Context In observational studies, elevated plasma total homocysteine levels
have been positively associated with ischemic stroke risk. However the utility
of homocysteine-lowering therapy to reduce that risk has not been confirmed
by randomized trials.
Objective To determine whether high doses of folic acid, pyridoxine (vitamin B6), and cobalamin (vitamin B12), given to lower total homocysteine
levels, reduce the risk of recurrent stroke over a 2-year period compared
with low doses of these vitamins.
Design Double-blind randomized controlled trial (September 1996–May 2003).
Setting and Participants 3680 adults with nondisabling cerebral infarction at 56 university-affiliated
hospitals, community hospitals, private neurology practices, and Veterans
Affairs medical centers across the United States, Canada, and Scotland.
Interventions All participants received best medical and surgical care plus a daily
multivitamin containing the US Food and Drug Administration's reference daily
intakes of other vitamins; patients were randomly assigned to receive once-daily
doses of the high-dose formulation (n = 1827), containing 25 mg of pyridoxine,
0.4 mg of cobalamin, and 2.5 mg of folic acid; or the low-dose formulation
(n = 1853), containing 200 µg of pyridoxine, 6 µg of cobalamin,and 20 µg of folic acid.
Main Outcome Measures Recurrent cerebral infarction (primary outcome); coronary heart disease
(CHD) events and death (secondary outcomes).
Results Mean reduction of total homocysteine was 2 µmol/L greater in the
high-dose group than in the low-dose group, but there was no treatment effect
on any end point. The unadjusted risk ratio for any stroke, CHD event, or
death was 1.0 (95% confidence interval [CI], 0.8-1.1), with chances of an
event within 2 years of 18.0% in the high-dose group and 18.6% in the low-dose
group. The risk of ischemic stroke within 2 years was 9.2% for the high-dose
and 8.8% for the low-dose groups (risk ratio, 1.0; 95% CI, 0.8-1.3) (P = .80 by log-rank test of the primary hypothesis of difference
in ischemic stroke between treatment groups). There was a persistent and graded
association between baseline total homocysteine level and outcomes. A 3-µmol/L
lower total homocysteine level was associated with a 10% lower risk of stroke
(P = .05), a 26% lower risk of CHD events (P<.001), and a 16% lower risk of death (P = .001) in the low-dose group and a nonsignificantly lower risk in
the high-dose group by 2% for stroke, 7% for CHD events, and 7% for death.
Conclusions In this trial, moderate reduction of total homocysteine after nondisabling
cerebral infarction had no effect on vascular outcomes during the 2 years
of follow-up. However, the consistent findings of an association of total
homocysteine with vascular risk suggests that further exploration of the hypothesis
is warranted and longer trials in different populations with elevated total
homocysteine may be necessary.
Homocystinuria, a rare condition in which plasma levels of total homocysteine
are very high, was first associated with cerebrovascular disease in 1962.1,2 In 1969, McCully3 suggested
that more moderate levels of hyperhomocystinemia might be associated with
atherosclerosis. Case-control studies have shown higher levels of total homocysteine
in patients with premature peripheral and cerebrovascular disease and atherosclerosis.4,5 Most but not all studies have demonstrated
an association between elevated levels of total homocysteine and stroke.6-11
In the European Concerted Action Project, the relative risk of vascular
disease for participants in the top fifth of fasting homocysteine level distribution
(>12 µmol/L) was 2.2 compared with the bottom four fifths.12 When
patients with coronary artery disease were stratified by total homocysteine
level, those with total homocysteine levels greater than 20 µmol/L had
an 8-fold increase in risk.13 A meta-analysis
of epidemiological studies of cardiovascular disease suggested that moderately
elevated homocysteine levels are associated with an increased risk of cardiovascular
disease independent of other established risk factors.14 A
recent meta-analysis found stronger associations with total homocysteine in
retrospective studies of stroke or ischemic heart disease than in prospective
studies of individuals with no history of stroke or cardiovascular disease.15 Boysen et al16 found
a significant difference in total homocysteine levels between patients with
ischemic and hemorrhagic stroke, suggesting that elevated total homocysteine
is not only a reaction to acute illness but also a risk factor for recurrent
stroke.
