Transcript Ocular Torsion Reveals the Mechanisms of Cyclovertical

Ocular Torsion Reveals the
Mechanisms of Cyclovertical
Strabismus
The Weisenfeld Lecture @ ARVO
2007
Guyton DL
IOVS, March 2008, Vol 49 (3):847 - 857
Introduction
• Torsion was not well understood in 1970s
• This lecture is the chronology of
understanding torsion and its implications
for cyclovertical strabismus
Topics to be covered:
• Measurement of ocular torsion
• Apparent Oblique muscle over /
underaction and ‘A’ and ‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Introduction
• Measurement of ocular torsion
• Oblique muscle over / underaction and ‘A’ and
‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Measurement of torsion
• Torsion cannot reliably be seen by external
landmarks unlike H and V deviations
• Subjective torsion: double Maddox rods,
Lancaster Red-Green Test [Hess Variant - uses
a line, not a dot]
• Objective torsion: blind spot mapping, fundus
photos, Indirect ophthalmoscopy
• In 1970s, abnormal fovea-disc relationship was
called foveal ectopia; considered pathological,
not a strabismic variant
Measurement of torsion
• von Noorden: why does a torted eye not
right itself on covering the other eye?..but
the torted image subjectively straightens?
• On comparing subjective vs objective
torsion significant adaptation was
discovered
Measurement of torsion
• Measurement of the blind spots on visual
fields and fundus photos were impractical
• Developed a scheme for measurement of
torsion on indirect ophthalmoscopic
examination
Grading of torsion
(LE, inverted view)
LK:
1/3: trace or slight
3/3: huge
2/3: in between
Introduction
• Measurement of ocular torsion
• Oblique muscle over / underaction and
‘A’ and ‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Oblique muscle over / underactions
‘A’ and ‘V’ patterns
• Fundus extorsion correlates with ‘V’ and IOOA;
intorsion with SOOA and ‘A’ [e.g. to follow]
• ‘A’ and ‘V’ patters can be explained by rotation of
the planes of action of the recti [e.g. to follow]
• The amount of torsion correlates with the pattern
and the abnormality of eye movements
BE IOOA
Fundus
Xtorsion
correlates
with ‘V’ &
IOOA
‘V’ pattern and Upshoot in
adduction
Oblique muscle over / underactions
‘A’ and ‘V’ patterns
• How did the torsion develop?
• How is it sustained?
Introduction
• Measurement of ocular torsion
• Oblique muscle over / underaction and ‘A’
and ‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Changes in strabismus over time
• Lack of fusion predisposes to H, V and T
deviations
• H: sensory exotropia [ET if hyperopic]
• T :sensory torsion
• Are these progressive changes due to change in
neurological tone or actual muscle length
change?
Changes in strabismus over time
• Skeletal muscles known to adapt lengths by
addition or removal of sarcomeres
• Shorter muscle:  sarcomere density
• Stimuli for length adaptation:
• Tension or lack of tension
• Mechanical stretch or lack of stretch
• Neurological stimulation : if increased, muscle shortens by
losing sarcomeres and and decreased stimulation muscle
lengthens by adding sarcomeres
Modeling of Ocular Alignment
Control System
• Each EOM has basic length and a basic
density of sarcomeres and
• Each EOM receives resting level of
stimulation (vergence tonus) → functional
muscle length → aligned eyes
Modeling of Ocular Alignment
Control System
• A change happens ( new glasses/ orbital
growth/ scar elongation etc) leads to
misalignment → retinal image disparity →
fast fusional vergence changes
stimulation → new functional lengths to
align eyes
Modeling of Ocular Alignment
Control System
Sustained fast fusional vergence →
vergence adaptation →
adjustments in basic level of vergence
tonus →
eases the burden on fast fusional vergence
Modeling of Ocular Alignment
Control System
• Change in vergence tonus → muscle
length adaptation → initial neurological
feedback mechanisms subside
Modelling of Ocular Alignment
Control System
Breakdown of Ocular Alignment
Control System
• Most common abnormality is absence of
fast fusional vergence
• Neurological feedback continues at basal
level, albeit without guidance
• New strabismus arises and progresses
• Cyclofusion loss leads to torsional
misalignment
Modelling of Ocular Alignment
Control System
ACCURATE FUSION
RESPONSE IS LOST
SENSORY XT
IF NO CONVERGENCE
STIMULATION REQU’D,
LESS CONV STIM 
PROGRESSIVE XT
Breakdown of Ocular Alignment
• [XT] who were overcorrected and lost
fusion: 43 % developed ‘A’ or ‘V’ patterns
• [XT] who had fusion after surgery: <5 %
developed ‘A’ or ‘V’ patterns
Introduction
• Measurement of ocular torsion
• Oblique muscle over / underaction and ‘A’
and ‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Dissociated Vertical Deviation
• Scleral search coil technique – DVD is
always associated with latent nystagmus
• DVD damps torsional component of MLN
• DVD is a learned, anticipatory response
when switching fixation to one eye
Dissociated Vertical Deviation
Dissociated Vertical Deviation
• Which muscles are responsible for DVD?
