Ординатура / Офтальмология / Английские материалы / Atlas of Glaucoma, Second Edition_Choplin, Lundy_2007

INTRODUCTION

From the most basic standpoint, the eye offers a unique opportunity to study hemodynamics. It is the only location in the body where capillary blood flow may be observed in humans non-invasively. Over 100 years ago, Wagemann and Salzmann observed vascular sclerosis in many of their glaucoma patients. Through the years, numerous other researchers have uncovered pieces of the ocular blood flow puzzle: documenting reductions in the capillary beds, sclerosis of nutritional vessels, vascular lesions and degeneration, and other circulatory pathologies in many eye diseases including glaucoma. A century of observation and circumstantial evidence suggesting a vascular component in the pathogenesis of glaucoma is now supported by direct experimental evidence. This transition from theory to fact took 100 years because the technology required to make such specialized measurements of hemodynamic function have only recently become available. Now that the link has been established, there has been a focus on ocular hemodynamics in glaucoma and the effect of intraocular pressure (IOP)-reducing medications on ocular perfusion.

OCULAR VASCULATURE

The ophthalmic artery is the source for blood flow to the eye but the left and right ophthalmic arteries derive blood from the heart through slightly different routes. The left carotid artery is one of three branches of the aortic arch. The right carotid artery is a branch of the brachiocephalic artery, itself one of the three branches of the aortic arch (Figure 13.1). Both left and right common carotids split to form the internal and external carotid arteries. The only branch of the internal carotid artery outside the

cranium is the ophthalmic artery (Figure 13.2). The ocular vasculature is exceedingly complex. The various layers of tissue in the retina receive nourishment from both the uveal and the retinal vasculature.

The ophthalmic artery (OA) supplies both major ocular vascular beds: the retinal and uveal systems. Its major branches include branches to the extraocular muscles, the central retinal artery and the posterior ciliary arteries (Figure 13.3). The uveal system, which supplies blood to the iris, ciliary body, and choroid, is supplied by one to five posterior ciliary arteries (PCA). They emerge from the ophthalmic artery in the posterior orbit. Short posterior ciliary arteries (SPCAs) penetrate the sclera surrounding insertion of the optic nerve (Figure 13.4). These vessels supply the peripapillary choroid, as well as the majority of the anterior

Figure 13.1 Origin and path of arteria carotis.

Arteria cerebri posterior

Arteria carotis interna

Arteriae insulares

Figure 13.2 Cerebral vasculature.

Arteria lacrimails

Arteria ciliaris posterior longa

VASCULATURE OF THE CHOROID

The outer choroid is composed of large nonfenestrated vessels, while the vessel caliber of the inner choroid is much smaller. The innermost layer of the choroid, the choriocapillaris, is composed of richly anastomotic, fenestrated capillaries beginning at the optic disc margin (Figure 13.5). The capillaries of the choriocapillaris are separate and distinct from the capillary beds of the anterior optic nerve. The SPCAs supply most of the optic nerve head

Arteria supratrochlearis

Arteria dorsails nasi

Arteria ethmoidails anterior

Outer choroid vessels

Figure 13.5 Cross section of choroidal vascular bed.

and the portion of the choriocapillaris posterior to the equator. The choriocapillaris anterior to the equator is supplied by the long posterior ciliary arteries (LPCAs) and the anterior ciliary arteries (ACAs). The LPCAs pierce the sclera and course anteriorly through the suprachoroidal space to branch near the ora serrata. Each LPCA then sends three to five branches posteriorly to supply the choriocapillaris anterior to the equator. The ACAs, branches of the OA, accompany the rectus muscles anteriorly to supply the major circles of the iris (Figure 13.6). Before reaching the iris, 8–12 branches pass posteriorly through the ciliary muscle to supply the anterior choriocapillaris together with the LPCAs (Figure 13.7). Functional anastomoses between the choriocapillaris anterior and posterior to the equator have not been demonstrated. This represents a peripheral choroidal watershed zone.

Venous drainage from the choriocapillaris is mainly through the vortex vein system. Minor

Figure 13.4 Choroidal and retinal vasculature.

