Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
Table 2.2
Types of eye movements and functions
Type of eye movement
Vestibular
Optokinetic
Saccade
Smooth pursuit
Vergence
Visual fixation
Function
Maintains image on retina during
head rotation (brief)
Maintains image on retina during
head rotation (sustained)
Captures fresh objects of interest
on fovea – refixation
Maintains image of moving target
on fovea – sustains fixation
Maintains image of a single
object through disjunctive
movement
Maintains clear visual image on
the fovea with head stationary
FEF
POT
LR MR
PC
riMLF
riMLF
SC
INC
INC
III
III
III
IV
IV
IV
PPRF MLF
PPRF MLF
PPRF
VI
VI
VI
VN
VN
VN
FEF frontal eye fields
MLF medial longitudinal fasiculus
POT parieto-occipital-temporal junction
III oculomolor nucleus
SC superior colliculus
MR medial rectus
riMLF rostral interstitial nucleus of
INC interstitial nucleus of cajal
the medial longitudinal lasciculus
PC posterior commissure
PPRF paramedian pontine reticular formation VN vestibular nuclei
VI abducens nucleus
IV trochlear nucleus
LR lateral rectus muscle
the control of smooth pursuit eye movements
through the flocculus and nodulus and the
enabling of accurate refixation saccades through
the dorsal vermis. There are also numerous
interneuron connections between both voluntary
and reflex gaze centres, mediated by pathways
for horizontal gaze (PPRF, paramedian pontine
reticular formation), vertical gaze (rostral
interstitial nucleus of the medial longitudinal
fasciculus) and vestibular systems (Figure 2.6).
The voluntary eye movements are either
saccadic (fast and visually capture objects of
visual interest) or pursuit movements (slower
and used to follow a visual object of interest).
The reflex controls add quality characteristics to
the movements; for example, the influence of the
cerebellum in smooth pursuit.
Most clinicians, when examining a patient,
will examine the pursuit system but many omit
to test the saccadic system. The slower pursuit
system is examined by getting the patient to
follow a specific target in the field of action of
the suspected paretic muscle. This is a routine
most clinicians use in testing the range of eye
movement. The saccadic or refixation system is
also readily examined. The patient is asked to
refixate from one target to a target in the field of
action of the suspected paretic muscle. The test
gives an indication of the accuracy of the
refixation mechanism and tests the ability to
generate normal speed of a saccade. It renders
12
Figure 2.6 Diagram showing integrated motor
control pathway for eye movement. (Reproduced with
permission from Miller N in Walsh & Hoyt’s Clinical
Neurophthalmology)
valuable information about muscle action. A
weakened muscle action will manifest as a
slowed saccade.
A number of terms are used in the description
of eye movements. They are important to
understand both in description of eye movement
disorders and in an eye examination and will be
briefly referred to.
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Saccades are rapid (300–700/sec) velocity
refixation movements to bring objects of
interest onto the macula.
Nystagmus movements are quick phases of eye
movement that occur during self-rotation.
They direct the macula toward the oncoming
visual experience and reset the eyes during
persistent rotations. The quick phases originate
in the pons and midbrain paramedian reticular
formation. Phylogenetically, this is an old system
and appears to share a common anatomic
substrate with the more recently evolved
voluntary saccadic system.
By contrast, there are a number of slow
eye movements (velocity 20–50º/sec). These
include smooth pursuit that serves to hold
the visual image of interest of a moving target