the underwater environment produces some pathophysiological problems due to the fast changing of its bounded pressure which may cause various illnesses that might be difficult to detect and treat . our nervous system is an organ which is very sensitive to underwater pressure changes , and a damaged nervous system can lead to hazardous disorders . hyperbaric condition can be considered as a risk factor for cerebral ischemic events and strokes in divers ( 1 - 4 ) . the trembling of hands , postural instability , gastrointestinal problems , somnolence and cognitive dysfunction are some of the diving related disorders developed by a hyperbaric condition called high pressure neurological syndrome ( hpns ) ( 5 ) . clinical symptoms of nitrogen narcosis are similar to alcohol intoxication which leads to a temporary dysfunction of neuromuscular and intellectual activity and the disturbance of behavior and personality . this condition generally occurs in depths of deeper than 30 meters ( 100 ft ) . nitrogen narcosis leads to hallucinations , unconsciousness and even death at a depth of 90 to 100 meters ( 5 ) . zeba et al showed that the cases of diving cns barotraumas are rare but its symptoms are diverse and include dysesthesias , complete quadriplegia and encephalopathy ( 6 ) . moreover , long term diving may cause various problems , the most common of them being the lack of concentration and numbness of hands and feet ( 7 ) . although there is much evidence emphasizing neurological effect of barotraumas in divers , there is not any data about the effect of high pressure condition on intracranial pressure . in some studies ultrasound ( us ) has been called as a noninvasive technique to detect increased intracranial pressure ( icp ) ( 8) in conditions such as hydrocephalus ( 9,10 ) and trauma ( 11 , 12 ) . however , in this study we investigated the effect of long term diving practice upon optic nerve and sheath diameters as a non - invasive indicator of intracranial pressure changes in professional divers . an informed consent was taken from all of the participants , and the study was approved by the tehran university medical sciences ( tums ) ethics committee . the study group consisted of 21 professional male scuba divers between 26 and 66 years of age having totally 42 eyes , and the control group consisted of 19 non - diving healthy male patients having 38 eyes . all of the participants had at least 2 years of scuba diving history without any previous history of eye disease or abnormal vision , head trauma or brain disorders . the diving depths were about 25 to 50 meters which was varied for different individuals . control group subjects were selected from healthy individuals with the following criteria : 1 ) not having any history of diving , working or living in very low or high atmospheric pressure conditions , 2 ) not having any history of eye or brain injuries or diseases , 3 ) not having any history of blurred vision , eye pain , and low visual acuity . all divers and the control group cases completed a questionnaire including demographic data , medical and diving history . all patients were examined in the supine position . 
 
