Background

Dehydration treatment is one of the main interventions that are adopted to prevent herniation of intracranial hypertension. Mannitol is widely used in China as a dehydrating agent in traumatic brain injury (TBI) patients with intracranial hypertension. Earlier studies indicated that dehydration therapy might decrease the intracranial pressure (ICP) of patients with TBI by reducing cerebral edema [1]. However, the excessive dosage of mannitol will penetrate from the blood into the brain, where it might cause the increase of the intracranial pressure (rebound phenomenon) and introduce the secondary brain cells damage accordingly [2, 3]. Therefore, the effectiveness of the treatment of mannitol for the intracranial hypertension in TBI is controversial [4, 5]. After the 1990s, a few new dehydration products have been introduced as the ICP reduction mediums, such as the hypertonic saline (HS) and the colloidal solution. Some clinical trials have shown that the hypertonic saline is more effective than mannitol in reducing the intracranial pressure and increase the cerebral perfusion pressure [5, 6]. However, there are studies with various results and conclusions [7, 8]. Single clinical trial usually has disadvantages by its sample size, design, or conduction. Clinical practitioners are puzzled by the chaos of the conflicted evidences and opinions to this end. The widespread use of mannitol is in great need of clarity optimal administration. There is uncertainty over the effectiveness of mannitol when compared to other ICP-lowering agents and other treatment without dehydrating agents. As a result, a systematic review is in urgent need, and this is why we present this study.

Methods

Strategy of data retrieving

Published literatures on the use of mannitol in severe traumatic brain injury patients were retrieved in the following databases: PubMed (US National Library of Medicine), The Cochrane Library (2014, Issue 3), ISI (Web of Science: Science Citation Index Expanded), Chinese Biomedicine Database (CBM), and China Knowledge Resource Integrated Database (CNKI). Letters have been sent to the first authors of the reports, asking them to assist in identifying any further trials that may have been conducted by them but not been reported publicly. Eligibility was determined by reading the reports of possible trials. The terms and strategies of retrieving are listed in the Table 1.

Table 1 Search terms and search strategy
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Inclusion criteria

Studies evaluating adult trauma patients with severe traumatic brain injuries and which included mannitol as an intervention were evaluated. The following data were required for inclusion: (1) study design: only RCTs set up with parallel control groups were selected, excluding self-control or crossover trials;(2) type of patients: adult trauma patients (age ≥18 years); (3) severe brain injures (Glasgow coma score <8) with cerebral edema. When indexes of the two groups such as genders, ages, pre-treatment ICP, osmolality levels, and Glasgow scores were matched, then the two groups were comparable; (4) intervention: the treatment group received mannitol in any dose for any duration, while the comparison group could be placebo controlled, different dose, different agent, or no agent; (5) reported one or more outcomes as following: (a) primary outcome: the mortality and (b) secondary outcome: (i) days on ICU and (ii) ventilator day; and (6) surrogate endpoints: ICP, cerebral perfusion pressure (CPP), and mean arterial pressure (MAP). The mortality is related with secondary and surrogate outcomes, so we collect them in the study.

Exclusion criteria

The following data were required for exclusion: (1) non-RCTs; (2) age ≤17 years, diagnosis of stroke, brain tumors, and non-traumatic brain injuries; (3) crossover studies; (4) did not address any primary or secondary outcomes as mentioned above; and (5) for those studies that did not describe the randomization methods, we attempted to contact the original authors. If the original authors did not provide a response or the randomization method proved inadequate, the articles were excluded.

Methodological quality evaluation

The methodological quality assessment table was based on the Cochrane Reviewers’ Handbook [9] and the modified Jadad scale [10, 11]. Data synthesis was conducted by R (R package version 3.7-0.) [12]. We followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement to report the research protocol, outcome, and relevant items in this systematic review [13].

