Posturography in the analysis of postural control in children with cerebral palsy : a literature review

Correspondence to: Guilherme Henrique Lopes – SQS 109, bloco B, apto 604 – CEP: 70372-020 – Brasília (DF), Brasil – E-mail: guilhermelopes@hotmail.com Presentation: Sep, 2012 – Accepted for publication: Jan, 2013 – Financing source: none – Conflict of interests: nothing to declare Posturography in the analysis of postural control in children with cerebral palsy: a literature review Posturografia na análise do equilíbrio em crianças com paralisia cerebral: revisão de literatura Posturografía en el análisis del equilibrio en niños con parálisis cerebral: revisión de literatura Guilherme Henrique Ramos Lopes, Ana Cristina de David


INTRODUCTION
The term cerebral palsy (CP) refers to a group of postural and movement disorders resulting from a nonprogressive and permanent brain lesion that occurs during the development of the immature brain, causing limitations in the daily life of the subject [1][2][3] .Studies point spasticity, joint deformities and muscle imbalance as the main causes for changes in the musculoskeletal system, which are able to interfere with the postural control of children with CP, as well as sensory changes observed in these children, such as visual, auditory and vestibular deficit 1,2,4,5 .
Therefore, it becomes important to assess the balance in these subjects.There are many evaluation protocols, from precise instruments to observational evaluations, with qualitative and quantitative methods.However, the most precise way to assess postural control is posturography, measuring body oscillation during the semi static erect posture by means of force platforms 6 .
However, few studies have used this instrument in children with CP 7 .Thus, the objective of this study was to identify published articles that used the posturography to assess postural control in children with CP and analyze the used protocols and parameters.

METHODOLOGY AND RESULTS
A systematic search of publications on postural balance in children with CP was conducted.The search included the databases LILACS, IBECS, MEDLINE, Cochrane library and SciELO.The following keywords were used: cerebral palsy AND postural control OR balance.In the initial search, by means of resources from the databases, studies performed with children aged less than two years old or with adults which have not been published in Portuguese or English and those published prior to 2002 (Stage I) were excluded.After this initial search, the assessment based on the title, abstract and keywords was performed, and the following inclusion criterion was added: the use of a force platform as an instrument to assess balance, and, as exclusion criteria, the studies of post-surgery evaluation and literature review (Stage II).The analysis of full texts (Stage III) was performed with the objective to find assessments of postural balance in children and adolescents with minimum conditions to be analyzed in the force platform, standing up or sitting down.
In Stage I, 165 articles were found.The evaluation of title, abstract and keywords reduced the sample to 47 studies.In Stage III, the full texts were analyzed, and at the end there were 16 articles that met the criteria (Table 1).

