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Bruch’s membrane opening analysis

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Michael Yapp
BOptom (Hons) MOptom GradCertOcTher FAAO
Chief Staff Optometrist, Centre for Eye Health Australia

Product
Spectralis OCT

Supplier
Heidelberg Engineering

 

Glaucoma is an optic neuropathy that provides a diagnostic challenge due in part to the variable appearance and nature of the gold standard markers of the disc/retinal nerve fibre layer (RNFL) appearance and visual fields. It is further complicated by the multitude of related ocular and systemic risk factors and other mimicking neuropathies.

There is an increasing number of other available tools and devices in the current glaucoma battery of tests designed to assist with the detection of structural glaucomatous damage. Understanding their relative strengths and weaknesses is critical to the interpretation and incorporation of their results into the clinical picture and management process.

This case study highlights a relatively newly released scan type and associated analysis on the Spectralis OCT, developed in an attempt to reliably and repeatably detect and monitor neural tissue loss in glaucoma.

A 40-year-old Asian male was referred to the Centre for Eye Health for a glaucoma assessment. His glaucoma specific historical risk profile included hypertension and previous myopia, approximately -3.00 DS, corrected with LASIK.

Gonioscopy and a dilated slitlamp examination showed open angles with no evidence of secondary glaucomas.

IOPs were 19 mmHg right and left at 2:00 pm. Pachymetry values were 536 µm right and 553 µm left.

Stereoscopic and red free optic nerve and RNFL assessment (Figures 1 and 2) showed average-sized round discs with no evidence of Drance haemorrhages or significant peripapillary atrophy (PPA).

E06 Figure 1 And Figure 2

Figures 1 and 2. Stereoscopic and red free optic nerve and RNFL assessment reveal average-sized round discs with no evidence of Drance haemorrhages or significant PPA

The inferior neuro-retinal rim in both eyes exhibited a sloped profile with the right inferior rim in particular being relatively thin with an associated loss of the RNFL (wedge defect).

Spectralis OCT RNFL assessment (Figure 3) showed a notable thinning infero-temporally OD, both in comparison to the left eye and the normative database. There was also a relative thinning supero-nasally in the right eye in comparison to the left.

EO6 Figure 3

Figure 3. Spectralis OCT retinal nerve fibre layer assessment

Macular asymmetry (Figure 4) analysis also showed a relative thinning inferiorly in the right eye, both in comparison to the corresponding superior sectors of this eye and the inferior sectors of the left.

E06 Figure 4

Figure 4. Spectralis OCT macular asymmetry analysis

Bruch’s membrane opening (BMO) analysis (Figure 5) suggested a scleral canal location notably different from the funduscopic appearance in particularly nasally and infero-temporally OU. The nasal minimum rim width (MRW) was consequently flagged as outside the normal range in both eyes.

E06 Figure 5

Figure 5. Bruch’s membrane opening analysis

Values are relatively symmetrical and at the lower end of the normative range apart from the infero-temporal rim OD, which was notably thinner than the corresponding left sector and considered outside the normal range.

HVFA 24-2 SITA Standard visual fields showed good reliability indices. The Pattern Standard Deviation (PSD) was significant (p < 0.05) and the Glaucoma Hemifield Test (GHT) outside normal limits OD with a superior nasal step present. The result was essentially clear OS.

Discussion

The gold standards of glaucoma diagnosis, management and monitoring still involve the stereo assessment of the disc and visual field testing. However, OCT devices continue to develop innovative methods of assessing the structural loss of neural tissue in glaucoma.

Currently, the most commonly-used OCT techniques involve assessment of the thickness of the ganglion cells at the macula and the RNFL thickness surrounding the disc, as well as a number of commercially-available methods of assessing the amount of neural tissue present at the optic nerve head itself. In this case, three different analysis techniques all show a deficit infero-temporally in the right eye, which correlates with the funduscopic and visual field results. However the Bruch’s membrane opening minimum rim width (BMO-MRW) analysis also flagged the nasal neural tissue in both eyes.