Mechanisms by which total homocysteine may cause vascular disease include
propensity for thrombosis, impaired thrombolysis,17 increased
production of hydrogen peroxide,18 endothelial
dysfunction,19,20 and increased
oxidation of low-density lipoprotein.21
Folic acid, pyridoxine (vitamin B6), and cobalamin (vitamin
B12) reduce plasma homocysteine levels22 and
may help to reverse endothelial injury associated with elevated total homocysteine.19,20 Vitamin therapy may lead to regression
of carotid plaque, even in patients with normal levels of homocysteine,23 and may reduce the number of vascular events and
revascularization procedures among patients who have undergone coronary angioplasty.24
The Vitamin Intervention for Stroke Prevention (VISP) trial was designed
to determine whether best medical and surgical management, risk factor modification,
and a multivitamin containing high-dose folic acid, pyridoxine, and cobalamin
given to lower total homocysteine levels would reduce the incidence of recurrent
cerebral infarction (primary outcome) as well as coronary heart disease (CHD)
and death (secondary outcomes) in patients with a nondisabling cerebral infarction
and fasting total homocysteine levels greater than the 25th percentile for
stroke patients.
This study was a multicenter, double-blind, randomized controlled clinical
trial performed at 56 centers across the United States (n = 45), Canada (n
= 10), and Scotland (n = 1). The protocol was approved by the ethics committees
of all study institutions and administrative sites. Written informed consent
was obtained from every potential participant prior to screening. The administrative
sites were an operations center, a statistical coordinating center, a central
laboratory for homocysteine and vitamin determinations, and a drug distribution
center.
Volunteers were recruited from university and community hospitals, private
neurology practices, and Department of Veterans Affairs medical centers. Screening
procedures are described elsewhere.25 Briefly,
patients with a presumptive diagnosis of acute ischemic stroke were screened
no sooner than 72 hours following stroke onset per our previous study,26 which confirmed that poststroke plasma total homocysteine
levels are unstable during the first 72 hours following stroke.
Investigators verified eligibility and obtained written informed consent
and a sample of plasma for quantification of total homocysteine by the central
laboratory. Total homocysteine includes homocysteine, homocystine, and mixed
cysteine-homocysteine disulfide.27 Efforts
were made to obtain fasting plasma samples to standardize testing conditions
and total homocysteine measurement. Patients with total homocysteine levels
that exceeded thresholds defined in the Box were qualified for random assignment to high- or low-dose vitamin
therapy. These thresholds were adjusted twice during the study as new information
was obtained regarding the 25th percentile of total homocysteine levels in
stroke patients.
Inclusion Criteria
Nondisabling ischemic stroke(Modified Rankin Stroke Scale ≤3):
Onset
≤120 days before randomization
Focal
neurological deficit of likely atherothrombotic origin, classified as ische
mic stroke by
questionnaire/algorithm or confirmed as new cerebral infarc
tion consistent with symptoms
by cranial computed tomography or brain mag
netic resonance imaging
Total homocysteine level ≥25th percentile
for North American stroke popula tion*
Age ≥35 years
Accessibility
for follow-up
Agreement to take study medication and not take other
multivitamins or pills containing folic acid
or vitamin B6
Written
informed consent
Exclusion Criteria
Potential sources of emboli (atrial fibrillation within 30 days of stroke,
prosthetic cardiac valve,
intracardiac thrombus or neoplasm, or valvular vegetation)
Other major neurological illness that would obscure evaluation of recurrent stroke
Life
expectancy <2 years
Renal insufficiency requiring dialysis
Untreated
anemia or untreated vitamin B12 deficiency
Systolic blood pressure >185 mm Hg or diastolic blood pressure >105 mm Hg on 2 readings 5
minutes
apart at time of eligibility determination
Refractory depression,
severe cognitive impairment, or alcoholism or other substance abuse
Use within the last 30 days of medications that affect total homocysteine level
(methotrexate, tamoxifen, levodopa, niacin, or phenytoin) or bile acid sequestrants
that can decrease folate levels
Childbearing potential
Participation
in another trial with active intervention
General anesthesia or hospital
stay of ≥3 days, any type of invasive cardiac
instrumentation, or endarterectomy,
stent placement, thrombectomy, or any other endovascular treatment of carotid
artery within 30 days prior to randomization or scheduled to be
performed within 30 days after randomization
*Twenty-fifth percentiles were ≥10.5 µmol/L at the beginning
of the study (November 1997); ≥9.5 µmol/L after April 8, 1998; and
≥9.5 µmol/L for men and ≥8.5 µmol/L for women after May
5, 1999.