Arteria and vena centralis retinae

drainage occurs through the ciliary body via the anterior ciliary veins. The vortex veins drain into the inferior (IOV) and superior (SOV) ophthalmic veins (Figure 13.8). The SOV exits the orbit through the superior orbital fissure and drains into the cavernous sinus. The IOV sends a branch to the SOV and then exits the orbit through the inferior orbital fissure into the pterygoid plexus.

The retinal system is supplied by the central retinal artery (CRA) and sustains the inner retina. The CRA, itself a branch of the ophthalmic artery, penetrates the optic nerve approximately 12 mm behind the globe. The CRA courses adjacent to the central retinal vein through the center of the optic nerve, then emerges from the optic nerve within the globe, where it branches into four major vessels.

The anterior optic nerve may be divided into four anatomic regions: the superficial nerve fiber layer, the prelaminar region, the lamina cribrosa, and the retrolaminar region (Figure 13.9).

The superficial nerve fiber layer, the anteriormost region, is continuous with the nerve fiber layer of the retina and is the only nerve head structure visible by fundus examination (Figure 13.9, yellow shaded). It is supplied by retinal arterioles arising from the branches of retinal arteries. These small vessels originate in the surrounding nerve fiber layer and run toward the center of the optic nerve head. They have been referred to as ‘epipapillary vessels’. The temporal nerve fiber layer may receive additional blood from a cilioretinal artery when it is present. No direct choroidal or choriocapillaris contribution is observed in the superficial nerve fiber layer.

Immediately posterior to the nerve fiber layer is the prelaminar region, which lies adjacent to the peripapillary choroids (Figure 13.9, red shaded). In this region, ganglionic axons group into

Figure 13.6 Anterior segment.

Circulus arteriosus iridis minor

Circulus arteriosus iridis major

Arteria ciliaris anterior (anterior ciliary artery)

Chorioidea

(choroid)

Figure 13.7 Anterior vessels.

bundles for passage through the lamina cribrosa. The prelaminar region is supplied primarily by branches of the SPCAs and by branches of the circle of Zinn–Haller, though some investigators have observed a vascular contribution to the prelaminar region from peripapillary choroidal arterioles. The amount of choroidal contribution may be difficult to ascertain, as there are branches from both the circle of Zinn–Haller and from the SPCAs which course through the choroid and ultimately supply the optic nerve in this region. These vessels do not originate in the choroid, but merely pass through it. The direct arterial supply to the prelaminar region arising from the choroidal vasculature is minimal. This minimal contribution from the choroidal vasculature is limited to small arterioles. There is no vascular contribution from the choriocapillaris.

More posteriorly, the laminar region is continuous with the sclera and is composed of the lamina cribrosa (Figure 13.9, green shaded). The lamina cribrosa is a structure consisting of fenestrated connective tissue which allows the passage of neural axons through the scleral coat. Like the prelaminar

Vena ophthalmica superior

Venae vorticosae

(vortex veins)

Sinus cavernosus

(cavernous sinus)

Vena angularis

Figure 13.8 Venous drainage of the orbit and globe.

region, the lamina cribrosa also receives its blood supply from branches of the SPCAs and branches of the circle of Zinn–Haller. These precapillary branches perforate the outer lamina cribrosa before branching centrally, forming a capillary network throughout the fenestrated connective tissue. Larger vessels of the peripapillary choroid may contribute occasional small arterioles to the lamina cribrosa region.

Finally, the retrolaminar region lies posterior to the lamina cribrosa and, marked by the beginning of axonal myelination, is surrounded by the

meninges of the central nervous system (Figure 13.9, blue shaded). The retrolaminar region has two blood supplies – the CRA and the pial system. The pial system is an anastomosing network of capillaries located immediately within the pia mater. The pial system originates at the circle of Zinn–Haller and may also be fed directly by the SPCAs. Its branches extend into the optic nerve to nourish the axons. The CRA may provide several small intraneural branches in the retrolaminar region. Some of these branches anastomose with the pial system.

Figure 13.12 Waveform produced by the oculo-ophthalmo- dynamamograph (OODG). This measures IOP in real time.

disappears. The OODG uses this phenomenon to directly quantify the perfusion pressure within the uveal and retinal beds.