Limitations of the study

Though there are many published literatures regarding the use of mannitol in severe traumatic brain injury patients, few of them have met the high quality standards of evidence-based medicine. There is insufficient reliable evidence to make suggestions on the administration of mannitol and many unanswered questions on the optimal use of mannitol in severe traumatic brain injury patients.

Results and discussion

Study identification and selection

There were in total 176 potentially relevant titles, abstracts, and articles that have been screened. Initial screening resulted in 42 candidate studies [4, 5, 8, 14–52]. Figure 1 shows the details of the selection process and the reasons for exclusion. There were four trails that met all inclusion criteria and were included in the final meta-analysis. The characteristics of these included trials are listed in Tables 2 and 3.

Fig. 1
figure 1

Literature searching and selection

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Table 2 Methodological characteristics of studies
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Table 3 The definition of secondary outcomes and surrogated endpoints of included trials
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Major outcome: mortality

Amongst all the enrolled studies, Vialet et al. [49] and Harutjunyan et al. [50] have provided the primary outcome (mortality) (Table 4, Fig. 2). There is no heterogeneity between two studies (I 2 = 0 %), and a fixed model was used for the meta-analysis (n = 53), pooled result indicated that the mortality was similar in mannitol intervention and control treatment, OR = 0.80, 95 % CI [0, 27, 2.37], P = 0.38.

Table 4 Mortality of included studies
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Fig. 2
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Comparison 1: mannitol versus hypertonic saline. Outcome 1 mortality

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Secondary outcomes

In terms of the report of the secondary outcomes and surrogate endpoints, four RCTs were not uniformed and the data synthesis was not available. Hence, we have conducted a qualitative analysis afterward. There was no difference between two groups in neurological outcome (Glasgow outcome scale (GOS) 5/10 vs. 6/10; P >0.05) and days on ICU (23.3 ± 14.8 vs. 22.8 ± 15.5; P >0.05). No trial reported ventilator day.

Surrogated endpoints

ICP

Mannitol therapy may have a detrimental effect on decrease ICP when compared to hypertonic saline therapy. Vialet et al. [49] compared 20 % mannitol group with 7.5 % HS group. The result suggested that the mean number (13.3 ± 14.6 vs. 6.9 ± 5.6 episodes) of intracranial hypertension episodes (ICP >25 mmHg) per day and the daily duration (131 ± 123 vs. 67 ± 85 min) of intracranial hypertension episodes were significantly higher in the mannitol group (P <0.01) than those in the HS group [49]. The rate of clinical failure was also significantly higher in the mannitol group (7 of 10 vs. 1 of 10 patients; P <0.01) than that in the HS group [49]. Harutjunyan et al. [50] compared 15 % mannitol to 7.2 % hypertonic saline hydroxyethyl starch 200/0.5 (7.2 % NaCl/HES 200/0.5). NaCl/HES 200/0.5 (7.2 %) caused a greater decrease in the ICP than mannitol did (57 vs. 48 %; P <0.01) [50]. Both groups decreased the ICP to below 15 mmHg; but the mean time was significantly longer in the mannitol group (8.7 vs. 6 min; P <0.01) than that in the HS group. Francony et al. [51] and Cottenceau et al. [52] have shown that the spectrum of the ICP decrease was significantly larger in the HS group than in the mannitol group, and the spectrum was about 10 % over the entire stage. The changes of ICP across the four studies are summarized in Table 5.

Table 5 ICP of included studies
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CPP