Dissociated Vertical Deviation
• DVD can be broken down into vergence
and version eye movements – both
obeyed Hering’s law
• Horizontal component of LN is dampened
by the convergence
• Cyclovertical component is dampened by
the oblique muscles
Dissociated Vertical Deviation
Vertical Vergence response
• Previous research by Guyton had shown
that the vertical fusional vergence is
mediated by the oblique muscles
• Superior oblique in the fixing eye recruited
to intort the eye and apply “brakes” on T
MLN (consequent depression)
• Inferior oblique in the other eye forms the
other part of vergence response and
causes extorsion, elevation of other eye
Dissociated Vertical Deviation
Vertical Version response
• Vertical version is mediated by the
superior recti of both eyes to elevate the
fixing eye to primary position
• Fixing eye = SO (↓ + in ∩) + SR (↑ + in ∩)
= net intorsion
• DVD eye = IO (↑ + ex ∩) + SR (↑ + in ∩)
= net elevation
Dissociated Vertical Deviation
• Head tilt can be used alternately to damp
LN
• DVD gets better when head tilt is used
• DVD frequently manifest when fine vision
is required or when fusion is suspended
• DVD solved!
Introduction
• Measurement of ocular torsion
• Oblique muscle over / underaction and ‘A’
and ‘V’ patterns
• Changes in strabismus over time
• Dissociated vertical deviation
• Congenital superior oblique paresis
Congenital SO Paresis
• Most common cyclovertical disturbance in
practice
• Are all unexplained extorsion + hypertropia
due to SO paresis?
• Where is the non-paretic analog of the
basic esotropia in the cyclovertical plane?
• Demer JL – 50 % of clinical SO palsies had normal
muscle thickness and contractility
Congenital SO Paresis
Mechanisms of Hypothetic “Basic Cyclovertical”
deviation
• Changes in strabismus are always
bilateral
Congenital SO Paresis
Mechanisms of Hypothetic “Basic Cyclovertical”
deviation
• EOMs do not adapt to their lengths to the
position in which they are held
neurologically
•
FIGURE 12. A particularly telling patient, a 33-year-old woman with
esotropia (crossed eyes) since birth. Her right eye, only, was operated for
the esotropia at age 21⁄2, but the esotropia recurred (top). The right eye
turned in again—or did it? She had fixed with her left eye only, as long as
she could remember, because of mild hyperopia and amblyopia in her right
eye. But the extraocular muscles of the two eyes had not adapted to these
positions, because when she was placed under deep anesthesia (bottom),
both eyes deviated rightward—the right eye because of the original surgery
turning it outward, and the left eye because of the original esotropia, with
still further esotropia developing later. When awake, her muscles were
stimulated neurologically to hold her eyes so that she could fix with her
better eye—her left eye, but the muscle lengths had not adapted long-term
to this type of stimulation
•
An 80-year-old woman with dense amblyopia in her left eye since childhood,
fixing with her right eye only, all her life. Note the left “sensory” exotropia
(top)—the poorly seeing left eye drifted out over the years because the
vision was not good enough for sensory fusion with the other eye. But under
general anesthesia, when the eyes reached their stable, deep-anesthesia
state (bottom), both eyes turned out, equally, and significantly farther than
the usual divergence seen under general anesthesia (in 110 patients with
exotropia, Apt and Isenberg26 documented an increased divergence under
general anesthesia of only 11.9 prism diopters). Both lateral rectus muscles
were equally and abnormally tight when operated. The strabismus had
occurred bilaterally.
Congenital SO Paresis
Mechanisms of Hypothetic “Basic Cyclovertical”
deviation
• EOMs have version stimulation and
vergence stimulations
• Version stimulations do not lead to length
adaptations
• Vergence stimulations help ocular
alignment hence lead to length
adaptations bilaterally
Congenital SO Paresis
Mechanisms of Hypothetic “Basic Cyclovertical”
deviation
• Evidence shows that the muscle fibres
involved in versions and vergence
responses are different
Congenital SO Paresis
Mechanisms of Hypothetic “Basic Cyclovertical”
deviation
• Chronic low-grade vergence response in the
absence of fusion leads to initiation and
progression of strabismus
• For example, hyperopic children develop
accomodative esotropia due to convergence
• Similar mechanism may operate in cyclovertical
plane involving obliques and recti
Congenital SO Paresis
Experimental evidence of hypothesis
• A haploscopic device was created to
induce vertical disparity and subjects were
adapted to fusion
• Pattern of vertical vergence and torsion
suggested a strong role of obliques in
vertical fusional movements
• Investigations are on to simulate the signs
of “Cong. SO palsy” in normal patients
with adaptation
Congenital SO Paresis
• “Though the mechanism explained has not
been confirmed, we are coming closer” Guyton
In the Hogwart of strabismus….
• If torsion is Lord Voldemort………David
Guyton is
P.S. Torsion did not kill Guyton’s parents