|NAVAL MEDICINE - ORIGINAL ARTICLE
|Year : 2018 | Volume
| Issue : 1 | Page : 44-49
A study of neurocognitive and executive function of divers
Nikita D Shah1, Sunil Goyal2
1 Consultant Psychiatrist, Sheth K C Parikh General Hospital and KLS Memorial Hospital, Mumbai, Maharashtra, India
2 HOD and Consultant, Department of Psychiatry, INHS Asvini, Mumbai, Maharashtra, India
|Date of Web Publication||9-Jul-2018|
Dr. Nikita D Shah
603-604 Chandan Building, Irla Railway Colony, Plot No 8, Irla Society Road, Vile Parle (West), Mumbai - 400 056, Maharashtra
Source of Support: None, Conflict of Interest: None
Background: Extant literature suggests that due to a variety of factors, diving causes long-term neurological sequelae. Effects on cognitive functions have been assessed in previous studies; however, the presence and extent of resultant cognitive impairment is still unclear. This study was done to test the presence of cognitive deficits in divers. Further, the deficits were correlated with the years and depth of diving. Materials and Methods: In this comparative study carried out at a tertiary care hospital, 46 healthy divers were exposed to neuropsychological tests to evaluate their attention, concentration, visuospatial processing, memory, and executive functions. Their performance was compared to that of 46 healthy-matched nondiving sailors. Their performance was also correlated to the years and depth of diving. Results: The study found that the divers performed worse than nondivers in the following indices in the psychometric tests; Stroop Word Component (P = 0.035), Trail A (P = 0.045), number of categories completed in Wisconsin Card Sorting Test (P = 0.007); and in delayed recall (P = 0.007) and visual recognition (P = 0.031) in the Memory Scale. It is inferred that the cognitive domains of visuospatial processing, executive function, and verbal and visual memory are affected. Increase in depth of diving showed worse performance in digit backward (P = 0.032) and verbal memory (P = 0.048) that points to impaired attention, concentration, working memory, and verbal memory. Conclusion: The study suggests that residual cognitive effects are seen in divers. Worsening of the cognitive function is correlated with depth of dives but not total years of diving. The study recommends that further research in the field of cognitive effects of diving is warranted.
Keywords: Divers, diving, executive functions, neurocognition
|How to cite this article:|
Shah ND, Goyal S. A study of neurocognitive and executive function of divers. J Mar Med Soc 2018;20:44-9
| Introduction|| |
Diving, an activity that has been known from the time of the ancient Greeks, is becoming increasingly popular, even as a recreational sport. Divers have been used by the military too from the time of the Trojan wars and they were also effectively employed during the World Wars.
With the widespread increase in the popularity of diving, studies that look into the health sequelae of this activity have also garnered interest. Acute adverse effects of diving are well established and caused mainly because of barotraumas, nitrogen narcosis, microbubbles, and gas embolisms. Some of the well-known detrimental effects are ear, sinus, and pulmonary barotraumas and decompression sickness., Most of these do not have long term implications.,
However, several studies have shown long-term health problems consequent to diving. The Examination of Long Term Health Impact of Diving study carried out in the UK found that divers were more likely than nondivers to report “forgetfulness or loss of concentration,” musculoskeletal symptoms, and “impaired hearing.” Manifestations of permanent neurological damage following acute spinal cord decompression sickness have been known for very long. Vaernes et al. found that 20% of saturation divers had neuropsychological deficits after a mean of 3–4 years of diving. Several studies found signs of long-term neurological sequelae in divers not only symptomatically  but also in neuroimaging , and electroencephalography.,
It has been hypothesized that diving leads to cognitive deficits. Over the last 40-odd years, there have been several studies addressing this hypothesis with mixed results. While there are several studies that have not only established the presence of these cognitive deficits but have also elucidated positive correlation with depth of diving and diving experience, there are also studies that have refuted this claim. Another relevant area of interest is whether these cognitive impairments are only limited to the actual dive and the time soon after or whether these changes are more sustained.
