Introduction
Strabismus, generally known as crossed eyes, is defined as an abnormal ocular alignment or any deviation in the direction of one or both eyes that may be congenital or acquired, horizontal, vertical, rotational, or combination of these modes (1). The deviation is expressed in degrees or prism diopters and have different etiologies. The most important and visible sign of strabismus is double vision and misalignment of the eyes, in which one eye turn in, out, up, down, or the eyes may be positioned obliquely. Strabismus may occur due to neurological or anatomical problems that interfere with the normal control and function of the muscles.
Intraocular pressure (IOP), generally known as eye pressure, is considered as the most important medical factors in ophthalmology, which can typically show the normal function of the eyes. The positive pressure within the eye is the result of balance between the production of aqueous humor by the ciliary process and outflow of aqueous humor from the trabecular meshwork and uveoscleral pathway, which is expressed in millimeters of mercury (mmHg). Moreover, different baseline values have been reported for IOP in different studies, but normal level of intraocular pressure (IOP) in 95% of the population (general population) is 10 to 21 mmHg (mean 15.5 mmHg) with a small variation (2). Many factors such as corneal thickness, gender, refractive error, and the presence of diabetes mellitus (DM) influence the IOP (3,4), and among these factors, external pressure on the globe and the orbital contents are of great importance (5). In addition, it is shown that some anesthetics and chemicals such as nitrous oxide may change IOP and there may be some diurnal and seasonal variation of IOP (6). Extraocular muscle tone are very important in IOP measurement. IOP changes have been reported following the histopathological abnormalities or various manipulations of extraocular muscles such as botulinum toxin injections or side effects of medications (7-9). There are evidences that confirm the fluctuation of IOP following strabismus surgery (2,10,11). Finding an association between IOP changes and strabismus surgery is of great interest, because IOP changes can be considered as an indicator for restrictive strabismus if this assumption is proven (12,13).
There is a few studies in this field and a trustful and thorough methodology has not been used in these studies. Hence, a comprehensive study is necessary to show the effect of extraocular muscle surgery on IOP. Thus, the main aim of this study is to systematically review the effects of strabismus surgery on variations of IOP, and to find out whether IOP changes during or after strabismus surgery reflex, and malpositioning of the ETT.

Methods
Literature search strategy
PubMed and Scopus were searched thoroughly for articles in which the fluctuation of IOP before and after strabismus surgery had been evaluated. Both database were searched for IOP and strabismus in the title, keywords, and abstract. Relevant articles with the following search terms: (Intraocular pressure OR IOP) AND (strabismus) were selected and reviewed by two independent reviewers with the last update on February 2015. Irrelevant articles were omitted in the first step by reviewing the titles and abstracts of all collected records. Afterwards, the full texts of the relevant papers were studied. Finally, the reference lists of relevant articles were also searched manually to minimize the possibility of missing relevant articles.

Study selection
There was no time limitation for the included articles. Articles in languages other than English were excluded to avoid problems during data extraction. All types of articles including case series, cross-sectional, clinical trials, and cohort studies were selected and included in this study. Review articles or previous meta-analysis were excluded. Inclusion criteria were articles in which the studied patients had undergone strabismus surgery and the IOP were measured pre- and postoperatively. Because two independent database were searched, duplicated articles were omitted in the first step. Exclusion criteria were the articles in which the fluctuation of IOP was due to anatomical problems other than strabismus, inflammation of the eye, genetic factors, or medication- related side effects.

Data extraction
Data including the name of first author, country of origin, publication date, study design, IOP before and after surgery and the concluded results were extracted. All available documents including total number of participants, demographic data and collected results were obtained as possible. Data were categorized based on the results recorded before and after surgery. The quality assessment of articles selected for this systematic review was performed according to the recommendations of CONSORT checklist 2010 by two independent reviewers (14).

Results
Over 323 articles were found in PubMed and about 1351 articles in Scopus, 1617 were excluded due to duplication or irrelevancy. After step by step process of article selection in the two database, 57 relevant articles were included for further evaluation. Finally, full text of 8 articles, which met the inclusion criteria were obtained and data were collected. Articles that met the inclusion criteria were ultimately excluded if their full texts or even abstracts were not found. Step by step selection process is shown in Figure 1.
information about association between strabismus surgery and fluctuation of intraocular pressure. the airway and lead to the incidence of postoperative respiratory complications (12).
According to the extracted data and based on the results of included articles, strabismus surgery results in a significant reduction in IOP, especially in early period after surgery (20). Several reports have shown that IOP changes related to the muscle resection could be treated with some medications such as steroids, dexamethasone, and diclofenac, reflecting the effect of surgery on IOP (21-23). These drugs often act by controlling inflammation and discomfort in the eyes, which shows that IOP changes might be the result of pressure on the eye and surrounding tissues during surgery.
On the other hand, these data might not reflect the actual correlation between IOP changes and surgical intervention due to limited number of data in the field, inappropriate design of studies, limited number of participants, and inaccuracy of methods designed for the measurement of IOP during studies. Although, several reports showed that IOP significantly increased from baseline several days after surgery (18), some studies concluded that there might be no direct correlation between the pressure changes and strabismus surgery. Moreover, temporary increase in IOP might be due to the intensity of massage of globe during surgery (15).

Conclusion
In sum, variation of intraocular pressure due to muscle recession is confirmed according to the extracted data; nonetheless, the changes might vary from very slight amounts to very high values. However, intraocular pressure reduces postoperatively at least for a few days after surgery according to reports.

Acknowledgement
We would like to thank Clinical Research Development Unit of Ghaem Hospital for their assistant in this manuscript.

Conflict of Interest
The authors declare no conflict of interest.