Chapter 4 - Extraocular movement
Questions
Define the following terms:
strabismus,
abduction,
adduction,
elevation,
depression,
convergence,
accommodation,
diplopia,
miosis,
mydriasis,
myopia,
hyperopia,
conjugate,
consensual,
extraocular,
amblyopia,
ptosis,
anisocorea.
Strabismus is a position of the eyes where they are not directed at the
same target (in some parts of the country this is termed a "squint").
Abduction is bringing the pupil away from the nose.
Adduction is bringing the pupil toward the nose.
Elevation is moving the pupil above the horizon.
Depression is moving the pupil below the horizon.
Convergence is directing both eyes toward the nose.
Accommodation is a combination of convergence, pupil constriction and
change in lens shape to permit focus on a near object.
Diplopia is double vision. This can be horizontal, vertical or skew.
Miosis is constriction of the pupil.
Mydriasis is dilation of the pupil.
Myopia is an inability to see at distance ("nearsighted") with light
focusing in front of the retina.
Hyperopia is an inability to see close up ("farsighted") with light
behind the retina.
Conjugate - "together". That is, the eyes moving in parallel to keep
images focused on the same part of each retina (preventing diplopia).
Consensual means "happening on both sides at the same time". A
consensual reflex is one where the response is bilateral when the
stimulus is unilateral (such as the papillary light reflex).
Amblyopia literally means "dim eye". This is a drifting or "lazy" eye
that usually happens because one eye has bad vision. The brain often
"turns off" control of that eye and the eye drifts. The patient usually
does not have diplopia because input from that eye is turned off. In
one of the most remarkable illustrations of plasticity, the eye can
become permanently blind in children if this is not treated.
Ptosis is a drooping of the upper eyelid.
Anisocorea is an inequality of pupil size. This is usually not
clinically significant unless it reaches one millimeter in difference.
4-1. Which muscles
would be active in the right and left eye when looking up and to the right?
Answer 4-1. In the right eye the lateral rectus and the superior rectus would
be the prime movers, while in the left eye, the medial rectus and the
inferior oblique would be most active.
4-2. Which muscles
would be active in the right and left eye when looking down and to the left?
Answer 4-2. In the right eye the medial rectus and the superior oblique would
be the prime movers, while in the left eye, the lateral rectus and the
inferior rectus would be most active.
4-3. What position
will the patient's head assume (in order to prevent diplopia) if their right
trochlear nerve is damaged?
Answer 4-3. Head tilted to the left and chin turned slightly to the right
("cockeyed").
4-4. When a patient
has double vision, in which position will they have the furthest separation
of the images?
Answer 4-4. The images will be furthest apart when the eyes look in the
direction that the weak muscle is most active.
4-5. What is the
significance of horizontal diplopia (where the images are side-by-side) as
opposed to vertical diplopia?
Answer 4-5. Horizontal diplopia results from weakness of the lateral or medial
rectus muscles; vertical diplopia is due to weakness of one of the
other muscles.
4-6. Which eye
(the one that is moving normally or the weak one) will see the image that
is furthest displaced from the center of vision?
Answer 4-6. The "bad eye" sees the image that is furthest toward the periphery
of vision.
4-7. Where is the
cortical center that controls lateral gaze? Where is the lateral gaze center
in the brain stem?
Answer 4-7. Lateral gaze centers include the frontal eye fields in the frontal
lobes of the cerebral cortex and the paramedian pontine reticular
formation.
4-8. Is there a
vertical gaze center in the cerebral cortex? Is there a brain stem vertical
gaze center?
Answer 4-8. Vertical gaze is a diffuse cerebral cortical phenomenon, while
there is a vertical gaze center in the rostral midbrain (the rostral
interstitial nucleus).
4-9. What are the
potential causes of ptosis?
Answer 4-9. Ptosis may be due to weakness of the levator palpebrae muscle (or
CNIII damage) or due to damage to the sympathetics (due to weakness of
the small, superior tarsal muscle).
4-10. What are
the components of Horner's syndrome?
Answer 4-10. Horner's syndrome (ptosis, miosis, anhidrosis and possibly
flushing) is from damage to sympathetics anywhere along their course.
4-11. What are
the functions of sympathetic and parasympathetic nerves to the orbit?
Answer 4-11. Sympathetics dilate the pupil, parasympathetics (CN III)
constrict the pupil to light and accommodation; there is balance
between sympathetics and parasympathetics.
4-12.
Where is the brain stem center for the pupillary light reflex?
Answer 4-12. The pretectal area of the dorsolateral midbrain is the center for
the pupillary light reflex.