Session 2: Orbit, Muscles And Eye

Question 1

What are the bony orbits formed from?

Answer 1

The two orbits are shaped like cones, with a broad opening at the front, tapering to a narrow part at the back. They are formed of several bones: the larger frontal, sphenoid, zygomatic and maxillary bones, and the smaller ethmoid and lacrimal bones.

Question 2

What structures are found within the bony orbits?

Answer 2

There are numerous structures found within the orbits:
• Eye (also known as the eyeball or globe) – spherical in shape and located at the front of the orbit.
• Extraocular muscles – six small but distinct muscles that move the eye to direct our vision where we want it, and a seventh muscle responsible for retracting the upper eyelid.
• Nerves – including the optic nerve for vision, the oculomotor, trochlear and abducens nerves that control the extraocular muscles, and the ophthalmic branch of the trigeminal nerve which supplies sensation to eye and forehead.
• Fat – fills the spaces left between the eye, muscles, and nerves.
• Lacrimal gland – found in the superior and lateral part of the orbit, this gland produces tears to lubricate the anterior surface of the eye.

Question 3

What is at the back of the orbit?

Answer 3

At the back of the orbit are the three foramina that transmit nerves and vessels in and out of the orbit: optic canal, superior orbital fissure and inferior orbital fissure.

Question 4

What are the extraocular muscles?

Answer 4

There are seven extraocular muscles. All have distinct functions and two have a unique nerve supply. Some muscles have additional actions on the eye due to the angle at which they attach to the eye. The dysfunction of one of the muscles or nerves supplying them will produce characteristic findings when examining the patient.

A useful mnemonic to remember the nerves that supply the extraocular muscles is LR6SO4 – lateral rectus is supplied by cranial nerve 6, superior oblique is supplied by cranial nerve 4. The rest of muscles are supplied by cranial nerve 3.

Question 5

What is the movement of the eye?

Answer 5

There are some new terms used here to describe the movement of the eye:
• Elevation – to look up.
• Depression – to look down.
• Adduction – to look medially.
• Abduction – to look laterally.
• Extorsion – to rotate the eye, so the top of the eye rotates laterally.
• Intorsion – to rotate the eye, so the top of the eye rotates medially.

Question 6

What are conjugate eye movements?

Answer 6

Both eyes do not necessarily perform the same movement when you change where you look. For example, to look to your left with both eyes, your left eye needs to abduct, but your right eye will need to adduct. This is known as conjugate eye movements.

Question 7

What are the muscles of the eyes, their nerve supply, action and findings if nonfunctional?

Answer 7

1. Levator palpebrae superioris - Oculomotor nerve (CN III) - Elevate the superior eyelid - Ptosis (drooping eyelid)
2. Superior rectus - Oculomotor nerve (CN III) - Elevate, Intort, Adduct - Unable to elevate
3. Inferior rectus - Oculomotor nerve (CN III) - Depress, Extort, Adduct - Unable to depress
4. Medial rectus - Oculomotor nerve (CN III) - Adduct - Unable to adduct
5. Lateral rectus - Abducens nerve (CN VI) - Abduct - Unable to abduct
6. Superior oblique - Trochlear nerve (CN IV) - intort, Depress, Abduct - Unable to depress if eye is adducted
7. Inferior oblique - Oculomotor nerve (CN III) - Extort, Elevate, Abduct -Unable to elevate if eye is adducted

Question 8

Where do the four recta extraocualr muscles originate?

Answer 8

The four recti extraocular muscles originate at the back of the orbit on a fibrous ring known as the common tendinous ring (or annular tendon, or annulus of Zinn).

Question 9

What are the functions for the superior and inferior rectus?

Answer 9

The superior and inferior rectus muscles have multiple functions. Their main function is to elevate or depress the eye respectively. The additional functions of intorsion, extorsion and adduction are secondary effects caused by the angle at which these two muscles pull on the eye. When looking at the orbits from above, if you draw a line through the centre of both orbits (which represents the angle at which these muscles pull on the eye) you’ll notice these two lines are not parallel and they do not point directly forwards. They diverge off to each side. However, the eye is normally angled to point directly forwards. This means that when the superior or inferior recti act on the eye, they are not only pulling it upwards or downwards, but they are also causing it to intort or extort and adduct.

Question 10

Where do the superio oblique originate?