 descriptive analysis of quantitative and qualitative variables were performed and presented in a table . possible association between onsd and scuba diving was examined by mann - whitney u - test . then , the relationship between qualitative data such as education and marriage , and quantitative data such as age and experience was determined using chi2 test and spearman correlation test , and after this , comparison of onsd and diving experience was made by linear regression model . an informed consent was taken from all of the participants , and the study was approved by the tehran university medical sciences ( tums ) ethics committee . the study group consisted of 21 professional male scuba divers between 26 and 66 years of age having totally 42 eyes , and the control group consisted of 19 non - diving healthy male patients having 38 eyes . all of the participants had at least 2 years of scuba diving history without any previous history of eye disease or abnormal vision , head trauma or brain disorders . the diving depths were about 25 to 50 meters which was varied for different individuals . control group subjects were selected from healthy individuals with the following criteria : 1 ) not having any history of diving , working or living in very low or high atmospheric pressure conditions , 2 ) not having any history of eye or brain injuries or diseases , 3 ) not having any history of blurred vision , eye pain , and low visual acuity . all divers and the control group cases completed a questionnaire including demographic data , medical and diving history . descriptive analysis of quantitative and qualitative variables were performed and presented in a table . possible association between onsd and scuba diving was examined by mann - whitney u - test . then , the relationship between qualitative data such as education and marriage , and quantitative data such as age and experience was determined using chi2 test and spearman correlation test , and after this , comparison of onsd and diving experience was made by linear regression model . most of them worked far from their place of residence , ( 13 ; 86.6% ) . the mean diameter of the two optic nerve sheaths was observed to be higher than 5.7 mm in thirty five divers . the mean of the left optic nerve and sheath diameters was significantly higher than the right ones in the diver group ( p=0.001 , ) . no difference was found between right and left onsd in the control group ( p>0.05 ) ; but age of divers had influence on the right onsd ( p=0.01 ; r=0.6 ) . thus a significant difference was witnessed in the optic nerve diameters of diver group and control group ( p= 0.01 , p=0.05 ) . mean diameter of left and right optic nerve sheaths in diver group was 6.420.87 , 6.520.63 respectively . it was significantly higher than the mean of the left and right optic nerve sheaths in control group which was 4.87 0.57 and 4.95 0.59 , respectively ( p=0.001 , p= 0.001).(see in   figs . 1 & 2 ) no relationship was found between optic nerve sheath and optic nerve diameters and bmi , but a significant relationship was noticed between both optic nerve sheath diameter and the diving , after age adjustment which is shown in table 2 . effect of hyperbaric condition on brain and nervous systems was already known to some extent . however , the result of this study gives evidence that divers have a higher onsd than that of the general population reported by related studies ( 13 - 17 ) . the ultrasound - based evaluation of the optic nerve sheath diameter is a highly suggestive method for the determination of increasing intracranial pressure especially in a critical situations . sensitivity and specificity of this method is estimated to be higher than 90% , whereas , that of tayal et al study was reported to be only 63% ( 11 ) 
 
 our research has also assessed the effect of atmosphere on onsd . onsd increase was seen at high altitudes , and so it is assumed that onsd increase may be associated with severe symptoms of acute mountain syndrome ( ams ) and may be related to intracranial pressure ( 18 ) . however , this claim could not be proved in a separate study undertaken for this purpose . it was shown that the development or amelioration of mild high altitude headache ( hah ) has no association with intracranial pressure ( 19 ) . therefore , there is a controversy over the claim that atmospheric pressure variations have affect on the optic nerve sheath diameter . 
 
 in comparison to the normal range of osnd of normal population which is estimated to be below 5.7 mm ( 11 , 16 , 20 ) , little research has been done on the effect of bar pressure or barotraumas on optic nerve ; and peripheral neuropathy after decompression sickness is a rare event . the mechanism of injury is assumed to be a manifestation of decompression illness with a gas bubble causing blood flow obstruction and an ischemic infarct ( 21 ) . in one case - report , sixth nerve palsy and sensory loss and ataxia was reported after decompression sickness ( 22 ) . thus the two possible mechanisms are : 1 ) high pressure conditions may cause temporary increase of intracranial pressure , and then increase of onsd ; 2 ) a blood flow obstruction or / and the effect of bar pressure on the performance of some enzymatic systems such as carbonic anhydrides enzyme , which may cause onsd increase . even though no study investigated the effect of age on onsd , a relationship was noticed between diving history and the optic nerve diameter even after making adjustments for age in our study . according to previous studies , increase of onsd is an indicator of severe increase of intracranial pressure and so in emergency cases it can be used for diagnosis in place of invasive methods . hence , on the basis of the results of this study , the effects of bar pressure on onsd increase can be used for the diagnosis of intracranial pressure increase . this study had some limitations too , such as intracranial pressure could not be measured by invasive method in healthy individuals , because invasive evaluation of normal population is unethical . another limitation was the low number of participants , as only a few divers had agreed to participate in this study . there was also one advantage by this study which is the origin of a new idea . however , more investigations are suggested for considering optic nerve and sheath diameters increase as a sure marker of intracranial pressure increase , especially in divers . in this study , we showed divers have a higher optic nerve diameter than that of the general population reported by other studies . although high optic nerve sheath diameter was previously seen in emergency situation caused by high intracranial pressure , we found that optic nerve sheath in normal divers is higher than the normal control group . however , our result can not yet be considered as a marker of intracranial pressure in divers as it was conducted on an insufficient number of subjects and so a bigger study should be undertaken for this purpose .