In a study of Australian abalone divers, it was established that divers had impaired learning and short-term memory and had increased tremors in hands when compared to controls. The reaction times of both groups were comparable, but the divers tended to make more errors. A study by Tetzlaff et al. also affirms the contention that diving may cause long-term neurological and intellectual impairment. Psychometry done in this study revealed that divers had impaired mental flexibility and visual tracking compared to controls. Moreover, impaired immediate and delayed recall of verbal material correlated significantly with mean maximum depth. The Geneva “memory dive” study found that depth and number of dives had a negative influence on cognitive functions – particularly those of speed, flexibility, and inhibition processing in attentional tasks.
However, there are several studies that have illustrated a lack of cognitive dysfunction in divers. In a study of breath-holding divers, neuropsychological tests for speed of visuomotor responding, speed of language comprehension, response inhibition, and visual and verbal attention, and recall tasks were carried out. No significant impairment was found. A meta-analysis of 14 studies describing neurological and motor/sensory studies, psychological functioning, and diving described that the broad cognitive domains that have been studied are attention, intelligence, memory, and executive functioning. Divers with a history of decompression sickness perform significantly worse on neuropsychological evaluations than divers without the same. These divers seemed to have global impairment, affecting several of the cognitive domains. The divers versus nondivers comparison demonstrated a positive effect size, suggesting that overall, the neuropsychological functioning of divers was greater than those of the selected control groups. This meta-analysis concluded that there was truth in the belief that significant cerebral incidents during diving affect long-term cognitive functioning, even when compared to noninjured divers. However, there is still a dearth of neuropsychological evidence and the exact nature of deficits is still not clear.
This current study was planned to shed light on neurocognitive functioning in divers in view of the mixed results seen in literature about the long-term effects on the same. It was also prudent to consider this kind of study in view of the paucity of Indian data on this subject. The objective of the study was to ascertain the presence of cognitive deficits in divers. We also wanted to observe if the deficits were restricted to any specific cognitive domain. Furthermore, we wanted to correlate the deficits to the years of diving and the depth of diving.
| Materials and Methods|| |
These objectives were explored by comparing the cognitive performance of divers on tests assessing various cognitive domains and comparing their performance with that of nondiving sailors. Furthermore, the divers' performances was correlated with various diving parameters. The study was conducted in the Psychology Laboratory of the Psychiatry Department of an Armed Forces Tertiary Care Hospital. The sample population included all divers who reported to the hospital during the study period. The total number of divers assessed was 46.
The comparison group comprised adult, male, healthy nondiver sailors sent to the Psychiatry Ward on attendant duty to care for admitted psychiatric patients with a similar age and education profile as the divers. Thus, 46 eligible attendant sailors were selected for the study from January 1, 2010, to December 31, 2010.
Demographic data were collected via interview after taking informed consent. During this interview, physical examination and a screening questionnaire were implemented to rule out physical or psychiatric morbidity, which may affect cognitive function.
Assessment of cognitive function
The cognitive assessment was carried out by a variety of psychometric tests. The Trail Making Test is a test of attention, speed, visuoperceptual abilities, and mental flexibility. The Stroop Color-Word Test was administered to test attention, working memory, conceptual ability, and speed of processing. A computerized version of the Wisconsin Card Sorting Test (WCST) was carried out to assess executive function in the form of abstract reasoning and flexibility. The digit span component of the Postgraduate Institute (PGI)-Memory Scale was used as it is a good index of attention, concentration, and working memory.
For academic interest, additional tests (not included in original protocol) were also used to test for further domains of memory. These tests were introduced after the data collection was partially complete and could only be done for 21 divers and 32 nondivers. These additional assessments were subtests of the PGI-Memory Scale and tested for delayed recall, verbal retention, and visual retention and recognition.
This was collected by perusing the diver's log book. The indices collected were total years of diving, average duration of dive, maximum depth of dive, and average depth of dive.
The data were analyzed using Statistical Package for the Social Sciences (SPSS) version 15 (SPSS Inc., Chicago: SPSS Inc.). Demographic data and indices for the various psychometric tests were compared between the two groups using Mann–Whitney U-test, whenever the data did not have normal distribution. For normally distributed data, unpaired t-test was used. To check for normality, the Shapiro–Wilk test was applied. Within the diving group, correlations between the psychometric indices and the diving parameters were carried out using Pearson's correlation. This was further tested for significance using Fisher's exact test. P < 0.05 was taken as statistically significant.
| Results|| |
The mean age for the divers was 26.43 years (standard deviation = 1.068) and that of the nondivers was 24.83 years (standard deviation = 3.492). The difference between the two groups was significant (P = 0.025). This significant difference in age is less likely to exert a significant effect on the cognitive performance as cognitive decline is usually seen after 50–55 years of age. There was no significant difference between the educational attainment (P = 0.397) of the two groups.
