Evidence for the initiation of decompression sickness by exposure to intense underwater sound

Evolutionary genetics of pulmonary anatomical adaptations in deep-diving cetaceans

BACKGROUND

Cetaceans, having experienced prolonged adaptation to aquatic environments, have undergone evolutionary changes in their respiratory systems. This process of evolution has resulted in the emergence of distinctive phenotypic traits, notably the abundance of elastic fibers and thickened alveolar walls in their lungs, which may facilitate alveolar collapse during diving. This structure helps selective exchange of oxygen and carbon dioxide, while minimizing nitrogen exchange, thereby reducing the risk of DCS. Nevertheless, the scientific inquiry into the mechanisms through which these unique phenotypic characteristics govern the diving behavior of marine mammals, including cetaceans, remains unresolved.

RESULTS

This study entails an evolutionary analysis of 42 genes associated with pulmonary fibrosis across 45 mammalian species. Twenty-one genes in cetaceans exhibited accelerated evolution, featuring specific amino acid substitutions in 14 of them. Primarily linked to the development of the respiratory system and lung morphological construction, these genes play a crucial role. Moreover, among marine mammals, we identified eight genes undergoing positive selection, and the evolutionary rates of three genes significantly correlated with diving depth. Specifically, the SFTPC gene exhibited convergent amino acid substitutions. Through in vitro cellular experiments, we illustrated that convergent amino acid site mutations in SFTPC contribute positively to pulmonary fibrosis in marine mammals, and the presence of this phenotype can induce deep alveolar collapse during diving, thereby reducing the risk of DCS during diving.

CONCLUSIONS

The study unveils pivotal genetic signals in cetaceans and other marine mammals, arising through evolution. These genetic signals may influence lung characteristics in marine mammals and have been linked to a reduced risk of developing DCS. Moreover, the research serves as a valuable reference for delving deeper into human diving physiology.

A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives

Sea turtles, like other air-breathing diving vertebrates, commonly experience significant gas embolism (GE) when incidentally caught at depth in fishing gear and brought to the surface. To better understand why sea turtles develop GE, we built a mathematical model to estimate partial pressures of N2 (PN2), O2 (PO2), and CO2 (PCO2) in the major body-compartments of diving loggerheads (Caretta caretta), leatherbacks (Dermochelys coriacea), and green turtles (Chelonia mydas). This model was adapted from a published model for estimating gas dynamics in marine mammals and penguins. To parameterize the sea turtle model, we used values gleaned from previously published literature and 22 necropsies. Next, we applied this model to data collected from free-roaming individuals of the three study species. Finally, we varied body-condition and cardiac output within the model to see how these factors affected the risk of GE. Our model suggests that cardiac output likely plays a significant role in the modulation of GE, especially in the deeper diving leatherback turtles. This baseline model also indicates that even during routine diving behavior, sea turtles are at high risk of GE. This likely means that turtles have additional behavioral, anatomical, and/or physiologic adaptions that serve to reduce the probability of GE but were not incorporated in this model. Identifying these adaptations and incorporating them into future iterations of this model will further reveal the factors driving GE in sea turtles.

Acute findings in an acquired neurosensory dysfunction

Background

In the Autumn of 2016, diplomatic personnel residing in Havana began to present with symptoms of dizziness, ear pain, and tinnitus that emerged after perception of high frequency noise and/or a pressure sensation. Understanding the acute symptoms of this disorder is important for better defining the disorder and developing optimal diagnostic, preventive, and treatment algorithms.

Objectives

To define the presenting symptoms in a cohort of patients in the acute time period after perceiving a noise/pressure exposure in Havana.

Design/Settings/Participants

Review of 25 symptomatic individuals who reported a localized sensation of noise/pressure and 10 asymptomatic individuals (roommates of those affected) who did not experience the sound/pressure.

Results

Immediately after the exposure, the majority of individuals reported intense ear pain in one or both ears and experienced tinnitus. All of the individuals noticed unsteadiness and features of cognitive impairment. On presentation to our center, dizziness (92%) and cognitive complaints (56%) were the most common symptoms. Formal testing revealed that 100% of individuals had an otolithic abnormality and evidence of cognitive dysfunction.

Conclusion and Relevance

This study focuses on the acute presentation of a phenomenon in which symptoms emerge after perception of a localized noise/pressure and in which the acute symptomology includes the universal nature of vestibular injuries and select cognitive deficits. The findings presented in this acute group of patients begin to provide a better picture of the initial injury pattern seen after this exposure and may allow for more accurate diagnosis of this disorder in future cases.

Level of Evidence

Retrospective review

High Pressure and [Ca 2+ ] Produce an Inverse Modulation of Synaptic Input Strength and Network Excitability in the Rat Dentate Gyrus

Hyperbaric environments induce the high pressure neurological syndrome (HPNS) characterized by hyperexcitability of the central nervous system (CNS) and memory impairment. Human divers and other animals experience the HPNS at pressures beyond 1.1 MPa. High pressure depresses synaptic transmission and alters its dynamics in various animal models. Medial perforant path (MPP) synapses connecting the medial entorhinal cortex with the hippocampal formation are suppressed by 50% at 10.1MPa. Reduction of synaptic inputs is paradoxically associated with enhanced ability of dentate gyrus (DG)’ granule cells (GCs) to generate spikes at high pressure. This mechanism allows MPP inputs to elicit standard GC outputs at 0.1–25 Hz frequencies under hyperbaric conditions. An increased postsynaptic gain of MPP inputs probably allows diving animals to perform in hyperbaric environments, but makes them vulnerable to high intensity/frequency stimuli producing hyperexcitability. Increasing extracellular Ca²⁺ ([Ca²⁺]_o) partially reverted pressure-mediated depression of MPP inputs and increased MPP’s low-pass filter properties. We postulated that raising [Ca²⁺]_o in addition to increase synaptic inputs may reduce network excitability in the DG potentially improving its function and reducing sensitivity to high intensity and pathologic stimuli. For this matter, we activated the MPP with single and 50 Hz frequency stimuli that simulated physiologic and deleterious conditions, while assessing the GC’s output under various conditions of pressure and [Ca²⁺]_o. Our results reveal that the pressure and [Ca²⁺]_o produce an inverse modulation on synaptic input strength and network excitability. These coincident phenomena suggest a potential general mechanism of networks that adjusts gain as an inverse function of synaptic inputs’ strength. Such mechanism may serve for adaptation to variable pressure and other physiological and pathological conditions and may explain the increased sensitivity to strong sensory stimulation suffered by human deep-divers and cetaceans under hyperbaric conditions.