Intraocular Pressure Measurement in Africa: A Review of Literature

Abstract

Ocular hypertension (OHT) is a pervasive condition easily mistaken for glaucoma. In Africa, glaucoma is reported the highest, hence there is a need to properly distinguish it from ocular hypertension, which is the rise in IOP in the absence of glaucomatous changes. Many methods exist for measuring IOP; however, recent focus has been on non-invasive techniques. This review aims to assess the measurement of IOP among non-glaucomatous Africans. The research adopts a systematic approach employing the five-step framework by Arksey and O Malley. We used the research question to formulate a search strategy used to extract the studies included in the review. Next, we used keywords in combination with Boolean operators to search the PubMed database. The study analyzed articles published in English between 2010 and 2023. The search identified 136 articles. Both authors checked the article for screening and eligibility. The instruments used include GAT, perking’s tonometry, rebound tonometry, tonopachy, and the value of IOP among Africans ranges from 11 to 16 mmHg and varies according to instruments and population. There is a wide variation in the value of IOP among Africans. Practitioners should consider the population mean for each instrument used and central corneal thickness during IOP measurements.

1. Introduction

Glaucoma is a major cause of irreversible blindness and a major public health concern on the African continent [1]. Intraocular pressure (IOP) is the only modifiable factor of glaucoma and ocular hypertension, which is regarded among other risk factors such as thinner corneas, older age, and greater cup-to-disk ratio [2]. Ocular hypertension (OHT) refers to eyes in which the IOP is higher than the normal range for the general population but is free of glaucomatous damage [3]. Noteworthy, the majority of people with OHT do not progress to glaucoma [2], nevertheless proper distinction is required to clinically detect OHT and glaucoma since glaucoma causes mechanical damage to the optical nerve thereby disrupting visual signals sent to the brain thereby affecting vision. OHT represents 75% of all forms of glaucoma [3].

The prevalence of OHT differs across populations. For instance, the cut-off value for OHT is 20.4 mmHg, 21.5 mmHg, and 22.6 mmHg for the Chinese, Indian, and Malay population groups [4]. However, OHT is highest among Africans compared to other continents, reflecting the effect of race and heredity [5]. Noteworthy, IOP measurements vary based on measurement technique and population. In general, the upper limit for “normal” IOP is two standard deviations above the population mean. Needless to say, practitioners need to consider the normal values during diagnosis and follow-up of glaucoma suspects and patients concerning measurement technique and population [6].

Despite that black people have thinner corneas, which predispose them to the risk of developing OHT and glaucoma [2], information regarding the normative values of different methods of IOP among people of African descent remains unknown. In addition, there are diverse ethnic groups in Africa, which calls for a need to assess the geographical distribution of IOP across the region. This chapter will summarize the current data on normative values of IOP including the current techniques employed for measuring IOP in Africa focusing on the challenges and opportunities associated with various measurement techniques employed in the region.

The paper is split into three sections. The first section will address the methodologies employed to answer the research question in this review, including the research design, search strategy, data retrieval, and analytical strategies. Next, the paper presents the results of the search summarizing the data retrieved in the previous section. Finally, the paper discusses the research findings in light of the available literature considering the situation of the African eye health care landscape discussing the advantages and disadvantages of using the diagnostic technique considering this setting.

2. Methods

The review adopted a systematic review approach postulated by Arksey and O’Malley [7]. The steps include formulating the research questions, identifying studies, selecting relevant studies, charting the data, and collating, summarizing, and reporting the findings.

2.1 Identifying research questions

The review is guided by the following research question.

• What is the distribution of intraocular pressure in nonglaucomatous Africans?

2.2 Eligibility of research studies

To assess the eligibility of the research question formulated, we employed the Population Concept Context framework as illustrated below

• Population: nonglaucomatous patients

• Concept: intraocular pressure or ocular hypertension

• Context: Africa

2.3 Identifying relevant studies

2.3.1 Inclusion criteria

As an inclusion criterion, we selected studies published from 2010 to 2023. In addition, we retrieved studies from the citations of our searched articles. Only studies published in the English language and conducted among nonglaucomatous Africans were entered into the review.

2.3.2 Exclusion criteria

We excluded studies conducted among nonhuman subjects. Furthermore, studies conducted to develop new instruments were not included in the review. In addition, we removed studies employing the following research designs retrospective and systematic reviews.

2.3.3 Electronic database search

We searched PubMed and Google Scholar databases for research on ocular hypertension or intraocular pressure among African populations. We used the following keywords and a combination of Boolean operators’ ocular hypertension OR intraocular pressure AND Africa AND nonglaucoma.

A manual search through the references of retrieved studies was conducted and all studies that satisfied the inclusion criteria were included in the review.

2.3.4 Selecting relevant studies

The remaining studies were assessed by reviewing the title and abstract. This was done by members of the study and they reached a consensus. When they failed to reach a consensus, a third independent reviewer was invited to help decide. The selected papers underwent review using a critical appraisal tool for cross-sectional studies to ensure quality studies are included in the review (Figure 1).

Figure 1.

Prisma.

2.3.5 Charting the data

We included relevant information, including the author’s name, year of publication, county of study, methods and design of the study, mean IOP, and standard deviation. The process was conducted by the two authors and meetings were held to compare the findings and agree on the charting.

2.3.6 Collating, summarizing, and reporting the findings

The charted data was summarized to communicate the values of IOP. The report followed the guidelines included in the Preferred Reporting Items for Systematic Reviews and Metanalysis (PRISMA) [7].

3. Results

The selected studies are illustrated in Table 1. Out of the 54 studies identified, 46 were excluded because they did not meet the inclusion criteria. About eight were sought for retrieval and only five out of eight were screened for eligibility and included in the qualitative synthesis.