The study inclusion and exclusion criteria are summarized in the Box. Most participants had routine
diagnostic tests such as carotid duplex ultrasonography and transthoracic
echocardiography. Previous atrial fibrillation was not exclusionary if an
electrocardiogram (ECG) performed within the 30 days preceding recruitment
showed normal sinus rhythm.
All eligible participants were given the low-dose vitamin formulation
for 1 month to determine compliance, assessed by pill counts. Only persons
taking at least 75% of the vitamins during the run-in phase were eligible
to be randomized.
Other baseline measurements included medical history, current medication
and vitamin use, physical and neurological examination, dietary inventory,
a stroke symptoms questionnaire, stroke severity determination (including
the National Institutes of Health Stroke Scale [NIHSS], Modified Rankin Scale,
and Barthel Index),28 the Mini-Mental State
Examination,29 the Rose angina questionnaire,30 and blood sampling for central laboratory determination
of plasma folate and B12 levels and local laboratory determinations
of plasma/serum B12 and creatinine. Another plasma total homocysteine
sample was obtained to serve as the baseline comparison measurement for all
subsequent samples. Cranial computed tomography (CT) or magnetic resonance
imaging (MRI), ECG, and a current lipid profile were required for randomization.
Participants were asked to fast for 12 hours before all clinic visits,
but blood was drawn regardless of fasting state and plasma total homocysteine
levels were determined in duplicate analyses. Concordance between duplicates
was within 10% by high-performance liquid chromatography, using the modified
method of Smolin and Schneider.31-33 Plasma
aliquots were protected from light for single radioassays of folate and vitamin
B12 (Bio Rad Quantaphase II, Bio Rad Diagnostics, Hercules, Calif).
For quality control, replicate blood aliquots were obtained from a sample
of participants (n = 283) and sent to the central laboratory with different
identifiers. Interrun coefficients of variation for analytes were as follows:
total homocysteine, 0.08; plasma folate, 0.14; and plasma vitamin B12, 0.08. The corresponding intraclass correlations between repeat measurements
of blind replicates were 0.94, 0.94, and 0.97, respectively. These results
were similar throughout the duration of the study.
Randomization, Intervention, and Follow-up
Participants were randomized to the high-dose or low-dose vitamin groups
within strata defined by clinic, sex, and age (≥70 vs <70 years). Permuted
block randomization (with block size randomly selected as 4 or 6) was used.34 The allocation of participants was programmed by
the statistical coordinating center, encrypted, and entered into a data entry
program installed on a study computer at each site. After computer verification
that all eligibility criteria had been met, participants were randomly assigned
1 of 20 medication codes. Allocation information was accessible only to the
drug distribution center, which bottled and distributed the vitamins to clinics,
and to selected coordinating center personnel who could assist with randomization
in case of computer failure. Both pill formulations were manufactured (Magno-Humphries
Laboratories, Tigard, Ore) to be indistinguishable by external color, weight,
or dissolution in water. No request was ever made to break the blind.
The multivitamin compositions contained the reference daily intakes
recommended by the US Food and Drug Administration for vitamins,35 varying
only in the content of folic acid, pyridoxine, and cobalamin. The high-dose
multivitamin formulation contained 25 mg of pyridoxine, 0.4 mg of cobalamin,
and 2.5 mg of folic acid; the low-dose formulation (control) contained 200
µg of pyridoxine, 6 µg of cobalamin, and 20 µg of folic
acid.25 Both doses contained at least 6 µg
of cobalamin to minimize the potential for neurological complications resulting
from vitamin B12 deficiency. Participants received once-daily doses
of these formulations.
Physicians provided best available medical and surgical management to
prevent recurrent stroke, which included risk factor control education and,
usually, administration of aspirin, 325 mg/d.
Participants were contacted every 3 months, alternating between telephone
contacts and in-clinic visits for up to 2 years after randomization. At every
contact, the stroke symptoms questionnaire was administered and patients were
asked about hospitalizations since the last contact. These forms along with
discovery of the death of a participant provided the triggers for end-point
determination. The 2-year visit (the exit visit) had an expanded clinical
examination including CT or MRI. When the study was closed, participants who
had not completed the 2-year enrollment or had not had an exit visit were
invited for an early exit visit.