DISCUSSION
The studies presented samples of subjects that ranged from 7 to 23 people, with mean of 12 subjects (±5.7), and mean age of 5 to 11 years old.Two articles that assessed postural balance in the sitting position and 14 that analyzed the standing position were found.
Even though we found only two studies analyzing children in the sitting position 8,9 , we recognize the importance to better understand body balance in this position.Due to balance changes and the global motor compromise, most subjects with CP remain sitting for a long time, and this is how they participate in the daily life activities.This situation happens because they cannot remain standing and, even if they can, they perform better when sitting down, or even because they spend too much energy by standing up.Liao et al. 9 emphasize as a priority in the rehabilitation of these subjects that they can get maximum function in the sitting position.
Both studies used groups with spastic CP in the sample, comparing them with children with typical development (TD), but with different methods to assess balance (Table 2).
Cherng et al. 8 assessed balance sitting down with the feet of the subjects against the force platform.The authors concluded that postural stability in the sitting down position goes through changes according to the seat inclination and in a comparable way between children with CP and TD.The anterior inclined seat provided better postural balance and functionality in the reaching task for both groups.However, with the posterior inclined seat, the displacement of the medial-lateral center of pressure (COP) was greater in the group with CP.
Liao et al. 9 used the platform as a supporting base for sitting down.It was observed that COP displacement in the medial-lateral and anterior posterior directions is greater in the group with CP, but there was statistical difference only in the lateral displacement of the dynamic balance.Such results suggest that children with CP acquired trunk stability and mechanisms of static balance similar to those of children with TD, and demonstrated that the tasks concerning static balance may not be sufficient to detect the difficulties of balance in this population.
The other studies assessed the balance in children with CP in the standing position (Table 3).The mobile platform was used to for the analyses and intervention in a study (divided into two publications) 10,11 , and only as an evaluation instrument in other two studies 12,13 .
The therapeutic use of the mobile platform seems to cause positive effects with practice, because the results showed that the postural control in children with CP can be changed after intense training (5 days with 50 anterior disturbances and 50 posterior disturbances/ day), also suggesting that children with spastic diplegia CP need more prolonged training than hemiplegic patients due to the different forms of motor or sensory and motor compromise found between these two groups 10 .Burtner et al. 12 and Chen and Woollacott 13 used the mobile force platform only to analyze balance and verified lower postural control in children with CP in comparison to children of the same age and level of motor development, but without CP.Therefore, the hypotheses of the authors that the balance variables can be considered in the differentiation between children with or without CP were corroborated.
From the other analyzed studies, five of them used the force platform aiming to compare the balance variables between groups of children with CP and with TD 7,[14][15][16][17] .These articles also point to the lower postural control in groups with CP.
Nobre found lower COP mean oscillation in the anterior posterior direction among children with CP in the conditions of eyes open and closed.His results suggest that children with TD use more ankle strategies to promote balance, causing more oscillation than children with no ability to use this strategy, besides mentioning that the group with CP was under constant intervention with focus on the balance training 7 .
The position of the body, arms and the support base, as well as the time in the force platform and the number of assessment attempts can influence the balance evaluation, be it for biomechanical reasons or for the subject's fatigue -that is why it is important to establish standardization criteria for these elements.In the analyzed studies, we observed there are different protocols of evaluation (Table 3).
In relation to the position of the support base, most studies use the feet distance self-selected by the research subjects 7,[13][14][15] .As to the position of the arms, we also observed convergence between most protocols, which follow the evaluation with relaxed arms along the body 7,14,15,[18][19][20][21][22] .Such preference for a more comfortable feet and arm position can be explained by the specific characteristics of the children population, with changes in movement and comprehension, thus facilitating the application of the protocol.
The time of evaluation in the force platform does not exceed 20 seconds in 50% of the studies 9,11,14,21 .In the others, the analyses are conducted with the limit of 60 seconds 16,19 , and at least 7 seconds 18 .In the articles, we did not find the theoretical base for the performance of short evaluations, but we believe that the reduced time to measure balance enables the participation of these subjects who, due to the characteristics of CP, demonstrate lower tolerance to long balance assessments.
Likewise, we believe that the specificities of CP, together with the aspects of childhood, in the sense that it is more difficult to apply extensive and detailed methods and protocols, justify the reduced number of task attempts.Despite the differences between protocols, we observed there are usually two to Five attempts in most studies 8,14,15,18,21 , but some make only one attemp 7,9,16,19 .
The balance parameters that are most used in the studies are the maximum medial-lateral and anterior posterior COP displacements.Other measures are also used, such as the displacement ratio (sway ratio), and speed of COP and RMS (root mean square) displacement.
Only one study with experimental design was found, with the randomization of groups and the assessment of balance in a force platform using the blind-condition strategy during evaluation 17 .The authors found that the experimental group of children with CP using a specific ankle-foot orthosis and practicing physical therapy exercises presented improvement in relation to the control group, which only practiced exercises.However, as shown in Table 3, this study did not report several aspects of the evaluation protocol, like time of evaluation, number of attempts, position of arms and legs at the time of evaluation, which makes it difficult to replicate the study and better understand it.

CONCLUSION
According to the research, there are still few studies that use the force platform to understand the behavior of balance in children with CP, especially the assessment of balance in the sitting position.Most articles perform comparative studies between children with CP and TD.Only one study with experimental design was found, which used the force platform to assess a specific intervention.
The sample size seems to be a limiting factor of the studies due to the reduced number of participants in the found articles.
The difficulties of movement and postural control of CP seem to be relevant aspects in the creation of assessment protocols for these children in the force platform, becoming less strict as to the position of the body, the number of attempts and the time of evaluation.
The analyzed articles showed that the linear parameters of medial-lateral and anterior posterior displacement in the COP and the ratio of these displacements have been more frequently used as parameters to assess balance from the force platform.
The evaluations of balance in the force platform demonstrated differences between the groups of children with TD and children with CP, and they were able to detect the improvements caused by interventions in the force platform or specific therapeutic practices.
Therefore, we believe that new studies using this assessment instrument and with larger samples can collaborate with the better understanding of aspects concerning postural balance of children with CP, leading to more efficient interventions and treatments.

Table 1 .
Description of studies that used the force platform to assess the postural balance in children with cerebral palsy IF: impact factor; CP: cerebral palsy; TD: typical development; OE: open eyes; CE: closed eyes; NI: not informed

Table 2 .
Procedures of studies that assessed postural balance in the sitting position AP: anterior posterior; ML: medial-lateral; COP: center of pressure AP: anterior posterior; ML: medial-lateral; NRMS: Normalized root mean square; EMG: electromiography; COP: center of pressure; COG: center of gravity; NI: not informed; CP: cerebral palsy; RMS: root mean square; NA: Non-applicable.

Table 3 .
Procedures of studies that assessed postural balance in the orthostatic position