COLOR DOPPLER IMAGING

Fundamentals

Ultrasound uses sound waves to locate structures in the body. By timing the delay between sound transmission and echo, ultrasound can measure the depth and location of an anatomic structure, e.g. A-scan ultrasound measurements of axial length. The time between transmission of a sound wave into the eye and the returning echo from the back of the eye provides a precise measurement of axial length. This measurement does not require clear optical media and can be performed in the presence of many ophthalmic diseases. By sweeping the A-scan in a line through the eye, a map of structural locations through a slice is obtained. This is commonly known as B-scan ultrasound and has been used to produce gray-scale images of ophthalmic structures. Color Doppler imaging (CDI) is based on B-scan technology, with an additional processing step (Figure 13.13). The frequency of the returning B-scan sound waves is analyzed. When a wave is reflected by a moving source, such as flowing blood, it is Doppler shifted to a different frequency. The amount of the shift is described by the Doppler equation (Figure 13.14), where VBlood is the blood flow velocity, Wavelength is the wavelength of the incident sound wave, and Cos is the cosine of the angle between the blood velocity vector and the incident sound wave vector. Doppler shifted sound is displayed using color-coded pixels within the gray-scale image. Red pixels represent movement toward the CDI probe, and blue represents movement away from the probe. Samples of Doppler shifts (or velocities as calculated using the

Figure 13.13 Color Doppler imaging ultrasound machine.

(Siemens Quantum 2000, Siemens Ultrasound, Isaquaah, WA.)

Figure 13.14 Formula for calculating blood flow velocities from Doppler shift.

equation in Figure 13.14) may be collected from specified vessels within the image. These data are collected in real time during the cardiac cycle. A number of data may be obtained from the resulting velocity waveform. The peak systolic velocity (PSV) is located by the ultrasonographer and is equal to the greatest observable flow velocity obtained by the blood during systole. The enddiastolic blood flow velocity (EDV) can also be located by the ultrasonographer (Figure 13.15). Using both of these measurements, Pourcelot’s resistive index (RI) may be computed (Figure 13.16); RI is an indication of the resistance to flow in the vasculature distal to the point of measurement.

Figure 13.15 Peak systolic velocity and end diastolic velocity. These are marked by the ultrasonographer and resistive index is then calculated from these values.

Figure 13.16 Formula for calculation of Pourcelot’s Resistive Index. This uses peak systolic and end diastolic velocities.

Hemodynamic measurements

CDI is used to measure blood flow velocities in the ophthalmic, CRA and SPCAs. Due to the large difference between the ophthalmic and smaller CRA and SPCAs, the system settings are changed to appropriate ranges of depth and velocity. The waveforms of the various vessels provide additional information. The dicrotic notch is clearly evident in the ophthalmic artery waveform (Figure 13.17), while still evident yet less pronounced in the CRA and missing from the SPCA.

In glaucomatous eyes with high resistance to flow, SPCA waveforms take on the appearance of a haystack with almost no end-diastolic flow. CDI thus allows quantification of flow velocities in the retrobulbar vasculature in research and clinical settings. The appearance of waveforms may also provide insight into the condition of the patient’s ocular vascular health.

Figure 13.17 Waveform produced by the ophthalmic artery.

This is distinct from the other vessels by a dicrotic notch.

ANGIOGRAPHY

Fundamentals

Ophthalmic angiography dates back to 1961 when, at the Indiana University School of Medicine, Novotny and Alvis first described a method for photographing fluorescein as it passed through the human retina. Their early technique was limited to one image every 12 seconds. Today fluorescein angiography can be performed with scanning laser ophthalmoscopes (SLO) which are capable of a wide range of imaging applications in ophthalmology. The Rodenstock SLO is an imaging device that produces fundus images (Figure 13.18). The Heidelberg retina angiography (HRA) unit is a digital imaging platform equipped with argon and infrared lasers (Figure 13.19).

Both the HRA and Rodenstock SLO utilize a confocal optical system (Figure 13.20). For each point of the image, light is focused to a point on the fundus. Light reflected from this point is quantified by a photodetector. Confocal systems create

Figure 13.18 Rodenstock Scanning Laser Ophthalmoscope.