Pupillary Light Reflex (PLR)

O Reflexo Pupilar à Luz (PLR) é um fenômeno bem conhecido há muitos anos. Em 1942, Lowenstein e Friedman demonstraram que a pupila se contrai em resposta à luz após um período de latência. Eles também descobriram que a duração desse período de latência, a amplitude da resposta e a velocidade da constrição pupilar dependem da intensidade do estímulo utilizado. Esses achados foram posteriormente validados por Alpern et al. em 1963, Feinberg e Podolak em 1965, e Lowenstein e Loewenfeld em 1969.

CONTROLE AUTÔNOMO DA PUPILA OLHO

Os olhos recebem inervação de fibras nervosas simpáticas e parassimpáticas, conforme representado nas Figuras 1 e 2. As fibras parassimpáticas pré-ganglionares originam-se no núcleo de Edinger-Westphal, que faz parte do terceiro nervo craniano. Essas fibras então viajam dentro do terceiro nervo até o gânglio ciliar localizado atrás do olho. Neste ponto, os axónios pré-ganglionares ligam-se aos neurónios parassimpáticos pós-ganglionares, que subsequentemente enviam as suas fibras para o globo ocular através dos nervos ciliares. Os nervos ciliares ativam duas estruturas principais do olho: 1) o músculo ciliar, que controla o foco do cristalino, e 2) o esfíncter da íris, que causa a contração da pupila.

A inervação simpática do olho surge das células do corno intermediolateral localizadas no primeiro segmento torácico da medula espinhal. A partir desta origem, as fibras simpáticas entram na cadeia simpática e viajam em direção ao gânglio cervical superior, onde se conectam com os neurônios pós-ganglionares. Depois disso, as fibras simpáticas pós-ganglionares movem-se ao longo da superfície da artéria carótida até atingirem o olho. Aqui, eles inervam as fibras radiais da íris, o que faz com que a pupila se dilate ou se abra.

Fig. 1

O diâmetro da pupila é controlado por dois mecanismos opostos. A estimulação dos nervos parassimpáticos desencadeia a ativação do músculo esfíncter pupilar, causando uma diminuição no tamanho da pupila, o que é chamado de miose. Por outro lado, a estimulação dos nervos simpáticos excita as fibras radiais da íris, resultando na dilatação da pupila, o que é conhecido como midríase.

Fig. 2

When light enters the eyes, the pupils naturally constrict as a reflex. The neural pathway responsible for this reflex is indicated by the black arrows shown in Figure 1. When light strikes the retina, a portion of the signal travels through the optic nerves to the pretectal region. From there, secondary impulses travel to the Edinger-Westphal nucleus and then return via the parasympathetic nerves to activate the iris sphincter, resulting in its contraction and subsequent decrease in pupil size. Conversely, in a dark environment, the reflex is inhibited, which leads to the dilation of the pupil.

The purpose of the Pupillary Light Reflex (PLR) is to assist the eyes in adapting rapidly to variations in light intensity. The diameter of the pupil has a minimum limit of about 1.5 mm and a maximum limit of about 8 mm. Since the brightness of light on the retina varies with the square of the pupillary diameter, the range of light and dark adaptation that can be achieved by the pupillary reflex is approximately 30 to 1. This implies that the PLR can accommodate up to a 30-fold shift in the amount of light that enters the eye, allowing the eyes to adjust to different lighting conditions more effectively.

PUPILLARY REFLEXES, CENTRAL NERVOUS SYSTEM AND PUPILLOMETRY

The Pupillary Light Reflex (PLR) has proven to be a useful tool in assessing an individual's ability to perform certain tasks, such as driving a motor vehicle (Monticelli et al, 2009, 2015). This is because the PLR can be significantly impacted by factors such as sleep deprivation, alcohol consumption, and drug use. Therefore, monitoring changes in PLR can help identify impairment due to these factors, providing valuable information for evaluating an individual's fitness to perform certain tasks safely and effectively.

When evaluating an individual's ability to perform a task, it is important to consider the three different neural processes that contribute to reaction time: (1) information processing, (2) response programming, and (3) motor control of muscles. The brain's response to sensory stimulation, particularly visual stimulation, is influenced by the demands of perception, while the reaction time is influenced by the combined demands of perception and motor activity. By measuring the peak latency of sensory potentials and the reaction time separately, it may be possible to reach different conclusions about the individual's performance. However, when used together, these measurements can provide valuable information for distinguishing between different sources that may affect task performance.

Visually evoked cortical potentials provide a direct and precise measurement of brain activity with high temporal resolution (Hall, 2016). This can be achieved through the non-invasive measurement of the PLR, which indicates the activity of the system responsible for regulating pupillary reactions, located between the diencephalon and the mesencephalon. This method provides valuable information about brain activity and can be used to evaluate the function of this system in a non-invasive manner.

In a study conducted on 19 normal subjects using infrared pupillography, the pupillary light reflex (PLR) was analyzed in response to different stimulus intensities. Results showed that as the intensity of the stimulus increased, there was an increase in the amplitude and maximum rate of contraction and redilatation of PLR, and the latency of the response decreased. These findings led to the proposal of PLR analysis for clinical evaluation of pupillary function.

Monticelli et al. (2009) suggested using PLR as an objective measurement method for evaluating vehicle driving safety, which would allow for reproducible, reliable, and verifiable data collection. To achieve this, healthy individuals (n=41) and individuals under the influence of drugs and/or medications (n=105) were exposed to different light stimuli, and their PLR responses were evaluated. The primary goal of the study was to assess the applicability and value of pupillography as an objective measurement method for evaluating people with central nervous system disorders with regards to their driving safety and ability to drive vehicles.

The study showed significant differences in almost all parameters when comparing healthy individuals and those under the influence of drugs/medications, demonstrating that pupillography is an objective method to measure pupillary function and can be used in routine police control of vehicle drivers. This was confirmed by a follow-up study that demonstrated the reliability of PLR as an indicator of previous medication and/or drug use. Alcohol has also been shown to affect pupillary measures.

PLR response has also been studied in relation to fatigue and lack of sleep. Lowenstein and Lowefnfeld (1963) demonstrated the use of PLR analysis for objective evaluation of tiredness, while Morad, Lemberg, Yofe, and Dagan (2000) found significant differences in all pupillary parameters between alertness and fatigue.