Mannitol therapy may have a small detrimental effect in increasing CCP when compared to hypertonic saline therapy. Mannitol therapy may only have a beneficial effect in improving the blood supply of local brain tissue. Vialet et al. [49] has indicated that there was no significant difference in the mean number (3.1 ± 3.6 vs. 4.0 ± 4.6 episodes) of cerebral perfusion hypotension episodes (CPP <70 mmHg) per day and the daily duration (62 ± 107 vs. 52 ± 83 min) of cerebral perfusion hypotension episodes between the two groups (P >0.05) [49]. Harutjunyan et al. [50] has shown that the CPP had significantly sharp increase in both groups after the start of the infusion (P <0.0001), and the HS group was significantly higher than the mannitol group at 30 min after the beginning of the infusion (P <0.05) [50]. The maximum increase occurred at 30 min after the beginning of the infusion in both groups, and the HS group increased more than the mannitol group did (27 vs. 18 %; P <0.05) [50]. Francony et al. [51] suggested that mannitol could only effectively improve the CPP and the blood supply of the local brain tissue [51]. Cottenceau et al. [52] suggested that the maximum of CPP increase occurred at 30 min after the beginning of the infusion in both groups, but there was no significantly difference between the 20 % mannitol and the 7.5 % HS group [52]. The changes of CCP across the four studies are summarized in Table 6.

Table 6 CPP of included studies
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MAP

Mannitol therapy may have no beneficial effect in increasing MAP when compared to hypertonic saline therapy. Harutjunyan et al. [50] shown that MAP had a significantly sharp increase in 15 % mannitol and 7.2 % NaCl/HES 200/0.5 after the beginning of the infusion (P <0.05). There was no significant difference in the increase of MAP between two groups (5.8 vs. 7.6 %), but the maximum increase occurred significantly shorter in the mannitol group than in the HS group (10 vs. 30 min) [50]. Francony et al. [51] and Cottenceau et al. [52] have shown in their research that there was no significant difference in MAP between the 20 % mannitol and the 7.5 % HS. The changes of MAP across the four studies are summarized in Table 7.

Table 7 MAP of included studies
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Plasma osmolality, urine, and serum sodium

Mannitol therapy may have no difference in elevating the plasma osmolality and serum sodium when compared to hypertonic saline therapy. Mannitol therapy may have a small beneficial effect in increasing the urine output. But the single trial was too small for reliable conclusion. Vialet et al. [49] suggested that the plasma osmolality and serum sodium elevated significantly in both the 20 % mannitol group and the 7.5 % HS group after the beginning of the infusion (P <0.05), but there was no significant difference between the two groups. Harutjunyan et al. [50] showed the serum sodium increased in both the 15 % mannitol and the 7.2 % NaCl/HES 200/0.5 (P <0.05), although there was no significant difference between the two groups. Francony et al. suggested that mannitol caused a significantly greater increase in urine output (P 0.05) than HS [51]. Cottenceau et al. suggested that the HS caused a significantly greater increase of serum sodium (P = 0.0000), although mannitol caused a significantly decrease of it (P = 0.0000) [52]. The changes of plasma osmolality and sodium across the four studies are summarized in Table 8.

Table 8 Plasma osmolality and serum sodium of included studies
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Conclusions

Published literatures regarding the use of mannitol in severe traumatic brain injury patients have rarely met the high quality standards of evidence-based medicine. This systematic review has revealed that the mannitol therapy cannot reduce death risk for TBI patients suffering from raised ICP.

Four RCTs retrieved by this study which have found reduced intracranial pressure (ICP): two RCTs suggested that hypertonic saline was superior to mannitol, and the time of maximized effect was earlier than that of mannitol; whilst the other two RCTs have proposed mannitol was equally on the impact of ICP comparing with hypertonic saline. In the elevation of CPP and MAP, one RCT found that hypertonic saline was superior to mannitol, whilst the other three RCTs considered no significant difference between the two agents. One RCT showed that mannitol might have a beneficial effect on cerebral hemodynamic when compare to hypertonic saline. However, some domestic studies have shown that the 23.4 % HS have faster and longer effect than the 20 % mannitol did in the treatment of raised ICP of severe brain injury [53–56]. Thus, the points of those researchers on this issue are varied and contradicted with each other.

Currently, mannitol is widely used as a part of conventional therapy of the TBI patients who suffer from intracranial pressure hypertension. Based on the most credible evidence from this study, mannitol therapy cannot reduce the mortality risk in this type of patients. Well-designed randomized controlled trials are urgently needed to demonstrate the efficacy of mannitol to acute severe traumatic brain injury.