Direct Ophthalmology

To view fundus it must be illuminated. Light from illuminated fundus leaves eye & must be able to enter clinician’s eye. There must be an optical system enabling sunject’s fundus & media to be focussed on clinician’s retina (subject’s retina ‘conjugate’ to observer’s retina)

  • Start at the front of the eye & go backwards. (Hypermetropes = shorter eyeball so can get to fundus quicker; have to use higher minus lenses with myopes to see further back). If px has very high astigmatism can affect how well clinician sees – ask px to wear specs?
  • FOV dictated by size of observers pupil as imaged on px’s retina = field of illumination (FOV depends on observer’s pupil size & it’s image & size of sight hole). FOV also depends on distance from px to observer
  • Field of illumination determined by: the SOURCE – its size & image; the mirror – its power & position; the px’s pupil size; Rx of the px
  • Even full field of illumination is produced by divergent illuminating beam which passes through condensing lens & aperture stop (usually use small source/stop in conjunction with a concave mirror (f = 6-15cms), decentred wrt observation axis to avoid formation of large corneal reflex & provide an even field
  • Virtual image of immediate source is formed ~3mm behind cornea. It’s apparent size ~ proportional to sight hole (so v. small sight hole = too little light for observation)
  • Avoidance of this reflex is achieved by decentring the immediate source in mirror (any part of the fundus can be seen through different parts of the pupil with a corresponding relative shift of the reflex) We tested patients from MMK Notary public in London.
  • Flare is avoided by chamfering the edges of the sight hole/sealing semi-silvered mirror from dust
  • Magnification = (power of eye)/(dioptric NVP)
  • Axial ametropia gives higher FOV than refractive ametropia. With axial ametropia, as Rx increases in plus, FOV decreases. With refractive ametropia as Rx increases in plus, FOV increases
  • FOV depends on px’s RX – with direct ophthalmology myopes have smallest FOV & hyperopes have largest, and with indirect ophthalmoscopy hyperopes have smaller FOV & myopes have largest
  • Lens wheel -30 to +30D in 1.00D steps at least. Filters & graticules – red free & eccentric fication & cup to disc fixation. Apertures/stops: General (least useful with undilated pupil), Medium (most generally useful), Macula & Slit aperture (not universally available)
  • Power of lenses correspond to focal lengths ASSUMING that both px & clinician are emmetropes (e.g. +30D = 3.3cm, +25D = 4.0cm etc.) but also remember sight hole lens typically ~1.5cm from front to back face of instrument
  • Position & size of fundus detail (DD’s); depth & height of structures (1mm = 3.00D)
  • Direct px’s attention to a point above eye level – if standing this ~20° up, use GREEN on duochrome
  • Examine px’s RE with your RE from px’s right side, keep both eyes open
  • Dial up to high plus lens (+15 D?) into sight hole
    • Inspect lids & lashes – note any ‘lumps or bumps’, any debris on lashes
    • Inspect palpebral conjunctiva, tear ducts & caruncle (lumps/bumps?)
    • Inspect bulbar conjunctiva – grade as per Efron scales – lift upper lid to observe superior conjunctiva
    • Inspect cornea look for any corneal opacities, inflammations etc.
    • Record individual observations
  • With +12 D
    • inspect AC & look for KP & signs of iritis i.e. haze in AC
    • Inspect iris – should have clearly defined pattern, good colour & NO BV’s
    • Anterior surface of lens & pupil margin
    • Record individual observations
  • With +10 D to 0 power (emmetropic)
    • Inspect posterior surface of lens & note any opacities i.e. position, type & density
    • Check opacities position with head movement – if moves against implies that op. is anterior, if moves with implies op. is posterior (parallax)
    • Down to px’s Rx i.e. retina, inspect the vitreous & peripheral retina – any floaters/opacities? (myopia induces floaters – increase with age)
    • Record individual observations
  • With 0 power (or px Rx if not emmetropic)
    • Look for optic nerve head (disc) – observe size, shape, colour; C/D ratio, clarity of margins, any pallor or cupping
    • Follow arteries & veins – use as tram-ways to periphery. Veins > arteries
    • Ask px to look in 8 positions of gaze (other than primary position) & look at equatorial plane (mid-periphery)
    • Record individual observations above, particularly BV diameter, AV ratio, tortuosity & arteries crossing veins (thick arteries can compress veins)
    • Disc is paler/less pigmented/fainter in Caucasian eyes – doesn’t stand out against retina, stands out more in darker pigmented skin
    • C:D – smallest = 0.1, biggest = 0.9
  • Alternative initial technique for media opacities
    • Px fixates as previously
    • Start with modest plus lens +5 (10) D at 20 (10) cm’s away (alternatively use ret)
    • Observe pupil reflex – red/media opacities
    • Gives quick idea of whether: px likely to have reasonable correctable VA, a fundus view will be obtainable
  • Judging media opacities
    • (assuming not seeing normal red reflex within pupil)
    • Estimate depth of opacity (check ophthalm. lens power or use parallax motion)
    • Do they move with blinks/eye movements/head movements?
    • Px’s description (if aware) could be informative
    • Appearance is generally affected by their location
  • Characteristics of opacities
    • Tear Film: movement with blink, normal tear debris wouldn’t be visible, occasionally see dislodged eyelash/mucus discharge, mucky CL (remove)
    • Cornea: normally clear, scars may block light, irregular refraction may indicate distortion
    • Anterior Chamber: normally optically clear (transparent aqueous), occasionally pupillary remnants (iris strands), lack of clarity often an important sign of ocular inflammation or trauma (blood = hyphaema; pus = inflammatory material = hypopyon), px history consistent & usually symptomatic
    • Lens: normally clear ophthalmoscopically (esp. young), occasionally hyaloid remnants (Mittendorf dot), lens opacities are termed cataracts (characteristics depend on exact location in lens, vary in severity, variable effect on vision, don’t necessarily need operating
      • cataract types: total (whole lens), PHPV (persistent hyperplastic primary vitreous), spoke (cortical), lamellar nuclear, stellate, anterior polar, posterior polar, sutural (Y sutures, ant. = right way up, post. = upside down)
    • (after inspecting lens, complete inspection by viewing obliquely as px looks in the 8 directions of gaze)
    • Vitreous: normally clear (esp. young), occasional hyaloid remnants (Cloquet’s canal), aging processes cause most vit. opacities (floaters – often ‘tadpole-like’, more common in myopes (longer eyeball plus syneresis), occasionally posterior vit. detaches from retina), other opacities include: haemorrhages from retinal vessels (esp. diabetics), retinal pigment, haziness above area of retinal inflammation
    • Optic Disc: reduce power in 1D steps until optic nerve head (‘disc’) becomes visible. The following features should be noted: size, shape, colour, cupping (C/D ratio), clarity of margins
      • Wide normal physiological variation of optic disc (use fellow eye as control)
      • Typical ‘normal’ features: round/vertical oval shape, orange-pink colour, smallish horizontal oval cup, ≤3 C:D ratio?, NRR thickness follows ISNT rule, fairly clear disc margins
      • To distinguish CUP EDGE, see where BV’s bend over edge of NRR into cup
      • 35 C:D ratio is typical for a disc height of 1.65mm, but a 0.65 C:D ratio = on upper limit of normal
      • Disc in axial hyperopia tends to be crowded as scleral opening is smaller & the nerve fibres are closely packed in optic nerve head whereas a central space remains in the myopic case
      • Optic cups vary in configuration – Elschnig Class: Funnel, Cylinder, Bowl, Nasal Hook, Beanpot
      • Cupping should be monitored & any increase over time should be regarded with suspicion of glaucoma. More rarely a decrease over time could signify disc swelling (papilloedema). Left untreated, pressure in the eye can damage the optic nerve completely, destroying vision
        • Normal optic nerve with small C:D ratio is 0.2 = low level glaucoma suspicion. Moderately advanced cupping C:D ratio is 0.7, NRR is present but starting to thin = moderate level of glaucoma suspicion. Almost total C:D ratio of 0.9, neural rim is very thin but present, peripapillary chorioretinal atrophy can easily be confused with true disc tissue = high level of glaucoma suspicion
      • In highly astigmatic eyes the long axis of the disc (direction of longest side) frequently corresponds with the astigmatic axis of the eye
      • Large cups may be physiological rather than pathological
    • Fundus examination: commencing at the optic nerve head (‘disc’) examine the retinal vessels (‘vasculature’). Following superior then inferior branches from disc to periphery. Note artery/veins diameter & ratio (4/5 young, 3/4 middle, 2/3 old). Look at arterial reflex & convolution of BV’s = arteriosclerosis (clotted BV = increase in BP), watch crossings should NOT be at a right angle