Recent studies have shown that what is seen funduscopically as the disc margin does not necessarily correlate to the scleral canal as delineated by the end of Bruch’s membrane.1

Bruch’s membrane is an acellular layer measuring between 2 µm and 4 µm thick, lying between the retinal pigment epithelium (RPE) and the choriocapillaris. It comprises an elastic layer sandwiched between collagenous layers and the basement membranes of the choriocapillaris and RPE.

In areas of alpha PPA (uneven RPE pigmentation due to non-uniform RPE cell size and melanin content) and beta PPA (complete atrophy of the RPE and choriocapillaris with visible choroidal vessels and sclera), the elastic layer of the Bruch’s membrane persists and becomes thickened with a continuous layer of ‘druse-like’ debris.2

The border tissue of Elschnig is an area of fibrous tissue running between the sclera and Bruch’s membrane, which separates the choroid from the RNFL as they pass through the anterior section of the neural rim.3 There are regional variations within individuals with respect to the border tissue of Elschnig involving different angles of incidence as well as extensions of Bruch’s membrane.

This implies that the thickness of the neuroretinal rim assessed funduscopically may not always be an accurate representation of the true amount of the corresponding neural tissue.4 As a result, if Bruch’s membrane is accurately identifiable with OCT imaging, the scleral canal can be more accurately identified, leading to a number of options for measuring the thickness of the overlying or adjacent neural tissue.

OCTs typically use the end of the RPE or Bruch’s membrane to determine the disc margin and variable methods to then determine optic disc parameters (NRR thickness, area and volume, cup-disc parameters and so on). Given the prevalence of beta PPA involving the RPE,2 Bruch’s membrane is a more anatomically correct determinate of the scleral canal.

However, the best methodology for measuring the volume and density of the neural tissue as it passes through the scleral canal and how this corresponds to the visible neural tissue funduscopically is still debatable.

The Spectralis OCT featured in this case uses a 24-line radial scan to locate the terminus of Bruch’s membrane, with the NRR then calculated on the basis of the minimum width from these points to the NRR surface. A normative database analysis is subsequently provided to assist with the analysis of the data.

There is a number of studies currently published5 and underway aiming to provide more information on the repeatability, reliability, sensitivity and specificity of this and other similar methods of glaucoma and glaucomatous progression detection. Further research on the implications and use of the apparent disparity between the funduscopically perceived disc margins and the OCT assessed scleral canal delineated by Bruch’s membrane opening with regards to assessing the NRR will also no doubt continue to evolve the available methods of glaucoma detection and monitoring.

In particular in this case, the clinical significance of the difference between the funduscopic nasal disc margin and BMO measured margin is yet to be established.

  1. Reis AS, Sharpe GP, Yang H, Nicolela MT, Burgoyne CF, Chauhan BC. Optic disc margin anatomy in patients with glaucoma and normal controls with spectral domain optical coherence tomography. Ophthalmology 2012; 119: 738-747.
  2. Curcio CA, Saunders PL, Younger PW, Malek G. Peripapillary chorioretinal atrophy: Bruch’s membrane changes and photoreceptor loss. Ophthalmology 2000; 107: 334-343.
  3. Strouthidis NG, Yang H, Downs JC, Burgoyne CF. Comparison of clinical and three-dimensional histomorphometric optic disc margin anatomy. Invest Ophthalmol Vis Sci 2009; 50: 2165-2174.
  4. Strouthidis NG, Yang H, Reynaud JF, Grimm JL, Gardiner SK, Fortune B, Burgoyne CF. Comparison of clinical and spectral domain optical coherence tomography optic disc margin anatomy. Invest Ophthalmol Vis Sci 2009; 50: 4709-4718.
  5. Chauhan BC, O’Leary N, Almobarak FA, Reis AS, Yang H, Sharpe GP, Hutchison DM, Nicolela MT, Burgoyne CF. Enhanced detection of open-angle glaucoma with an anatomically accurate optical coherence tomography-derived neuroretinal rim parameter. Ophthalmology 2013; 120: 535-543.


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