The two groups showed a significant difference in performance in the Word part of the Stroop test, with the divers performing worse (P = 0.035) [Table 1]. This could be interpreted to reveal a relative deficit in visual processing. There was no significant difference in the performance of the two groups in the Stroop Color (P = 0.587), Color-Word (P = 0.403), and Interference (P = 0.141) components.
Trail making test
The divers performed worse than the nondiving group on the Trail A part of the Trail Making Test (P = 0.045) [Table 2]. This test is essentially a test of attention and visual scanning. The two groups had comparable performances on Trail B (P = 0.888) and the mathematically calculated index B-A (P = 0.177).
Wisconsin card sorting test
A number of indices were collected from the WCST. The divers could complete lesser categories than the nondiving group (P = 0.007) [Table 3]. The remaining indices were the T-scores of the percentages of errors made, of perseverative responses, of concept level responses, and the number of times there was failure to maintain set and the trials needed to complete the first category. There was no significant difference between the performances of the two groups on these remaining indices.
|Table 3: Comparison of Wisconsin Card Sorting Test categories completed among study groups|
Click here to view
There was no significant difference in the between-group performance on digit forward (P = 0.670) and digit backward components of the digit span test (P = 0.164).
The additional tests done to test for more memory domains among the cognitive domains revealed significantly worse performance in the divers when compared to the nondivers in the subtests for delayed recall (P = 0.007) [Table 4] and visual recognition (P = 0.031) [Table 5]. On the other subtests, the two groups had comparable performances.
Relationship between diving indices and cognitive performance
The correlation between the total years of diving, maximum depth of diving, average duration, and depth of diving with each of the cognitive indices was computed. The mean of the total years of diving was 6.76 years (standard deviation = 0.95). The mean of the maximal depth of diving was 57.54 m (standard deviation = 18.51). The mean of the duration of the average dive was 43.91 min (standard deviation = 41.25) and that of depth was 16.30 m (standard deviation = 8.81).
There was no significant correlation between the diving indices and the various components of the Stroop Test and the parameters of the Trail Making Test and the WCST.
Digit span test and diving indices
A significant negative correlation was found between average depth of dive and digit backward score (r = −0.316, P = 0.032) [Table 6]. Thus, one can say that digit backward scores worsen with increasing average depth of diving. Among the correlations between the digit forward and the computed total of digit forward and backward scores with the diving parameters, no significant correlations were seen.
No significant correlations were found between measures of delayed recall, verbal retention for dissimilar pairs, visual retention and visual recognition, and the various diving parameters. However, as the maximal depth of diving increased [Table 7], the performance on verbal retention of similar pairs got significantly worse (r = −0.437, P = 0.048).
|Table 7: Correlation between diving indices and verbal retention for similar pairs|
Click here to view
| Discussion|| |
To summarize the between-group analysis, lower levels of functioning was found in the divers as compared to the nondivers in Stroop Word Test, Trail A, and categories completed on the WCST. These could be interpreted to mean that the visuospatial processing (Stroop Word, Trail A) and executive function (categories completed on WCST) among the divers were lower than that of the nondivers. Furthermore, as part of the additional tests, it was found that the divers had poorer verbal and visual memory than the controls (delayed recall, visual recognition). Their functioning on tests of working memory was comparable to that of the controls.
To recapitulate the findings of the correlations of the cognitive indices with those of diving, the results show worsening of digit backward scores with depth of diving. This may indicate worsening attention, concentration, and working memory with increased depth of diving. In addition, the additional tests of memory showed a worsening of verbal memory as correlated with the maximal depth of dive.