Author (year)	Country	Methods	Mean IOP (standard deviation)	Technique usage
[8]	Ethiopia	Prospective cross-sectional study	12.1 mmHg (SD = 5.0 mmHg)	Rebound tonometer (iCare)
[9]	Southwest Nigeria	Comparative cross-sectional study	15.18 ± 4.26 mmHg (GAT) 16.32 ± 4.48 mmHg (Rebound iCare tonometer)	GAT & iCare
[10]	Southwest Nigeria	Hospital-based case control study	14.2 ± 2.6 mmHg	GAT
[11]	Egypt	Prospective cross-sectional study	11.5 ± 2.34 mmHg	Perkins tonometer
[12]	South Africa	Cross-sectional study	14.79 ± 3.09 mmHg (GAT) 14.32 ± 3.57 mmHg (Tonopachy) 16.64 ± 4.38 mmHg (iCare)	GAT, Tono-pachymeter (Tonopachy) & iCare

Table 1.

An overview of reviewed studies.

Out of the five studies included in the synthesis, two were from Nigeria [9, 10], 1 from Ethiopia [8], 1 from South Africa [12], and 1 from Egypt [11].

3.1 IOP variation according to instruments

The reviewed studies showed that there are differences in the mean IOP based on the instruments used. For instance, Ashano et al. [9] found that IOP measured with a rebound tonometer differed significantly from the mean IOP measured with GAT although the two were significantly correlated. The I Care rebound tonometer shows higher results compared with GAT measured by one practitioner on the same set of patients. In agreement, the South African study [12] found that tonopachy and GAT findings were in tandem while the rebound tonometer produced higher IOP2021 values. Therefore, the two procedures cannot be used interchangeably. Nevertheless, the rebound tonometer is relatively easy to use as it requires minimum training, consumes less time and does not need anapestic eye drops, and is portable ideal for outreach programs [8].

3.2 Central corneal thickness and IOP

Furthermore, it was discovered that there is a positive correlation between central corneal thickness and intraocular pressure as evidenced by an increase in IOP associated with CCT in a study in Nigeria [10]. In contrast, a study conducted in South Africa [12] found no relationship between CCT and IOP measured with GAT, rebound tonometer and tonopachy. This relationship highlights the need to exercise caution with thinner corneas which may underestimate IOP and delay initiation of treatment among the continental Africans.

4. Discussion

The chapter summarizes the common methods used to measure the modifiable risk factor of OHT and glaucoma namely IOP in the African setting. The chapter has achieved this by systematically reviewing research that has been conducted on the African continent to assess IOP among normal populations. The following narrative provides some of the common themes emerging from the literature on the topic.

4.1 Instruments used to measure IOP

This review has discovered that the most common instruments used for measuring IOP are Goldman applanation tonometry, Perkins tonometer, and rebound tonometry. The results of this paper are not surprising considering that GAT is considered the gold standard of measurement worldwide. On the other hand, Perkins and eye care could be mostly utilized for their compatibility. Unlike the Goldman applanation tonometry, which requires sophisticated equipment such as slit lamp biomicroscope, the Perkins and rebound tonometry are portable and therefore easy to use during outreach programs. In most African regions, access to eye care services is complimented by community outreach programs, especially through cataract camps. The major drawback of GAT also lies in the requirement of fluorescein and local anesthesia as part of diagnostic drugs. This becomes a challenge in the African context against the backdrop of insufficient and unavailability of drugs and medicines coupled with issues of pilferage. In Africa, the delivery of eye care services continues to be undermined by health system performance bottlenecks [13]. The medical practice in Africa is constrained by the unavailability of equipment, a lack of diagnostic skills, and expensive treatment regimens [1].

4.2 IOP and central corneal thickness

The review found that there is a discrepancy in the relationship between IOP and central corneal thickness. This also concurs with Osman et al. [14] who postulated that corneal thickness and irregularities may affect GAT. We attribute the differences to different corneal thickness parameters across populations. Accordingly, ocular parameters differ significantly with ethnicity. The findings of this review suggest that practitioners in Africa should consider the corneal thickness when measuring IOP. IOP measurements are heavily dependent on corneal parameters and can be a major source of error. However, accuracy in IOP measurements is integral [15].

The range of CCT across Africa is 519–550 μm. The highest CCT value was reported among a Nigerian sample (550 μm) [16]. The lowest was reported among the Ethiopian and South African populations (519 μm) [12, 17]. Instruments such as the Keeler Pulsair Eye non-contact tonometer are prone to changes in corneal thickness and should be used together with CCT [18]. Modern techniques allow for the CCT measurements to be incorporated in the calculation of the final result of IOP measurement. For example, the non-contact automated Reichert’s Ocular Response Analyzer (ORA; Reichert, Inc., Depew, NY) produces two measures of IOP: the Goldman correlated IOP and the corneal compensated IOP. Moreover, it assesses corneal resistance factor and corneal hysteresis, two biomechanical properties [19].

5. Summary

In conclusion, the chapter has reviewed the current techniques of measuring intraocular pressure that have been applied on the African continent. There is a paucity of population-based studies, especially with a nationally representative sample measuring the distribution of IOP in Africa. The majority of studies conducted are comparative studies. The commonly used techniques include GAT and handheld devices such as Perkins tonometry and rebound tonometry. Despite the scarcity of these instruments mainly associated with high purchasing prices, they offer reliable methods of monitoring ocular hypertension and glaucoma. African eye care practitioners should familiarize themselves with the methods and their drawbacks while exploring novel techniques to aid in the fight against ocular hypertension and glaucoma.