Answer 10

The superior oblique muscle originates at the back of the eye but passes through a fibrous sling called the trochlea, which is located in the superior and medial corner of the orbit. The muscle then inserts onto the top of the eye, so it’s action will pull the top of the eye medially, causing it to rotate. The top of the eye rotating inwards is called intorsion.

Question 11

Where does the inferior oblique resonate?

Answer 11

The inferior oblique muscle originates from the medial orbital wall and inserts on the bottom of the eye. Its action will pull the bottom of the eye medially, also causing it to rotate, but in the opposite direction. With the top of the eye rotating outwards, this is called extorsion.

Question 12

Why does the eye need the oblique muscles to be able to intort and extort?

Answer 12

The reason that the eye needs the oblique muscles to be able to intort and extort is to counter the secondary effects of extorsion and intorsion caused by the inferior and superior recti muscles respectively. This helps maintain steady vision when looking up or down. The oblique muscles also help with maintaining steady vision when we tilt our heads.

Question 13

What is the eye responsible for?

Answer 13

The eye is responsible for providing us with sight. As light enters the eye, it is focused to converge onto the retina where it is detected by specialised cells (rods and cones). These cells generate nerve impulses which are transmitted along the optic nerve and optic tracts towards the primary visual cortex in the occipital lobe. The anatomy of the internal eye will be covered in your lectures. Two important functions of the eye will be covered here.

Question 14

What is accommodation? How is it achieved?

Answer 14

To achieve good vision, the eye must be able to focus light to varying amounts depending on how far away the object is that is being visualised. This is called accommodation. To achieve this, the eye adjusts the thickness of the lens within it. A thicker lens will cause greater refraction of light and is useful for looking at near objects. A thinner lens will cause less refraction of light and is useful for looking at distant objects.

Question 15

What do the ciliary muscles do?

Answer 15

The ciliary muscles are responsible for adjusting the thickness of the lens. They are supplied by parasympathetic fibres that travel within the oculomotor nerve (CN III). Because of this, accommodation is an autonomic function that we are unable to control.

Question 16

What do constrictor pupillae do with too much light?

Answer 16

To protect the retina from over-exposure to light, the eye must be able to limit how much light enters the eye. To achieve this, a circular muscle within the iris known as constrictor pupillae will constrict the pupil when too much light is detected on the retina. This muscle is also supplied by parasympathetic fibres that travel within the oculomotor nerve (CN III). Because of this, pupillary construction is another autonomic function that we are unable to control.

Question 17

What does the dilator pupillae do when it is dark?

Answer 17

Conversely, when it is dark, the eye must be able to allow more light in to allow us to see adequately. To achieve this, a radial muscle within the iris known as dilator pupillae will dilate the pupil when not enough light is detected on the retina. This muscle is supplied by sympathetic fibres that originate from the sympathetic chain and enter the skull alongside the internal carotid artery. Because of this, pupillary dilation is another autonomic function that we are unable to control.

Question 18

How do the pupils work?

Answer 18

When there is stimulation of the body’s parasympathetic or sympathetic nervous systems, the pupils may change size. For example, in states of fear, excitement or pleasure, the sympathetic nervous system is highly active, and the pupils will dilate. In states of resting, the parasympathetic nervous system is active, and pupils will constrict. The pupillary response to light will override these effects though, so, for example, even when in a state of sympathetic activation, bright light will still cause the pupils to constrict.

Question 19

What is the pupillary light reflex?

Answer 19

The pupillary light reflex, which is responsible for automatically adjusting the amount of light entering the eye, has an afferent (sensory) and efferent (motor) nerve. The afferent nerve is the optic nerve, and it carries information about the amount of light being detected on the retina to the midbrain. Here, there is a synapse with the Edinger-Westphal nucleus, which instructs the efferent nerve (the oculomotor nerve) to initiate constriction of the constrictor pupillae muscle in the iris, thereby narrowing the pupil and reducing the amount of light entering the eye.

Question 20

What is the pupillary light reflex?

Answer 20

The pupillary light reflex, which is responsible for automatically adjusting the amount of light entering the eye, has an afferent (sensory) and efferent (motor) nerve. The afferent nerve is the optic nerve, and it carries information about the amount of light being detected on the retina to the midbrain. Here, there is a synapse with the Edinger-Westphal nucleus, which instructs the efferent nerve (the oculomotor nerve) to initiate constriction of the constrictor pupillae muscle in the iris, thereby narrowing the pupil and reducing the amount of light entering the eye.

Question 21

How does a healthy patient react with light in eye?

