocretin
activation.


The PIPR after blue light will be assessed in the following six groups: (1)
type 1 narcolepsy; (2) type 2 narcolepsy; (3) idiopathic hypersomnia; (4)
epilepsy with only daytime seizures; (5) epilepsy with only nighttime seizures
and (6) healthy controls, using a validated pupillometry paradigm. In addition,
vigilance levels in these groups will be assessed using the *Sustained
Attention to Response Task* (SART) (Fronczek et al., 2006) and questionnaires.
As previously stated, the functionality of the ipRGCs may be mediated
through hypocretin signaling. Since type 1 narcoleptic patients are hypocretin
deficient, it is hypothesized that this may directly affect ipRGC function. It
is thus expected that the PIPR in type 1 narcolepsy patients differs from the
PIPR of type 2 narcolepsy and the other abovementioned disorders that are all
characterized by decreased vigilance, but not by hypocretin deficiency; and
that all groups may differ from healthy controls and epilepsy patients with
only daytime seizures.

Primary question
Are changes in vigilance in type 1 and 2 narcolepsy, idiopathic hypersomnia and
epilepsy with only nocturnal seizures reflected in differences in the
responsiveness of the pupils to light as measured by pupillometry (PIPR) versus
healthy controls and epilepsy with only daytime seizures?

Secondary question
What are the differences in vigilance as measured by the SART between type 1
and 2 narcolepsy, idiopathic hypersomnia, epilepsy with only daytime or
nocturnal seizures and healthy controls? Is there a difference between
hypocretin deficient subjects (type 1 narcolepsy) and the other groups that are
not hypocretin deficient?

In a cross-sectional study design, the functionality of the melanopsin-pathway,
i.e. responsiveness to light, will be assessed in six groups (see below), using
pupillometry and the SART.

The experiment exists of the following parts (figure 1):
1. Color deficiency test (1 minute)
2. Questionnaires (25 minutes)
3. SART instructions and exercise (5 minutes)
4. SART 1 (5 minutes)
5. Post-illumination pupil response (PIPR) (15 minutes)
6. SART 2 (5 minutes)


Color deficiency test: The Richmond HRR 2002
In order to make sure the participants do not have a color deficiency, the
Richmond Hardy-Rand-Rittler (HRR) 2002 (Cole et al., 2006) plates are used. The
entire test consists of 24 plates: the first four plates serve as practice, the
next six plates (screening series) determine possible color deficiency and the
last 14 plates (diagnostic series) grade the severity and type of deficiency.
Since we merely want to investigate whether or not the participants suffer from
a form of color deficiency, the first 10 plates suffice. A correct response to
all these 10 plates is defined as having *normal color vision*.

The plates, placed in a light box (1280lux), perpendicular to the line of sight
at 400mm fixed distance, display one or two symbols (a cross, a circle and/or a
triangle) constructed of colored dots on a background of grey dots. The
participants are asked to name the shape and location of the symbol(s), which
can be in one of the four quadrants of each plate, within three seconds.

Questionnaires
This experiment continues with the following questionnaires;
• The Munich Chronotype Questionnaire (MCTQ) (Roenneberg et al., 2003)
• The Pittsburgh Sleep Quality Index (PSQI) (Buysse et al., 1988)
• The Epworth Sleepiness Scale (ESS) (Johns, 1991)
• The Fatigue Severity Scale (FSS) (Krupp, 1989)
The MCTQ gives insight into everyday life and circadian rhythm and consists of
seven chronotypes, differing from *extreme early* to *extreme late*. The PSQI
assesses subjective sleep quality in the form of a self-report. This
questionnaire consists of seven components forming a *global PSQI score*,
ranging between 0 and 21. Higher global PSQI scores indicate worse sleep
quality. Lastly, the ESS score [range 0-24] is used in order to measure daytime
sleepiness. A score between 0 and 9 is considered normal, whereby a score
between 10 and 24 gives reason to ask for medical advice. The FSS is used to
determine daytime fatigue, in order to differentiate between daytime sleepiness
from fatigue. It consists of nine questions that are answered on a scale of 1
to 7, with a higher score indicating a greater level of fatigue.

The sustained response to attention task (SART)
The SART will be used as an objective tool to measure vigilance. The task will
be performed twice. In random order and different size, white numbers ranging
from 1 to 9 will appear on a black computer screen 225 times for a period of 4
minutes and 20 seconds in a quiet room with dimmed lights. Each number is
presented for 250 ms, followed by a black screen for 900 ms. The participants
are instructed to react to appearances of these numbers by pressing a key,
except for the number 3. Accuracy is as important as speed. The SART primarily
measures the total error score, expressed as the sum of the *no-go
trial* (times a key was pressed when a 3 was presented) and a *go trial* (times
when no key was pressed when it should have been pressed). The SART has been
shown to detect decreased vigilance in narcolepsy and other sleep disorders
(Van Schie et al., 2012).

Pupilometry
During the pupil measurements the eyes will first be exposed to mesopic
(dimmed) lighting for 3 minutes. The right eye will then be exposed to baseline
darkness for 1 minute, followed by red light for 20 seconds and 2 minutes of
darkness, followed by another three minutes of mesopic lighting. The right eye
will be exposed to 1 minute of darkness again, followed by 20 seconds of blue
light, and finally 2 minutes of darkness. The left pupil diameter will be
continuously recorded using an infrared LED light and a digital camera. To
avoid any risk, the maximum intensities of the light sources in the set-up are
well below the recommendations of the American National Standard (ANSI-2007)
for red, blue and infrared illumination.

From the baseline and post-blue pupil diameter we calculate two primary PIPR
outcome parameters (Van der Meijden et al., 2015):
1. PIPR-mm = baseline pupil diameter - post-blue pupil diameter
2. PIPR-% = 100 * PIPR-mm/baseline pupil diameter

Not applicable