Eye movements

Muscle	Actions	Neural control
Medial rectus	Adducts	Oculomotor nerve (CN III)
Inferior rectus	Mainly depresses, also extorts adducts	Oculomotor nerve (CN III)
Superior rectus	Mainly elevates, also intorts, adducts	Oculomotor nerve (CN III)
Inferior oblique	Mainly extorts, also elevates, abducts	Oculomotor nerve (CN III)
Superior oblique	Mainly intorts, also depresses, abducts	Trochlear nerve (CN IV)
Lateral rectus	Abducts	Abducens nerve (CN VI)
Levator palpebrae	Elevates upper lid	Oculomotor nerve (CN III)
Müller’s muscle	Elevates upper lid	Sympathetic nerve
Orbicularis oculi	Closes lids	Facial nerve (CN VII)

Optic nerve, cranial nerve (CN) II	The axon of the retinal ganglion cell which transmits visual impulse from the eye to the brain
Oculomotor nerve (CN III)	Innervates	Action
Motor (1–5)	1 Medial rectus muscle	Adducts
	2 Inferior rectus muscle	Mainly depresses, also extorts, adducts
	3 Superior rectus muscle	Mainly elevates, also intorts, adducts
	4 Inferior oblique muscle	Mainly extorts, also elevates, abducts
	5 Levator palpebrae muscle	Elevates upper lid
Parasympathetic (6 and 7)	6 Pupil constrictor muscle	Responds to light and near focus
	7 Ciliary muscle	Focuses lens for near
Trochlear nerve (CN IV)	Superior oblique muscle	Mainly intorts, also depresses, abducts
Trigeminal nerve (Fig. 108)	CN V (Fig. 108); CN V branch 1: upper lid, orbit, and nose	Sensory
	CN V branch 2: lower lid
Abducens nerve (CN VI)	Lateral rectus muscle	Abducts
Facial nerve (CN VII, Fig. 110)	Orbicularis muscle	Closes upper and lower lids
Sympathetic nerve (Fig. 126)	1 Müller’s muscle	1 Elevates upper lid
	2 Pupil dilator muscle	2 Opens pupil in response to stress, “fight or flight,” and adrenergic drugs
	3 Skin of lid	3 Sweat glands

Complications of strabismus

Amblyopia

Also called lazy eye, amblyopia is decreased vision due to improper use of an eye in childhood. The two common causes are an eye‐turn (strabismic amblyopia) or a refractive error (refractive amblyopia), uncorrected before age 8. In strabismus, children unconsciously suppress the deviated eye to avoid diplopia.

Strabismic amblyopia is treated by patching the good eye (Fig. 83), thereby forcing the child to use the amblyopic eye. The better eye is patched full time: 1 week for each year of age. It is repeated until there is no improvement or if the vision drops off again on cessation of patching.

Refractive amblyopia is treated by correcting the refractive error with glasses and patching the better eye. Both types must be treated in early childhood because after age 5 it is difficult to improve vision. After age 8, improvement is almost impossible, but should be tried.

Loss of fusion (binocular vision)

Fusion occurs when the images from both eyes are perceived as one object, with resulting stereopsis (three‐dimensional vision). Many patients with tropias never gain the ability to fuse. Finer grades of fusion are assessed by using the Wirt stereopsis test (see Fig. 84).

While wearing polarized glasses, the patient views a test card. The degree of fusion is determined by the number of pictures correctly described in three dimensions.

Near point of convergence (NPC) (Fig. 85)

The NPC is the closest point at which the eyes can cross to view a near object. It is measured by having the patient make a maximal effort to fixate on a small object as it is moved toward his or her eyes. The distance at which the eyes stop converging and one turns out is recorded as the NPC. Convergence insufficiency must be considered if the NPC is greater than 8 cm. These patients may complain of diplopia or other difficulties while reading and is common in patients with Parkinson’s disease. Exercises or prism glasses may help.