Data from participants who had a follow-up assessment of likely stroke
from the stroke symptoms questionnaire; who had a hospital discharge International Classification of Diseases, Ninth Revision, Clinical
Modification (ICD-9-CM) code of 433, 434, or 436,36 or
a discharge diagnosis of stroke, cerebral infarction, cerebrovascular accident,
or other synonyms; who had an increase in score from the previous examination
in specified sections of the NIHSS; or who died, with stroke as underlying
cause of death, were entered into stroke end-point review.
All relevant information regarding potential stroke end points was reviewed
by the local neurologist and 2 external review committee neurologists. Recurrent
stroke was diagnosed only with evidence of sudden onset of focal neurologic
deficit lasting at least 24 hours accompanied by an increased NIHSS score
in an area that was previously normal. When the sudden onset of symptoms lasting
at least 24 hours was not accompanied by an increased NIHSS score in an area
that was previously normal, then recurrent stroke was diagnosed using cranial
CT or MRI evidence of new infarction consistent with the clinical presentation.If
the reviewers disagreed, the casewas adjudicated by the full review committee.
Silent cerebral infarction after treatment was not analyzed as an outcome
measure, but each participant underwent cranial CT or brain MRI at exit to
assist in the validation of stroke end points with sudden onset of stroke
symptoms but without confirmation by increased NIHSS score in areas previously
normal.
Coronary heart disease events included myocardial infarction (MI) requiring
hospitalization, coronary revascularization, cardiac resuscitation, and fatal
coronary heart disease. Coronary heart disease event end-point review was
implemented when the hospital discharge diagnosis included terms suggestive
of MI, unstable angina, or coronary atherosclerosis. Review was also conducted
for deaths with underlying cause of death related to CHD, including sudden
death, or with ICD-9-CM codes 410, 411, 414.1, 429.2,
36.0, or 36.1.36 Myocardial infarction was
defined by new ECG changes including Q waves or marked ST-T changes plus abnormal
cardiac enzymes, cardiac symptoms plus abnormal enzymes, or symptoms plus
hyperacute ECG changes resolving with thrombolysis. Fatal CHD was defined
by CHD as underlying cause of death on the death certificate and either (1)
(a) prior hospitalization with MI, autopsy evidence of MI, or death resulting
from an invasive coronary procedure or (b) history of angina, MI, or coronary
revascularization when no cause of death other than CHD could be determined;
or (2) death was sudden and no cause of death other than CHD could be determined.
If the 2 external reviewers who reviewed the data disagreed, a third reviewer
adjudicated.
The planned sample of 1800 in each treatment group gave the study 80%
power to detect a 30% reduction in recurrent ischemic stroke over 2 years
of follow-up at a .05 significance level for a 2-sided test. The calculations
assumed probability of recurrent stroke of 8% in the first year and 4% in
the second year and that 20% of participants would be lost to follow-up or
noncompliant or would die of other causes.
Predetermined interim analyses were conducted when the study had obtained
approximately 16%, 30%, 50%, 70%, 85%, and 95% of its total information (ischemic
stroke end points). The Lan-DeMets spending rule, which approximates the O'Brien-Fleming
stopping rule, was used to guide a decision to stop the study early for efficacy.34 Conditional power calculations were also presented
for consideration of futility.37,38
The primary test statistic for interim and final analyses was the log-rank
test,39 based on intention to treat including
all randomized patients as randomized.34 Time
was defined as number of days from randomization to the first end point (if
one occurred), death, date of last contact, or the last day of 2002. Data
collection continued through March 17, 2003, to ascertain and validate events
through 2002. In addition, survival curves were fit by the Kaplan-Meier method.39 In a secondary analysis, tests of treatment group
differences in time to end point were also performed by adjusting for stratification
variables and baseline covariates using a Cox model.39 Rate
ratios are cited as the ratio between persons in the high-dose group vs those
in the low-dose group. SAS software, version 8 (SAS Institute Inc, Cary, NC)
was used for all analyses and P<.05 was considered
statistically significant for all analyses.
Recruitment began in August 1997 and was completed in December 2001.
In December 2002, the performance and safety monitoring board recommended
to the funding agency that the study be terminated because the chance of showing
any difference between the 2 treatment groups in the remaining follow-up period
was close to nil. Exit assessments were to be completed by March 17, 2003,
after all participants had completed at least 1 year of follow-up. The centers
were asked to collect information on potential end points and all hospitalizations
that had occurred prior to January 1, 2003.
