The optic II nerve

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Examination of the optic nerve involves testing:

• pupillary reflexes

• visual fields

Much of the examination of the optic nerve is described in Chapter 7. This section deals with the pupillary reflexes and with the visual lields.

Anatomy

The optic nerve transmits the axons of the retinal ganglion cells. It begins at the back of the globe and passes through the optic canal of the sphenoid bone into the cranium, where it joins with the contralateral nerve to form the optic chiasma. The optic tract then passes posteriorly to the lateral geniculate bodies of the thalamus and then to the primary visual cortex (area 17) in the occipital lobe. This tract is termed the geniculocalcarine tract or the optic radiation. The visual association cortex and areas IS and 19 are in close proximity in the occipital lobe.

The afferent limb of the pupillary reflex involves the retina, optic nerve, chiasma and tract. These fibres terminate in both sides of the midbrain pretectum. The axons then pass to the accessory oculomotor nuclei of Edingcr-Westphal. The efferent limbs of the reflex arc arc preganglionic parasympathetic fibres in the III nerve, the ciliary ganglion and postganglionic fibres that innervate the constrictor muscle of the iris.

The normal visual field extends 160° horizontally and 130" vertically, with the physiological blind spot 15" from fixation in the temporal field. The field for red colour vision is much smaller than that for monochrome.

Pupillary reflexes

If a light is directed at one eye. both pupils will normally constrict. The reaction of the pupil on the side stimulated is called the direct light reflex, and the constriction of the other pupil the consensual light reflex.

When a near object (e.g. at 10 cm) is viewed, convergence of the eyes is accompanied by bilateral pupillary constriction referred to as the accommodation reflex.

Examination sequence

P Examine the pupils in a dimly lit room for size and symmetry.

□ Check the direct and consensual reflex in each eye in turn by shining a bright pen torch from the side and from below with the patient looking into the distance in order to avoid an accommodation response.

□ To test the reaction to accommodation ask the patient first to look into the distance and then at an object held close to the face; observe any change in the ptipil size.

Common abnormalities

Abnormal pupillary reflexes. Impairment or absence of the pupillary reaction to light may be due to damage to either the afferent or efferent sides of the reflex arc. Since both pupils constrict in response to light directed into one eye, afferent lesions can easily be distinguished from damage to the efferent pathway.

With afferent pupillary defect, the dysfunction is oil the afferent limb of the reflex arc (retina or optic nerve). The pupil does not have a direct light reflex but constricts when light is shone into the opposite eye (i.e. the consensual reflex is preserved).

With efferent pupillary defect, the lesion is on the efferent limb of the unreactive eye (oculomotor nerve, ciliary ganglion). One pupil is fixed and dilated and does not respond to light directly, but the contralateral pupil responds consensually.

TABLE 6.14 Drugs affecting the pupil

Dilated (mydriasis)

Mode of action

Atropirie/homatropine

Anticholinergic

Amphetamine and derivatives

Sympathomimetic

Constricted (miosis)

Neostigmine, morphine and derivatives

Parasympathomimetic

Horner Syndrom Mit Miosis

Fig. 6.9 Right Horner's syndrome in a patient with bronchogenic carcinoma. Note the ptosis and enophthalmos. (Courtesy of Dr D.H.A. Boyd).

Other abnormalities of pupillary reflexes include Ihe Parinaud syndrome (p, 202), oculomotor paresis (see p. 202) and following structural damage to the iris. In the Holmes-Adie syndrome absencc of ankle jerks and olher-^' deep tendon reflexes is seen in association with myotonic pupils. (See Table 7.2, p. 249.)

Abnormalities of pupillary size Some abnormalities in pupil size arc shown in Table 7.2. Symmetrically small or dilated pupils are commonly drug induced (see Table 6.14).

Pontine haemorrhage causes bilateral pupil constriction, and in anxiety the pupils are symmetrically dilated.

In Horner's syndrome (Fig. 6.9) unilateral miosis is associated with ptosis, impaired sweating and enophthalmos (recession of the gtobe in the orbital fossa). Paresis of the light reflex and accommodation as a result of a lesion in the ciliary ganglion is termed an internal ophthalmoplegia.

Visual fields

Testing fields assesses the function of the peripheral and central retina, the optic pathways and the cortex. Since

Contralateral Hemianopsia Meningioma

Fig. 6.10 Testing the visual fields.

Causes Visual Field ConstrictionRed Hat Pin For Visual Field Exam

Fig. 6.11 Testing for a central field defect using a red hat-pin,

TABLE 6.15 Common features in optic nerve lesions

Abnormalities

Abnormal pupillary response Enlargement of blind spot Scotomas

Impaired visual acuity Impaired colour vision Abnormal fundoscopy

Common causes to consider Multiple sclerosis, optic neuritis Vascular disease

Retrobulbar and parasellar neoplasms (e.g. pituitary adenomas, craniopharyngioma and meningioma)

Fig. 6.10 Testing the visual fields.

