You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
thrombotic risk factors for budd - chiari syndrome ( bcs ) have been clearly recognized in western countries [ 1 , 2 ] .
major causal factors include myeloproliferative neoplasm ( mpn ) , factor v leiden ( fvl ) mutation , prothrombin g20210a gene mutation , antiphospholipid antibodies , hyperhomocysteinemia , paroxysmal nocturnal hemoglobinuria , antithrombin ( at ) , protein c ( pc ) , protein s ( ps ) deficiencies , and others [ 1 , 2 ] .
several european cohort studies have demonstrated that at least one underlying thrombotic risk factor is found in more than 80% of western bcs patients [ 3 , 4 ] .
accordingly , the recent american association for the study of liver diseases ( aasld ) practice guideline has recommended that routine screening for these risk factors should be performed in bcs patients . by contrast , a recent systematic review and meta - analysis of observational studies did not confirm the role of inherited at , pc , or ps deficiency in the pathogenesis of bcs .
additionally , the prevalence of other thrombotic risk factors for bcs appears to be very low in chinese patients [ 6 , 7 ] .
for example , jak2v617f mutation , a critical diagnostic marker of mpn , exists in approximately 37% of western bcs patients , but in only 4% of chinese bcs patients .
other studies have reported that fvl mutation , prothrombin g20210a gene mutation , or paroxysmal nocturnal hemoglobinuria is extremely rare in chinese patients with sporadic bcs [ 1012 ] .
although the congenital theory for the pathogenesis of bcs is hypothesized by early histopathological studies [ 1315 ] , it has never been advocated by the current practice guideline and consensus due to the rarity of bcs in childhood and the anatomic and topographic inconsistency of bcs [ 1618 ] .
several scattered case reports have demonstrated familial clustering of bcs , suggesting the possibility of congenital etiology [ 1922 ] . in 1988 ,
the age at their first symptoms was 1.2 and 1.8 years old . in 1990 ,
an israeli study also demonstrated that one 30-year - old man and his older brother were simultaneously diagnosed with bcs , but his other first - degree relatives were asymptomatic without any imaging evidence of hepatic vein or inferior vena cava obstruction . in 1995
, a dutch study further showed that 3 of 11 siblings from a single family were diagnosed with membranous obstruction of the inferior vena cava during early adult life .
in addition , a japanese national survey revealed that one of 157 japanese bcs patients had a family history of bcs .
recently , a simultaneous occurrence of bcs was reported in two american twin sisters of 13 years of age with heterozygous fvl mutation .   in this paper , we have conducted a systematic review of the published literature to better understand the familial occurrence of bcs in chinese patients .
one database of english journals ( i.e. , pubmed database ) and two databases of chinese journals ( i.e. , chinese scientific and technological journal database ( vip database ) and china national knowledge infrastructure database ( cnki database ) ) were employed for the review .
vip and cnki databases were subscribed by our university library . for the pubmed database ,
search items were listed as follows : (  budd chiari  [ all fields ] ) and (  china  [ all fields ] ) . two chinese names of bcs ( i.e. ,  bu jia zong he zheng  and  bai cha zong he zheng  ) were also used as search items in the two chinese databases . for the vip database , search items were listed as follows : (  budd chiari 
[ all fields ] ) or (  bai cha zong he zheng  [ all fields ] ) .
because there is no tag term of [ all fields ] in the search strategy of cnki database , search items for the database were listed as follows : (  budd chiari  [ keywords ] ) or (  budd chiari  [ title ] ) or (  bu jia zong he zheng  [ keywords ] ) or (  bu jia zong he zheng  [ title ] ) or (  bai cha zong he zheng  [ keywords ] ) or (  bai cha zong he zheng  [ title ] ) . the last search was performed on february 20 , 2012 . two authors ( x. qi and w. ren ) independently reviewed abstracts and full texts of all identified papers to avoid omitting the information of family history .
inclusion criteria were as follows : ( 1 ) the bcs patients were chinese and ( 2 ) at least two siblings or first - degree relatives in the same one family were simultaneously diagnosed with bcs .