Participant Recruitment Data
Participant flow is illustrated in Figure 1. Of the 6860 potential participants screened for total
homocysteine eligibility, 70% (4772) had total homocysteine levels above the
eligibility cut point. After adjusting to final total homocysteine cut points
(8.5 µmol/L for women and 9.5 µmol/L for men), 74% of women (1178/1589)
and 73% of men (1711/2336) screened for total homocysteine were eligible,
close to the 75% study target. Of those with eligible total homocysteine levels,
77% (n = 3680) were randomized (75% of women [1379/1828] and 80% of men [2301/2860]),
1853 to the low-dose vitamin group and 1827 to the high-dose vitamin group.
For the 1092 patients with eligible total homocysteine levels who were not
randomized, the most frequent reasons included "refusal after screening" (48%);
ineligibility, including but not limited to stroke beyond eligibility window
and noncompliance with run-in vitamin therapy (25%); administrative errors
(3%); and "unknown" (24%).
Baseline Descriptive Data
Selected baseline participant characteristics are shown in Table 1 and characteristics of the qualifying
stroke are shown in Table 2. Of
96 baseline characteristics, only 4 treatment group differences were significant
at the .05 level: current cigarette use (16% vs 18%), history of diabetes
(31% vs 27%), chest pain or discomfort (35% vs 38%), and distribution of right
patellar reflex response (data not shown; P = .009).
Of the 3680 randomized patients, for the purpose of the primary analysis,
31 had no follow-up after randomization, (high= 13, low = 18 [high=
number in high-dose group, low = number in low-dose group]), 300 proceeded
to stroke (high= 152, low = 148), 161 died without recurrent stroke
but with follow-up (high= 70, low = 91), and 234 had some but not
complete follow-up (high= 120, low = 114). The remaining 2954 exited,
reached the end of the study (December 31, 2002), or reached the end of 2
years of follow-up without stroke (high= 1472, low = 1482) (Figure 1). Mean follow-up time for all participants
with follow-up was 20.4 months in the high-dose group and 20.2 months in the
low-dose group. For all living participants, 94% of planned contacts were
completed for each treatment group.
Follow-up Descriptive Data
Participants returned their vitamins for pill count at approximate 6-month
intervals, and the percentage of those doing so was similar between groups
(86% for each group), as was compliance among those returning pills, with
94% of each treatment group taking at least 75% of their pills.
Selected patient characteristics after 1 year of follow-up are shown
in Table 3. More than 150 patient
characteristics were tested for treatment group differences during the 6-,
12-, 18-, and 24-month follow-up examinations. Three (not including the expected
differences in total homocysteine and plasma vitamin levels) had statistically
significant differences between treatment groups at the .05 level at study
end: a 1.4-mg/dL higher high-density lipoprotein cholesterol level at the
1-year examination in the high-dose group compared with the low-dose group,
a 6% more frequent use at 1 year of estrogen/progestin among women in the
high-dose group, and 4.2% more patients with reduced or absent right-side
temperature sensation in the high-dose group compared with the low-dose group
from the neurological examination at 18 months (P =
.02; data not shown).
Figure 2 shows the mean levels
of plasma total homocysteine, folate, and B12 at each clinic examination,
by treatment group. Mean values of each were virtually identical between treatment
groups at randomization (13.4 µmol/L for total homocysteine in each
group). After randomization, both groups experienced a decrease in total homocysteine,
but the decrease was greater for the high-dose group by 2.0 µmol/L at
1 month (2.4 µmol/L in the high-dose group and 0.3 µmol/L in the
low-dose group), by 2.2 µmol/L at 1 year, and by 2.3 µmol/L at
2 years.
Between-group differences in decrease in total homocysteine for a particular
visit relative to baseline were somewhat smaller in late 2001: the mean 1-month
difference was 2.7 µmol/L in 1997 through early 1998 and 1.5 µmol/L
in late 2001. Likewise, the 12-month difference decreased from 2.4 µmol/L
in 1997 through early 1998 to 1.8 µmol/L in late 2001. The large treatment
group differences in plasma vitamin levels did not vary substantially over
time.
Seventy-five participants had a baseline B12 level of less
than 150 pmol/L. After study centers were notified and patients received treatment,
all but 2 had subsequent levels greater than 150 pmol/L. At 6 or 18 months
of follow-up, an additional 9 participants (all but 1 were in the low-dose
group) had a B12 level at or below the alert threshold of 150 pmol/L.