Answer 21

When a light is shone in one eye of a healthy patient, both pupils will constrict. The constriction of the pupil which is having a light shone into it is called the direct pupillary response, and the constriction of the other pupil is called the consensual pupillary response. The reason that the other pupil constricts as well is because there is a connection between the right and left Edinger-Westphal nuclei such that if one side is activated, then both sides are activated.

Question 22

What does the lacrimal gland do?

Answer 22

The lacrimal gland produces tears to lubricate and moisten the surface of the eye. It is located in the superior and lateral corner of the orbit. The tears flow across the eye and drain into the lacrimal ducts in the inferior and medial corner of the orbit. Through here, they then drain into the nasal cavity via the nasolacrimal duct. This is why our nose runs when we cry. The secretomotor nerve supply to the lacrimal gland is via the parasympathetic fibres within the facial nerve (CN VII).

Question 23

What are orbit wall fractures?

Answer 23

Direct traumatic blows to the eye (as may be sustained during a fist fight) may fracture the walls of the orbit. These are known as ‘blow out’ fractures and a fracture of the inferior orbital wall is most common. The inferior rectus muscle can become trapped inside the fracture, tethering the eye in position and patients will be unable to look up.

Question 24

What are cataracts?

Answer 24

A cataract is a common ocular condition characterised by clouding of the lens that is responsible for focusing light onto the retina. Cataracts develop slowly and painlessly, and whilst they cannot be prevented, they are relatively easily treated surgically, by removing the affected lens and replacing it with a new and clear intraocular lens.

Question 25

What is oculomotor nerve palsy?

Answer 25

If the oculomotor nerve is not functioning on one side, it produces a very characteristic set of findings on clinical examination. The affected eye will rest in a ‘down-and-out’ position (depressed and abducted). This is because the lateral rectus and superior oblique muscles are unopposed so pull the eye into that position at rest. The affected eye’s pupil will also be dilated due to loss of parasympathetic nerve supply to the constrictor pupillae, leaving dilator pupillae unopposed. Finally, due to the loss of motor nerve supply to levator palpebrae superioris, the eyelid will droop. This is known as ptosis. Additionally, on asking the patient to look left and right, the affected side will be unable to adduct.

Question 26

What is abducens nerve palsy?

Answer 26

If the abducens nerve is not functioning on one side, the affected eye will be unable to abduct as the lateral rectus muscle is no longer working. The lateral rectus muscle may be overpowered by the medial rectus, which is still functional, to pull the eye medially at rest causing strabismus (sometime referred to as a ‘squint’).

Question 27

How do you test eye movements?

Answer 27

Testing the different movements of the eye is a vital part of a neurological examination of the cranial nerves. The do this, a clinician may ask the patient to follow their finger as they draw the shape of the letter ‘H’ in front of the patient. The clinician observes the patient’s eyes to ensure they are moving together and asks the patient if they are experiencing any diplopia (double vision).

The four recti muscles are relatively straightforward to test as they are functioning if the eyes can elevate, depress, abduct and adduct. To isolate the superior oblique muscle and ensure the patient is moving their eye in such a way that only the superior oblique can perform that movement, a clinician observes if the patient can depress an adducted eye. Whilst adducted, the inferior rectus, which is normally responsible for depression, is kinked in such a way that it is rendered weak and unable to depress the eye. Because of the orientation of the fibres of the superior oblique and where it attaches to the top of the eye, it becomes a powerful depressor of the eye in the adducted position. This is how clinicians can test the function of the trochlear nerve.

Question 28

What is the pupillary response to drugs?

Answer 28

Certain therapeutic and recreational drugs will cause changes to the size of the pupil. Opiates, for example, such as morphine and heroin (diamorphine), will cause pupillary constriction in higher doses. Sympathomimetics, such as ecstasy (MDMA) and cocaine will cause pupillary dilation. The size of the pupil can help clinicians during assessment of unconscious patients.

Question 29

What is the pupillary response to head injuries?

Answer 29

Head injuries and the pupillary response
If a patient suffers a significant head injury and has bleeding inside the skull (for example, an extradural haemorrhage), intracranial pressure will rise. This may lead to compression of the oculomotor nerve on one or both sides. If compressed, the oculomotor nerve may be unable to function properly and an early sign of this is dilation of the ipsilateral pupil. This is another reason why clinicians often check the size of a patient’s pupils during clinical examination of unconscious patients. A fixed and dilated pupil is a concerning sign.