Introduction

Plateau iris is most often an anatomical variant found in Asian populations, particularly in Mongolia, an area with a particularly high prevalence of PACG. But this trait has followed the migration pattern of Asian populations, extending to Greenland, as well as North America. Analyses of mitochondrial DNA and the nonrecombining portion of the Y-chromosome have tracked these migration patterns from the evolution of Homo sapiens in Africa 200 000 years ago to present (Figure 1). These analyses revealed a northbound migration pattern from Asia into Siberia approximately 30 000 years ago. The Inuit populations inhabiting the northern region from Alaska to Greenland are considered descendants of a migrating population that crossed the Bering Land Bridge from north-east Asia approximately 13 000 years ago [4].

Figure 1: Migration Patterns Preceding the Evolution of PACG in Inuit Populations [4].

The four Inuit groups that resulted from this migration from Asia (Greenland Inuit, Siberian, Alaskan, and Canadian), have remained largely genetically isolated from the time of migration. As a result, the shallow anterior chamber, consistent with plateau iris associated with PACG, is considered most shallow in this population. (Figure 2) [4].

Figure 2: Anterior Chamber Depth in Various Populations [4].

One proposed evolutionary explanation for this genetic drift suggests that the shallow anterior chamber observed in this population may be an adaptation to cold climates. Alsbirk proposed that corneal freezing and desiccation keratitis results from prolonged exposure to extreme cold with high wind speeds, as in Arctic climates. He suggested that a shallow anterior chamber decreases the distance between the cornea and the vascular iris, preventing blindness as a result of freezing. He suggested that prevention of this morbidity produced an effective enough advantage to allow proliferation of this trait over time [5]. Unfortunately, this trait carried the disadvantage of PACG, seen in these populations today.

In 1921, Knud Rasmussen left Copenhagen, Denmark, on an expedition to ascertain the origin of the Eskimo race. He, along with a group of ethnographers, travelled to Greenland, and across the Canadian Arctic. The only population he did not visit inhabited the coast of Labrador, a group that one of his colleagues later visited to collect data for the report. Rasmussen’s most significant discovery was a common language known or used by the Eskimo throughout all of Alaska and Canada [6]. Linguistic and medical anthropological evidence reveal a connection between Northern China specifically, and Inuit populations. Therefore, the incidence of PACG among these populations can be dated back thousands of years.

A 2006 review proposes that PACG originates from a different disease process in East Asian populations in comparison with European populations. The study reports the predominant clinical manifestation of PACG as chronic angle closure in Asian populations, while this is not a common finding in European patients. They also report anatomical differences that may account for an etiological basis for PACG in Asian populations. The risk factors cited with respect to eye anatomy were small size, shallow anterior chamber, small corneal diameter, short axial length, steep curvature, thick, anterior lens, shallow limbal chamber, and plateau iris. Finally, and perhaps most importantly, the two mechanisms by which PACG occurs differ between populations. In European populations, pupil block is considered the primary etiology in the majority of patients, whereas this mechanism accounts for only 38% of PACG in Chinese patients. 54% of disease in this population could not be attributed to pupil block alone, but was considered a result of pupil block and flat iris, taking into account the anatomical differences described previously. It is of particular consideration that these anatomical differences were maintained throughout migration patterns that led to Inuit populations today.

This anatomical commonality has led to a significant burden of care in China, where blindness from PACG remains a source of morbidity in the population. Decreased quality of life as a result of PACG has even been reported as increased incidence of mental illness among patients with PACG. One study revealed an increased report of depression and anxiety in this population. This finding was specifically attributed to PCAG in contrast with POAG and controls [7].

In Canadian Inuit populations, a similar burden of disease is apparent. A large population study investigating ophthalmic conditions in Nain, Labrador revealed that 10.8% of 52-85 year olds in the study population were legally blind. This is in stark contrast to the Framingham Eye Study, where only 0.6% prevalence of legal blindness was found in the same age group in a population from Massachusetts. It was determined in 1984 that the prevalence of angle closure glaucoma in the Labrador Inuit population aged 40 years and over was 3.0% [8]. Similar prevalence rates were found in all Inuit populations along the migratory path described previously, in Greenland (4.8%), the Canadian Arctic (2.9%), and Alaska (2.65%) [9].

Decreasing burden of disease in Inuit populations is of high priority in Canadian health care at this time. The 2006 Canadian Census revealed that Canadian Inuit are the youngest and fastest growing population in Canada, with a 26% increase (total 50,485) in the number of Canadian citizens who identify as Inuit from the previous census in 1996. The census also found that the median age of the Inuit population was 22 years, compared with 40 for the non-Aboriginal population [10]. This evidence was corroborated by data specifically from the Nunatsiavut Inuit population in Labrador. In 2011 with the National Household Survey, revealing that at that time, the Nunatsiavut Inuit population was comprised of 2,330 individuals, increasing by 8%. Nearly half (44%) of that population were under the age of 25 at that time [11]. Given that this population is young and quickly growing, it demands early intervention for prevention of PACG, to decrease the high burden of ophthalmic disease in this population in the future.

The literature surrounding prevention and treatment of PACG is growing, particularly with the introduction of technologies allowing for improved diagnosis and treatment. Johnson and Foster reported in 2005 that screening for prevention should be determined specifically for individual at risk populations [9]. Therefore, we propose a screening and prevention program for PACG in the Nunatsiavut Inuit population of Labrador.

Case Presentation

A 67-year-old male of Inuit descent in Labrador was seen for a routine diabetes mellitus screening in ophthalmology clinic. Screening in this clinic includes tonometry, gonioscopy and fundus imaging with swept source optical coherence tomography (DRI OCT-1 Triton). There was no evidence of diabetic retinopathy in this patient. Increased pressure was observed on tonometry and a decreased angle was noted on ‘Radial Anterior Segment’ scan protocol, performed limbus to limbus using a 6mm, 2D scanning modality, producing scan resolution of 1024x12 of the cornea. Figure 3 and Figure 4 used this scanning modality prior to intervention to demonstrate the shallow anterior chambers of this patient, consistent with plateau iris commonly seen in the Inuit population. The patient provided consent for the utilization of this case for research.

Figure 3: Radial Anterior Segment, OD, Pre-Treatment.