Site investigators were notified of these low values, the affected participants
were treated with replacement cobalamin, and all subsequent B12 levels
exceeded 150 pmol/L when assayed locally or in the central laboratory. At
12 months, 29% of participants had neurological findings (diminished Achilles
reflex, reduction of vibration sense in the great toe, or an extensor plantar
sign) that could represent cobalamin deficiency but also might be observed
in stroke, diabetes, or other conditions. The percentage of participants showing
1 or more of these neurological signs after randomization did not differ significantly
between treatment groups, confirming that physical evidence suggestive of
cobalamin deficiency was not confined to the low-dose group and was of no
consequence (Table 3).
At each follow-up clinic visit, participants were asked about potential
adverse effects of the vitamins. There were no statistically significant differences
between treatment groups for itching, skin rash, gastrointestinal upset, for
the overall question on any adverse effects, or for any of the most frequently
cited other adverse effects. No statistically significant differences were
found for any additional self-reported adverse effect thought to be due to
the vitamins, hospital admissions overall and by diagnostic category, or death
overall and by underlying cause.
In an intention-to-treat analysis of the primary end point, 8.1% of
the low-dose group (148/1835) and 8.4% of the high-dose group (152/1814) had
a recurrent ischemic stroke (Table 4).
The Kaplan-Meier curves by treatment group were nearly identical (Figure 3), with P =
.80 by log-rank test. The high-dose group had a 0.4% (95% confidence interval
[CI], −1.6 to 2.4) greater probability of ischemic stroke within 2 years
(by Kaplan-Meier method), and the 2-year risk ratio was 1.0 (95% CI, 0.8-1.3).
Analysis of fatal or disabling ischemic stroke gave similar results.
The intention-to-treat analysis for CHD events included 6.7% of cases
in the low-dose group (123/1835) and 6.3% (114/1815) in the high-dose group.
The high-dose group had a 0.5% (95% CI, –1.3 to 2.2) lower probability
of CHD events within 2 years (by Kaplan-Meier method), and the 2-year risk
ratio was 0.9 (95% CI, 0.7-1.2). Results of separate analyses of hospitalized
MI and fatal CHD were similar.
In the low-dose group, 6.3% (117/1847) died compared with 5.4% of the
high-dose group (99/1821). The high-dose group had a 1.0% (95% CI, –0.7%
to 2.7%) lower probability of death within 2 years (by Kaplan-Meier method);
the 2-year risk ratio was 0.9 (95% CI, 0.7-1.1).
In all analyses, adjusting for characteristics in which the treatment
groups differed at baseline or accounting for the stratification in randomization
had little effect on the results.
Analysis of an end point combining the ischemic stroke, CHD events,
and death end points (whichever event came first) yielded an observed 17.2%
event rate in the low-dose group (316/1838) and 16.7% in the high-dose group
(303/1819), with a relative risk of 1.0 (95% CI, 0.8-1.1). In similar analyses
for ischemic stroke, CHD events, and death within various participant subgroups
defined at baseline (eg, age ≥70 vs <70 years, race/ethnicity, sex,
current smoking, diabetes, history of stroke prior to qualifying stroke, history
of MI, blood pressure, history of chest pain, baseline total homocysteine
level, and fruit, vegetable, or grain intake), no effect of treatment was
found. Of particular interest is the treatment effect among those who began
with high baseline total homocysteine levels. In the top third of the baseline
total homocysteine distribution (total homocysteine >14 µmol/L), the
2-year risk ratios were 0.9 (95% CI, 0.7-1.3) for stroke, 0.9 (95% CI, 0.6-1.3)
for coronary events, 0.9 (95% CI, 0.6-1.3) for death, and 1.0 (95% CI, 0.8-1.2)
for the combined end point including all 3 outcomes. Analyses limited to participants
with compliance of at least 75% showed similar results as the intention-to-treat
analyses.
To determine whether treatment effect might occur only after a longer
interval, we conducted an analysis limited to participants with at least 1
year of follow-up. These results were not significant (hazard rate ratios
for stroke, 1.0; 95% CI, 0.7-1.5; P = .81]; for CHD
events, 1.0; 95% CI, 0.7-1.5; P = .97; and for death,
0.7; 95% CI, 0.5-1.1; P = .12).