Colour vision tends to fail early in many disorders of tlie retina and optic nerve, a red hat-pin is a particularly useful test object for detecting scotomas. Visual fields can be estimated quickly by the method of confrontation or measured accurately using perimeters, Roth methods require a cooperative patienl.

If a patient is unable to cooperate (because of an impaired conscious state or dysphasia) a crude test of visual field integrity can be made by moving the hand quickly towards the patient's face, This menacing stimulus will usually evoke reflex blinking if the incoming stimulus is detected.

Exam/nat/on sequence

□ Sit directly opposite and facing Ihe patient.

□ Examine each eye separately first.

□ Ask the patient to cover one eye and look at the examiner's opposing eye (Fig. 6.10).

□ Examine the outer aspects of Ihe visual fields with a waggling finger or preferably a fine probe with a large red or white head (e.g. a hat-pin).

□ Bring the test object into the field of vision in a curve, not a straight line. Approach from the periphery at several points on the circumference of the upper and lower, nasal and temporal quadrants of the visual fields.

□ Ask the patient to respond as soon as the movement of the test object is observed.

□ Map out central field defects by moving the test object across the visual field (Fig. 6.11).

□ When using a red test object instruct the patient to report as soon as the colour is perceived.

Fig. 6.11 Testing for a central field defect using a red hat-pin,

Fig, 6.12 Testing for visual inattention.

□ Test for visual inattention by asking the patient to report when the fingertip is moved on one or both sides simultaneously (Fig. 6.12).

Common abnormalities

Impaired visual acuity can be due to lesions of the cornea, lens, vitreous, retina or optic nerve and the more distal visual pathway. Characteristics of optic nerve lesions are shown in Table 6.15.

The common patterns of visual defect are illustrated in higure 6.13. Lesions proximal to the optic chiasma cause monocular dysfunction. Lesions of the optic chiasma and distill visual pathways will produce binocular visual field defects. Lesions of the optic tract and lateral geniculate body (LGB) region are frequently vascular in origin.

Visual field defects

Visual ^ fields

o c9

Visual Field Defect Lgb Lesion

Fig. 6.13 Visual field defects. 1. Total loss ol vision in one eye because of a lesion of the optic nerve. 2. Bitemporal hemianopia due to compression of the optic nerve. 3. Right homonymous hemianopia from a lesion of the optic tract. 4. Upper right quadrant hemianopia from a lesion of the lower fibres ol the optic radiation in the temporal lobe. 5. Less commonly a lower quadrantic hemianopia occurs from a lesion of the upper fibres of the optic radiation in the anterior part of the parietal lobe. 6. Right homonymous hemianopia with sparing of the macula due to lesion of the optic radiation in the posterior part of the parietal lobe.

Fig. 6.13 Visual field defects. 1. Total loss ol vision in one eye because of a lesion of the optic nerve. 2. Bitemporal hemianopia due to compression of the optic nerve. 3. Right homonymous hemianopia from a lesion of the optic tract. 4. Upper right quadrant hemianopia from a lesion of the lower fibres ol the optic radiation in the temporal lobe. 5. Less commonly a lower quadrantic hemianopia occurs from a lesion of the upper fibres of the optic radiation in the anterior part of the parietal lobe. 6. Right homonymous hemianopia with sparing of the macula due to lesion of the optic radiation in the posterior part of the parietal lobe.

Common lesions in the optic radiation and occipital cortex are vascular {e.g. posterior cerebral artery infarction), inflammatory (e.g. multiple sclerosis) or neoplastic (e.g. glioma, metastatic neoplasia).

Visual field delects are termed homonymous if the same part of the visual field is affected in each eye. I lomonymous defects are due to lesions distal to the optic ehiasma (see Fig. 6.13). The field defect may be hemianopic (i.e. half of the visual field is lost) or quadrantinopic (i.e. one-quarter of the visual field is lost). Such a defect will be either upper or lower depending upon which part of the visual field is affected. If the visual field loss is not identical in both eyes it is termed incongruous; this type of defect is seen in lesions of the oplic tract. Homonymous hemianopia due to occipital cortical lesions tends not to involve the central part of vision (macular sparing). When inattention is present the patient will be able to detect single targets on both sides, but will ignore objects on one side when two fields are stimulated simultaneously.

The oculomotor, trochlear and abducent (III, IV and VI) nerves

These three cranial nerves innervate the muscles controlling eye movement and pupillary si/e. Although the nerves subserve discrete actions they are examined together because of their close functional interrelationships.

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Responses

  • demi-lee
    How to check colour vision with a red hat pin?
    8 years ago
  • marcello
    What is a nerve examination using objects?
    8 years ago
  • Cettina
    Is optic radiation afferent?
    6 years ago

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