Accommodative esotropia (Figs 86 and 87)

When the lens of a normal eye focuses, it simultaneously causes the eyes to converge. Patients with hyperopia who are not wearing glasses must focus the lens of their eye (accommodation) to see clearly near and far. This focusing stimulates the accommodative reflex, causing convergence of the eyes. When the ratio of convergence to accommodation is abnormally high, an esotropia results, which corrects with lenses.

Nonaccommodative esotropia (Figs 89 and 90)

This is due to a defect in the brain not related to the accommodative reflex. It is corrected by surgically weakening the medial rectus muscle by recessing its insertion posteriorly on the sclera or by tightening the lateral rectus muscle by resecting part of it (Figs 88 and 89). Less often, botulinum toxin is injected to weaken eye muscles. Adjustable sutures with slip knots could enable the tension on the muscle to be altered during the early postoperative period.

An epicanthal skin fold connects the nasal upper and lower lids (Fig. 91) and is common in infants and Asians. It gives the false impression of a cross‐eye, called pseudostrabismus.

Measurement of the amount of eye‐turn with prisms

Ocular deviations are measured in prism diopters. When light passes through a prism, it is bent toward the base of the prism. One prism diopter (1 Δ) displaces the image 1 cm at a distance of 1 m from the prism. Do not confuse prism diopters (Δ) with lens diopters (D).

In a right esotropia, the right fovea is turned temporally. To focus the light on the right fovea, a prism (apex‐in) is placed in front of the right eye (Fig. 92). For an exotropia, use apex‐out. Rule: point the prism apex in the direction of the tropia.

Prism cover test for measurement of eye‐turn (Fig. 93)

The patient fixates on an object at 20 ft (6 m). When the fixating eye is occluded, the deviated eye must move to look at the target. Increasing amounts of prism are placed in front of the deviated eye until no movement is noted when the cover is moved back and forth over each eye.

Hirschberg’s test

When the cover test is difficult to perform on infants, the angle of strabismus can be estimated by using Hirschberg’s test (Figs 94–96). As the child fixates on a point source of light, the position of the corneal light reflex is noted. Each 1 mm of deviation from the center of the cornea is equivalent to approximately 14 Δ of deviation. A reflex 2 mm temporal to the center of the cornea indicates an esotropia of approximately 28 Δ.

Demonstration of paralytic strabismus (Table 5)

In paralytic strabismus, the amount of deviation is greatest when gaze is directed in the field of action of the weakened muscle. To demonstrate underaction of any of the 12 external ocular muscles, the patient fixates on an object moved into each of the six cardinal fields of gaze (Fig. 97). Each position tests one muscle of each eye (e.g., position 3 tests the right inferior rectus and the left superior oblique muscles). In addition to observing for underaction or overaction of the muscles, ask the patient where diplopia is greatest. For exact measurements, use the prism cover test.

Most often the cause for cranial nerve (CN) III, IV, and VI paralysis cannot be confirmed, since it is due to ischemia from small‐vessel closure. In adults, ischemia from diabetes is the most common cause and often resolves within 10 weeks. Testing is done to rule out causes such as multiple sclerosis, aneurysms, neoplasms, and other rarer conditions, especially in younger individuals where vessel closure is not likely.

Oculomotor nerve (CN III)

CN III paralysis (Figs 98–100) results in underaction of the inferior oblique and medial, inferior, and superior rectus muscles, resulting in an eye turned down and out. Since this nerve also innervates the levator palpebral muscle, which elevates the lid and the pupillary constrictor muscle, the lid is drooped and the pupil is dilated. CN III paralysis due to diabetes often spares the pupil.

Always examine for a dilated pupil after head trauma. CN III parallels the posterior communicating artery (see Fig. 81) so that ruptured aneurysms in the circle of Willis are a common cause of paralysis with a dilated pupil and an explosive headache (Figs 101 and 102). Also, CN III passes under the tentorial ridge in the brain and is highly susceptible to uncal herniation of the brain. Herniation may follow increased intracranial pressure from cerebral edema, hematoma, tumor, abscess, or cerebral spinal fluid obstruction. Although a dilated pupil is a more common ominous sign after head injury, small or unequal pupils could indicate serious insults to other parts of the brain.