Because we found no treatment effect despite several observational studies
that found an association between baseline total homocysteine and cardiovascular
disease in follow-up, we considered such associations within each treatment
group. Based on baseline measurements we found persistent and graded associations
between baseline total homocysteine level and outcomes (Figure 4), which were significant for stroke (P = .02) for the low-dose group but not significant (P = .24) for the high-dose group; for CHD events (P = .001 for the low-dose and P = .002 for
the high-dose group); and for death (P = .001 for
the low-dose and P = .001 for the high-dose group).
For comparison with other observational studies, the above model was recomputed
using baseline total homocysteine as a continuous variable. For the low-dose
group, a 3-µmol/L lower total homocysteine level was associated with
a 10% lower risk of stroke (P = .05), a 26% lower
risk of CHD events (P<.001), and a 16% lower risk
for death (P = .001). For the high-dose group, the
risk was lowered by 2% for stroke, 7% for CHD events, and 7% for death, but
these effects were not significant.
In this randomized double-blind trial, high-dose vitamin therapy had
no effect on the outcome measures of stroke, CHD events, or death. Noncompliance
cannot explain the null findings because the reported good compliance was
corroborated by the consistently higher blood levels of vitamins and the consistently
lower total homocysteine in the high-dose group vs the low-dose group. In
addition, the results were similar when limited to high compliers.
One possible reason our treatment was not effective may have been that
patients enrolled in this study had levels of total homocysteine that were
too low to show a large effect. The very high stroke risk associated with
hyperhomocystinemia involves total homocysteine levels in the hundreds of
micromoles per liter. At levels approaching the normal range, there is a steep
relationship between total homocysteine and risk: a prospective study in Norway40 showed that levels of total homocysteine above 20
µmol/L carried a 9-fold increase in risk, whereas the European Concerted
Action Project41 showed that levels above 10.2
µmol/L were associated with a doubling of risk.
Wald et al42 estimated that reducing
total homocysteine by 3 µmol/L is associated with a 24% reduced risk
of stroke (95% CI, 15%-33%) and a 16% reduced risk of ischemic heart disease
(95% CI, 11%-20%). This implies a 13% combined stroke/coronary event reduction
for a difference of 2 µmol/L, which our trial had 31% power to detect
compared with 80% power for the 30% effect size used in the sample size calculations.
To detect a statistically significant 10% reduction in all-cause mortality
(the nonsignificant result we observed), a sample size of 20 000 participants
would be required for 80% power, assuming 10% dropouts.
The modest reduction in total homocysteine observed in our study may
be due in part to the folate fortification of the US grain supply that coincided
with the initiation of our trial. Folate fortification, which began in 1996
and was mandated by January 1998, profoundly reduced the prevalence of low
folate and high total homocysteine levels. For example, in the Framingham
Offspring Study, the proportion with folate deficiency declined from 22% before
fortification to 1.7% after fortification.43 During
the course of our trial, the mean difference in total homocysteine levels
between the treatment groups narrowed: the 1-month difference was 2.7 µmol/L
at baseline and 1.5 µmol/L at the end of the trial. Fortification probably
reduced the number of participants with high total homocysteine who might
be most likely to benefit.44 Furthermore, the
correction of low serum B12 levels in the low-dose group may have
blunted the vitamin effect. Thus, other determinants of total homocysteine
may have been more important in this setting, suggesting that other regimens,
including betaine (trimethylcholine) and higher doses of B12, might
be more effective.
Another consideration is that a longer duration of treatment may be
necessary. The baseline levels of total homocysteine that were linked to risk
in this trial and in many observational studies likely represent many years
of elevated total homocysteine; the 2 years of treatment in this trial may
have been insufficient to reverse those effects.
An alternative interpretation is that elevated total homocysteine levels
are a marker but not a cause for vascular disease risk. A previous randomized
trial of total homocysteine lowering with vitamins found a significant reduction
in adverse outcomes among patients with successful angioplasty.24 However,
another trial, using folate alone, showed no reduction in adverse outcomes
for patients with coronary artery disease.45 That
trial, like ours, also found baseline total homocysteine to be an independent
predictor of outcome.
In summary, the VISP trial showed that moderate reduction of total homocysteine
level after ischemic stroke had no effect on vascular outcomes during the
2 years of follow-up. However, because of the consistent findings of an association
of total homocysteine level with vascular risk, further exploration of the
hypothesis is warranted and longer trials in different populations with elevated
total homocysteine may be necessary.
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