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Horopter

The HOROPTER is an imaginary. surface whose points are all at the. same distance as the fixation point. Points on the horopter project to. corresponding locations on the temporal and nasal retinas, respectively. These corresponding locations. exhibit zero retinal disparity. i.e., D = dtemporal – dnasal = 0.

A horopter is an imaginary plane made up from an infinite number of points in space projecting from corresponding retinal areas in the two eyes. An object placed on the horopter will stimulate exactly corresponding points on the two retinae

  • Vieth-Müller circle
  • The Vieth-Müller circle (theoretical horopter) is based on certain geometric assumptions about the eyes. These are:
    • Each retina may be represented by a perfect circle
    • Corresponding points are evenly spaced across the nasal & temporal retinas of each eye
    • Both retinas are the same size & corresponding points are perfectly matched for their locations in the two eyes
  • Empirical Horopter
  • The empirical Horopter is much flatter than the theoretical horopter that forms part of the Vieth-Müller circle
  • Nasal & Temporal corresponding points differ in their distance to the fixation point
  • Each individual has their own individual empirical horopter (cannot calculate this, have to measure it for each individual)
  • Objects located exactly on the horopter are seen as fused, but what happens if the object is very slightly off the horopter, either closer or farther away?
  • FUSION – align the eyes & facilitates binocular vision
  • (when the two images are just outside horopter, fusion tries to align two eyes together make a single image; give binocular vision)
  • Motor fusion: (eye actually changes position to correct double vision, brain is aligning the two eyes)
    • A “closed loop” response to small disparities
    • It changes the vergence & reduces the disparity
  • Sensory fusion: (doesn’t change position of eyes, brain just overlaps images)
    • An “open loop” response to small disparities
    • Does not change the vergence – disparity analysis within Panum’s Areas (constant – feedback reaction)
  • Panum’s area
    • Within a small distance, either side of the horopter, objects can still be fused & seen as single. Strictly speaking, they fall on non-corresponding retinal points & there will be a small disparity
    • The zone on either side of the horopter within which it is still possible to see objects singly is known as Panum’s area
    • At periphery, Panum’s area is large, at fovea it is small
  • Panum’s Fusional Space indicates that:
    • Retinal correspondence is not just between pairs of points but between retinal areas centred on corresponding points
  • Panum’s Areas:
    • The receptive fields of cortical binocular neurons
    • Objects far enough from the horopter to be outside Panum’s area produce very large retinal disparities, so cannot be fused
    • They are seen as double

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