exclusion criteria were as follows : ( 1 ) the patients were not diagnosed with bcs ; ( 2 ) animal study ; and ( 3 ) the patients were not from china . in cases of disagreement among the two reviewers , a consensus was achieved through discussion .
a data extraction sheet was developed that included the journal , publication year , authors , affiliation , provinces , or regions where the data was collected , period of enrollment , demographic data ( age and sex ) of patients , standards of living , clinical presentations , types of bcs , results of family study , results of thrombotic risk factors for bcs , and outcomes of these patients .
if the data was unavailable in some included papers , one reviewer ( g. han ) would contact these principal investigators or their colleagues who participated in these studies by telephone .
if these investigators agreed to provide accurate data , the requisite data for the present review were sent to them via a mobile phone short message .
the studies were considered of higher quality if they fulfilled the following predetermined criteria : ( 1 ) the demographic data ( age and sex ) were clearly described ; ( 2 ) the diagnostic methods of bcs were clearly described ; ( 3 ) the type and location of obstruction were recorded ; and ( 4 ) screening for bcs in family members was done .
overall , 112 , 1332 , and 1561 papers were identified in pubmed , vip , and cnki databases , respectively . among them , 18 papers ( i.e. , one english - language paper and 17 chinese - language papers ) were eligible for the review .
after duplicated publication was excluded , a total of 30 siblings or first - degree relatives from 14 chinese families were diagnosed with bcs at 9 different centers ( table 1 ) .
these centers were in 6 provinces or regions ( i.e. , beijing , guangdong , henan , liaoning , shaanxi , and shandong provinces ) .
seven studies including only 3 families , in which demographic data , diagnostic methods of bcs , type and location of obstruction , and family screening were available , were considered of higher quality . the standards of living were available in only two patients from the 3rd family . both of them were farmers with low incomes .
they included abdomen distension ( n = 10 ) , swelling of lower limbs ( n = 8) , variceal bleeding ( n = 2 ) , and no symptom ( n = 2 ) .
they included varicosity of chest and abdominal wall and/or lower limbs ( n = 9 ) , hepatomegaly ( n = 9 ) , edema of legs ( n = 6 ) , ascites ( n = 6 ) , and splenomegaly ( n = 3 ) . type and location of obstruction were reported in 22 bcs patients from 10 families .
inferior vena cava ( ivc ) and hepatic vein ( hv ) obstruction was found in 19 and 7 patients , respectively . among them , both ivc and hv were obstructed in 4 patients .
generally , anatomical and topographic lesions were similar among these patients from the same one family .
familial screening for bcs was performed in 3 families ( 8th , 9th , and 10th families ) . in the 8th family ,
27 additional family members from 4 generations were screened for bcs by color doppler ultrasound .
except for two symptomatic probands , two asymptomatic sisters were further diagnosed with bcs ( figure 1(a ) ) . in the 9th family ,
14 additional family members from 3 generations were screened for bcs by color doppler ultrasound .
no other family member was diagnosed with bcs ( figure 1(b ) ) . in the 10th family ,
24 additional family members from 4 generations were screened for bcs by b - mode ultrasound .
except for two symptomatic probands , no other family member was diagnosed with bcs ( figure 1(c ) ) .
factor v leiden ( fvl ) mutation was detected in 10 patients from the 7th , 8th , 12th , and 13th families .
three of 4 patients from the 7th family had heterozygous fvl mutation , one of two patients from the 8th family had heterozygous fvl mutation , and none of patients from the 12th and 13th families had fvl mutation .
prothrombin g20210a gene mutation was detected in 4 patients from the 12th and 13th families .
to date , the etiological factors for most of chinese bcs patients are still unclear because thrombotic risk factors for bcs are rarely found in china [ 6 , 7 , 912 ] .
the present study shows familial occurrence of bcs in china , which potentially suggests that family history of bcs patients should be carefully inquired and the patient 's first - degree relatives might be screened .
but these available data can not support the previous hypothesis that familial bcs is associated with the congenital vascular malformation .