The findings of this study can be compared to previous studies on cognition in diving. Our findings were similar in part with those of Tetzlaff et al. in that both studies found impairment in nonverbal memory and worsening of memory function with increasing depth. However, the lack of correlation between visual scanning and diving parameters is at odds with the findings of Tezlaff et al., who found decreased visual tracking with total years of diving.
Some of our findings also were similar to the study on Australian abalone divers  where impaired vision was found which mirrors our finding of impaired visual scanning. Another similarity between the findings of the study on abalone divers and our study was that the digit forward and backward performance was comparable between the two groups. Difference between the two studies was that our study found that verbal and visual memory was relatively impaired in the diver group as opposed to the control group. However, the findings can be explained to be divergent because of the differences in diving conditions between abalone divers and Indian divers. The abalone divers have extremely strenuous diving conditions. Hence, it is difficult to extrapolate their findings to studies on divers that use stricter diving norms.
In the Geneva memory dive, the study design was different from our study. Here, 215 recreational divers underwent testing for a variety of neurocognitive domains, but there was no control group. They found that depth and total number of dives negatively correlate to processing speed, mental flexibility, inhibition processing, and attentional tasks. Our findings differed from the ones in this study as we found that although the working memory and attention was found to worsen with depth of diving, no deterioration in cognitive functioning could be found with increasing years of diving. In a recent meta-analysis of cognitive function in diving, an analysis of the diver versus control group studies revealed that the neurocognitive functioning of divers was better than that of controls. This was different from our findings which showed the divers to be more impaired than controls in terms of visuospatial processing and executive function.
A possible explanation for the poorer performance of divers as compared to the nondiving group in terms of visuospatial scanning and mental flexibility and verbal and visual memory is the various insults that the brain undergoes due to gas toxicities, barotraumas, decompression, and embolisms. Although none of the divers had a history of decompression sickness, it shows that these changes are manifested even without overt neurological signs. This suggests that cognitive damage may occur even in the absence of overt clinical signs.
The worsening of verbal memory, attention, and concentration with an increase in depth of diving is another finding which adds further proof to the belief that diving could cause cognitive dysfunction. This is generally in keeping with the findings of most other studies as discussed above.
The strength of this study is that it is among the rare Indian studies and few international studies to assess cognitive function in divers. Reflecting the trend worldwide, of an increase in interest in diving, this study helps shed light in a relatively unknown field of study and creates awareness about residual deficits which could result from this upcoming sport, recreational activity, and occupational activity.
The other strength of this study is that it not only establishes the presence of these residual cognitive differences but also correlates them with various diving parameters. The tests for neurocognitive functioning involve a variety of domains which adds to the appeal of the study.
The study also has its fair share of limitations. Due to the sampling used, the spread of the years of diving was not large which may have affected the correlations. However, this sampling technique serves a purpose too. Diving is a trade with few numbers. A randomized sampling carried out in any of the diving schools would have yielded total number of subjects which would have been few, which might also hamper the findings. An amendment was made to the initial study protocol halfway through the study to accommodate testing for the various domains of memory which were not tested for in the original protocol. Thus, these tests could only be administered to a part of the divers and the controls. The power for the study could not be generated as the sample size was not calculated. Thus, one cannot know how accurate the generalizations made on the basis of these additional tests are. The other limitation of this study was that it was not blind to the two groups, either at the time of data collection or analysis. The age difference between the study and the control group was significant. While this may not have exerted a clinical significance, it still remains a confounding variable.
| Conclusion|| |
This study found that the performance of the divers was poorer than that of the nondivers in terms of visuospatial processing, executive function and verbal and visual memory. Increase in depths of diving showed a significant trend toward worse attention, concentration, working memory, and verbal memory. The study suggests that residual cognitive effects are seen in divers. Worsening of the cognitive function is correlated with depth of dives but not duration or total years of diving. It is recommended that future studies are done with a longitudinal cohort study that follows the cognitive functioning of divers over their tenure of diving. Baseline neuropsychiatric testing should be done at the start of a diver's career and repeated at regular intervals.
We would like to acknowledge Director General Armed Forces Medical Services for providing the requisite permission and funds for this research.
We would also like to acknowledge Surg Rear Admiral (Retired) VSSR Ryali for being mentor and guide.
Financial support and sponsorship
The study was supported by Director General Armed Forces Medical Services for funds.
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]