Parapapillary Atrophy: Histological Gamma Zone and Delta Zone

Abstract

BackgroundTo examine histomorphometrically the parapapillary region in human eyes.Methodology/Principal FindingsThe histomorphometric study included 65 human globes (axial length:21–37 mm). On anterior-posterior histological sections, we measured the distance Bruch's membrane end (BME)-optic nerve margin (“Gamma zone”), BME-retinal pigment epithelium (RPE) (“Beta zone”), BME-beginning of non-occluded choriocapillaris, and BME-beginning of photoreceptor layer. “Delta zone” was defined as part of gamma zone in which blood vessels of at least 50 µm diameter were not present over a length of >300 µm. Beta zone (mean length:0.35±0.52 mm) was significantly (P = 0.01) larger in the glaucoma group than in the non-glaucomatous group. It was not significantly (P = 0.28) associated with axial length. Beta zone was significantly (P = 0.004) larger than the region with occluded choriocapillaris. Gamma zone (mean length:0.63±1.25 mm) was associated with axial length (P<0.001;r2 = 0.73) with an increase starting at an axial length of 26.5 mm. It was not significantly (P = 0.24) associated with glaucomatous optic neuropathy. Delta zone (present only in eyes with axial length of ≥27 mm) was associated with axial length (P = 0.001) and scleral flange length (P<0.001) but not with glaucoma (P = 0.73).Conclusions/SignificanceParapapillary gamma zone (peripapillary sclera without overlying choroid, Bruch's membrane and deep retinal layers) was related with axial globe elongation and was independent of glaucoma. Delta zone (no blood vessels >50 µm diameter within gamma zone) was present only in highly axially elongated globes and was not related with glaucoma. Beta zone (Bruch's membrane without RPE) was correlated with glaucoma but not with globe elongation. Since the region with occluded choriocapillaris was smaller than beta zone, complete loss of RPE may have occurred before complete choriocapillaris closure.

Beta and Gamma Peripapillary Atrophy in Myopic Eyes With and Without Glaucoma

“…23 Beta-and gamma-PPA areas were mostly associated with different covariates, and not related to each other, reinforcing the hypothesis that they could be the result of different physiological mechanisms. [11][12][13][14] Gamma-PPA was significantly associated with oval discs and oblique configuration of the border tissue of Elshnig, similar to what was previously reported. 13,14 Kim et al 32 documented development of PPA and progressive tilting of the optic disc in children with myopic shift, demonstrating that PPA might be caused by globe elongation.…”

“…2,3 Although both the prevalence and size of beta-PPA is increased in glaucoma, [4][5][6] these features are also observed in myopia [7][8][9] and with increasing age. 4,8,10 Recent histologic 11 and spectraldomain optical coherence tomography (OCT) [12][13][14][15][16] imaging studies proposed a new classification for beta-PPA, dividing it into two subsets that are indistinguishable by clinical examination: an area with intact Bruch's membrane, still termed beta-PPA, and an area lacking Bruch's membrane, termed gamma-PPA. Recent studies showed that gamma-PPA had a higher association with myopia, [11][12][13]15 and could represent anatomical changes resulting from globe elongation, whereas beta-PPA with intact Bruch's membrane has a higher association with glaucoma, 11,14,15,17 reduced choroidal volume, 17 and older age, 13,16,17 possibly representing degenerative changes.…”

Macular Bruch's Membrane Defects and Axial Length: Association with Gamma Zone and Delta Zone in Peripapillary Region

“…These findings agree with a recent histomorphometric study in which gamma zone and delta zone of the parapapillary region were significantly associated with axial length but not with glaucoma, whereas beta zone of the parapapillary region was significantly associated with glaucoma but not with axial length. 11 The macular Bruch's membrane defect in highly myopic eyes would be second to the primary hole in Bruch's membrane as the inner layer of the optic nerve head, as the optic nerve head can be considered to be a two-layered structure: an inner hole in Bruch's membrane and an outer hole in the sclera (i.e., the optic nerve scleral canal). The primary Bruch's membrane hole present in all eyes allows the passage of the retinal ganglion cell axons out of the eye, and the passage of the large retinal vessels in and out of the eye.…”

“…3,8,11,13,14 The question arises as to which reasons led to the development of an MBMD in highly myopic eyes. One may assume that the myopic enlargement of the globe mainly occurring posterior to the equator is associated with a stretching of Bruch's membrane parallel to the thinning of the sclera.…”

Histological changes of high axial myopia

“…It leads to an overhanging of the Bruch's membrane in the nasal region of the optic nerve head or optic disc (as also shown clinically by optical coherence tomography) and to an absence of the Bruch's membrane at the temporal disc border. 21 The absence of the Bruch s membrane in the parapapillary region has recently been examined and measured in histomorphometric studies. The region without the Bruch's membrane (and correspondingly, without retinal pigment epithelium and choriocapillaris) has been called parapapillary gamma zone.…”