first , although bcs patients from the same one family have the similar anatomical and topographic lesions , nearly all of these familial bcs cases are identified during adulthood rather than childhood .
however , we would like to emphasize that imaging evaluation for hv and ivc in these patients during the neonatal period is lacking and unrealistic .
therefore , we can not exclude the possibility that bcs is unrecognized in childhood and the overt clinical presentations occur in adulthood .
indeed , if screening for bcs was not performed in the 8th family , two asymptomatic bcs patients would not be identified until their clinical presentations occurred .
second , an incomplete screening for inherited thrombotic risk factors in four families have demonstrated that fvl mutation is found in 4 of 6 familial cases from the 8th and 9th families .
certainly , a complete work - up for thrombophilia should be adopted in further studies to evaluate the frequency of thrombotic risk factors in familial bcs patients .
third , the most possible causes for membranous obstruction of ivc ( movc ) in developing and underdeveloped countries so far are often considered an extremely poor standard of living and suspected bacterial infections in such poor living environment [ 18 , 25 ] .
thus , it can be expected that familial aggregation in patients with movc may be associated with such factors .
only one included study reported that two bcs patients from the same one family were farmers with low incomes .
therefore , whether or not a poor standard of living plays a role in familial occurrence of bcs deserves further investigation .
this question is very important because health policy should focus on improving the standard of living in these patients ' offspring , thereby preventing the development of bcs . strengths of the study are that literature searches were comprehensive , and all full - texts were reviewed to avoid omitting the information of familial bcs . additionally , several limitations of this study need to be clarified .
the fact was primarily because the objectives of most studies were not to demonstrate familial occurrence of bcs , but to explore the outcome of bcs patients .
additionally , although we have tried our best to contact with the authors or their colleagues to collect more adequate information about these patients , only a few data could be acquired .
second , common causes of familial thrombophilia include fvl mutation , prothrombin g20210a gene mutation , and inherited at , pc , and ps deficiencies . if these familial bcs patients had a common inherited thrombophilia , the congenital malformation theory would not be approved . but screening for familial thrombophilia was incomplete in these included studies , and thrombotic risk factors were detected in only a minority of cases ( i.e. , fvl mutation was detected in 10 patients from 4 families , and prothrombin g20210a gene mutation was detected in 2 patients from 2 families ) .
it might be explained by the fact that the role of thrombotic risk factors for bcs was not well established in china .
further , because fvl mutation and prothrombin g20210a gene mutation are rarely found in chinese healthy population or patients with venous thromboembolism , at , pc , and ps deficiencies are more common than the western populations [ 2730 ] ; we recommend that screening for inherited at , pc , and ps deficiencies appears to be more necessary in these patients . in conclusion ,
our study confirmed the possibility of familial bcs in china , but it appeared that familial occurrence of bcs was not related to congenital vascular malformation .
familial thrombophilia and standard of living in these patients should be evaluated in further observational studies .
familial occurrence of budd - chiari syndrome ( bcs ) has been reported in scattered cases , which potentially favors the congenital theory . 
 a review of the literature was conducted to demonstrate this phenomenon in china . 
 pubmed , vip , and cnki databases were searched for studies describing at least two chinese bcs patients from the same one family . in the 18 eligible papers , 30 siblings or first - degree relatives from 14 families were diagnosed with bcs at 9 different centers . 
 common clinical presentations included varices of abdominal wall and lower limbs , edema of legs , and ascites . 
 type and location of obstruction were similar among these patients from the same one family . 
 screening for bcs was conducted in 65 family members from 3 families , demonstrating that 2 asymptomatic siblings from one family were further diagnosed with bcs . 
 factor v leiden mutation was found in 3 of 4 patients from one family and in one of 2 patients from another one family . 
 prothrombin g20210a gene mutation was found in none of the 4 patients from the 2 families . in conclusion , 
 our study showed the possibility of familial aggregation in chinese bcs patients , but these available data can not support the previous hypothesis that familial bcs originates from congenital vascular malformation .