Olfactory Testing in Perinatal Asphyxia: Enhancing Risk Assessment

Olfactory Memory in Infants With Signs of Asphyxia: An Early Indicator of Neurodevelopmental Outcomes

Neonatal asphyxia remains a leading cause of neurodevelopmental disabilities despite advancements in perinatal care. Hypoxic-ischemic encephalopathy (HIE), a severe outcome of asphyxia, impacts 1-3 infants per 1,000 live births annually in industrialized nations, causing long-term neurological impairments such as cognitive dysfunction, motor deficits, and sensory impairments. Early identification of at-risk newborns is critical to initiate timely interventions and improve outcomes.

Olfactory perception, crucial for newborns' adaptation to extrauterine life, involves odor identification and memory. Odor perception is known to be impaired in adults with neurological disorders and in animal models of brain injury. However, no clinical studies have assessed olfactory function in newborns with signs of asphyxia. Olfactory memory, which can be evaluated through habituation to repeated odors, may provide insights into early brain function.

This study aims to evaluate whether olfactory memory can serve as an early marker of neurodevelopmental outcomes in newborns with signs of asphyxia. By assessing physiological, behavioral, and neurological responses to olfactory stimuli, the study seeks to explore the differences between infants with mild asphyxia and those with moderate-to-severe asphyxia.

Neonatal asphyxia is the leading cause of neurodevelopmental disability. Despite recent advances in perinatal medicine, which have improved associated mortality, the neurological outcomes of hypoxic-ischemic events in newborns remain significant. The incidence of neonatal asphyxia is estimated to be 1.5 per 100 live births per year in industrialized countries with advanced obstetric-neonatal care. Between 1 and 3 infants per 1,000 live births suffer from severe neurological impairment due to perinatal asphyxia, developing hypoxic-ischemic encephalopathy.

Early identification of newborns with signs of asphyxia who are at high risk for hypoxic-ischemic encephalopathy and brain damage can improve outcomes by enabling the early initiation of rehabilitative interventions. Hypoxic-ischemic injury often results in impairments in audiovisual perception, cerebral palsy, cognitive dysfunction, memory difficulties, and other neurological sequelae.

Olfactory perception (the sense of smell) is one of the most important sensory functions in humans. In newborns, the sense of smell plays a crucial role, allowing them to perceive maternal odors and breast milk, guiding them in adapting to their environment and completing the physiological transition from intrauterine to extrauterine life. Olfactory neurosensory function involves several abilities, particularly odor identification and memory. Olfactory perception is also linked to olfactory memory, which can be tested as a habituation response. Olfactory memory is defined as the recollection of odorants. In recent studies, exposure to pleasant odors, such as breast milk or vanilla, has been shown to reduce perceived pain during invasive procedures in healthy full-term newborns. Olfactory stimuli can generate various physiological changes, regulating heart and respiratory rates, as well as triggering motor responses that guide head movements and initiate orofacial responses.

Some studies have shown that brain injury impairs odor perception: smell is altered in several neurological conditions, including both neurodevelopmental and neurodegenerative disorders. In Alzheimer (AD), Parkinson disease (PD) and in stroke, several cortical and subcortical areas directly involved in olfactory function are damaged or show signs of atrophy, including the olfactory bulb (OB), primary olfactory cortex (POC), hippocampus, orbitofrontal cortex (OFC), amygdala, and olfactory tract. Moreover, reduced odor detection early in life, along with anatomical and functional alterations in olfactory and higher-order cortical networks, has been reported in neurodevelopmental disorders such as Autistic Spectrum Disorder (ASD) and Attention Deficit / Hyperactivity Disorder (ADHD).

Olfactory dysfunction has also been studied and demonstrated in animal models through the induction of brain damage in rabbits. So far, no study has evaluated olfactory function in the clinical setting of brain injury due to neonatal asphyxia.

In cases of perinatal asphyxia, early identification of hypoxic-ischemic encephalopathy is achieved through continuous video-electroencephalographic monitoring, which is currently considered the gold standard for evaluating brain functions and detecting subclinical seizures. Although EEG does not directly assess olfactory function, certain electroencephalographic alterations may suggest impairment in brain regions involved in olfactory perception (piriform cortex, insular cortex, and amygdala).There are numerous studies in the literature that adopt various methods of olfactory testing in healthy newborns, but no validated tool or test exists to evaluate olfactory memory in newborns.

This study aims to assess olfactory function in newborns exhibiting signs of asphyxia at birth and to investigate whether infants with mild asphyxia differ from those with moderate to severe asphyxia.

The hypotheses of this study is that olfactory stimulation with odors such as rose, vanilla, and breast milk induces changes in heart rate, respiratory rate, peripheral oxygen saturation, EEG activity, and neuroimaging measures in asphyxiated newborns. These changes also occur following repeated olfactory stimuli. Olfactory memory, or habituation to odors, is delayed or absent in newborns with moderate-severe asphyxia.

Inclusion Criteria:

• Term newborns (37-41 weeks of gestational age) with signs of asphyxia at birth (cord pH < 7.10 and/or BE > -12).
• Maternal age > 18 years.
• No medication use during pregnancy (e.g., antipsychotics, antidepressants, sedatives, anticonvulsants, anxiolytics).
• Absence of maternal infections.
• Apgar score < 5 at 10 minutes of life.
• Newborns with mild asphyxia at birth.
• Newborns with moderate asphyxia at birth, at risk of developing hypoxic-ischemic encephalopathy, who don't need hypothermia treatment.
• Newborns with severe asphyxia at birth, at risk of developing hypoxic-ischemic encephalopathy who don't need hypothermia treatment.

Exclusion Criteria:

• Post-term infants (gestational age > 42 weeks).
• Preterm infants (gestational age < 37 weeks).
• Infants with genetic syndromes or congenital anomalies.
• Infants from mothers using drugs of abuse.
• Infants with scalp injuries or lesions.
• Infants with microcephaly.
• Infants who underwent therapeutic hypothermia.