Richard W. Hertle MD
Chief of Pediatric Ophthalmology
The UPMC Eye Center and Children's Hospital of Pittsburgh
412.692.7220 Phone 412.692.8940
richard.hertle@chp.edu
Fax
Eye Muscle Surgery and Infantile Nystagmus Syndrome
Description
Infantile Nystagmus Syndrome (formerly congenital nystagmus CN)
is an ocular motor disorder of unknown etiology, which presents at birth or
early infancy and is clinically characterized by involuntary oscillations of the
eyes. These movements most commonly have a slow and fast phase
although they may be purely pendular. They are usually horizontal with a
small torsional component and may (rarely) have a vertical component.
Other clinical characteristics, with variable association, include: increased
intensity with fixation and decreased with sleep or inattention; variable
intensity in different positions of gaze (usually about a null position);
changing direction in different positions of gaze (about a neutral position);
decreased intensity (damping) with convergence; anomalous head
posturing; strabismus; and the increased incidence of significant refractive
errors. INS can occur in association with congenital or acquired defects in
the visual sensory system (e.g., optic nerve hypoplasia, albinism,
achromatopsia, and congenital cataracts). Children with this condition
frequently present with a head turn, which is used to maintain the eyes in
an “eccentric” position of gaze (point of minimum nystagmus). This is
particularly prominent when the child is concentrating on a distant object,
since this form of nystagmus tends to worsen with attempted fixation. The
head turn is an attempt to stabilize the image under these conditions.
Head oscillations are common in INS, but are not used as the strategy to
improve vision. Head oscillations in most patients with INS probably reflect
underlying instability of neck muscle control.
The world moving (oscillopsia) is almost never constantly present in
INS. In those rare patients with intermittent oscillopsia, the oscillopsia
tends to occur at gaze angles in which the nystagmus is maximal or a new
sensory system defect develops (e.g., retinal disease). Numerous studies
of INS in infants and children confirm an age-dependent evolution of eye
movement waveforms during infancy from pendular to jerk. This is
consistent with the theory that jerk waveforms reflect modification of the
INS oscillation by growth and development of the visual sensory system.
These studies reemphasize that continued clinical classification of INS as
either "sensory" or “motor" is confusing and often inaccurate. Accurate
and repeatable classification and diagnosis of nystagmus in infancy as INS
is best accomplished by a combination of clinical and eye movement
recording findings; in some cases, the latter are indispensable for
diagnosis.
Etiology
Eye movement calibration/development is an active process that may
start in utero and continues at least through early infancy. The ability to
see “sensory-system development” is a parallel visual process that has
been more thoroughly studied and also continues to develop through the
first decade of life. Previous studies have documented connections
between these two visual processes (cross-talk) that modify, instruct, and
coordinate these systems. INS probably results from abnormal cross-talk
from a defective sensory system to the developing motor system at an
early time during the motor system’s sensitive period. This theory of the
genesis of INS incorporates a role of VISION in its genesis and
modification. The primary ocular motor instability underlying INS is the
same in all patients with this syndrome but it’s clinical and eye movement
recording expressions are modified by both initial and final developmental
integrity of all other visual processes.
Treatment
Numerous treatments have been described for INS. These include
dietary manipulation, drugs, contact lenses, prisms, biofeedback,
intermittent photic stimulation, acupuncture, transcutaneous vibratory or
electronic stimulation of the face and neck, injection of botulinum toxin and
a variety of surgical procedures. Excepting those treatments which directly
improve visual acuity (spectacle and contact lens correction of refractive
errors), all these treatments have the common goal of reducing the
nystagmus intensity directly, and indirectly showing an increase in visual
acuity.
Surgery
Extraocular muscle surgery has been performed routinely for about
150 years and is the third most common eye surgery performed in the
United States (about 1.2 Million/Year). Although common indications for
extraocular muscle surgery in patients with INS include associated
strabismus and “convergence damping”, surgery is most commonly
performed on those patients in whom there is a null position in eccentric
gaze and the patients assumes an anomalous head posture enough to be
“clinically significant” (a combination of a certain amount of head posture
plus a certain amount of time posturing). Common technical names eye
muscle surgical procedures include; recession, whereby the muscle
tendon is cut (tenotomy, “otomy” comes from “to cut”) at its insertion on the
globe and the entire muscle is moved backwards on the eye, resection,
whereby the muscle tendon is cut (tenotomy) at its insertion on the globe
and a piece of muscle and tendon is removed (tenectomy and myectomy,
“ectomy” comes from “remove”) thus shortening the muscle,
transposition, whereby the muscle tendon is cut (tenotomy) at its insertion
on the globe and the entire muscle in displaced upward, downward or the
right or left of its normal location (“transposed”). The purpose of
recessions, resections and transpositions are to “move” the eye ball to
correct an eye deviation or a head posture. All eye muscle procedures
have in common that they begin with cutting the tendon off the eye or a
“tenotomy.”
Research accomplished over the last 25 years has shown that INS
patients who had eye muscle surgery performed for strabismus or a head
posture had “unanticipated” beneficial effects due to reducing their
nystagmus. There was no ready explanation for these observed effects,
which could be related to simply mechanically shifting the position of the
eyes until an animal model appeared in members of a family of Belgian
sheep dogs. It was shown in this animal model that tenotomy with
reinsertion of the muscle directly back at the original insertion (no eye
positioning change) duplicated the damping effects first documented in
human patients after undergoing the full recession, resection or
transposition procedures.
About 40-60% of all INS patients will have a ”traditional” indication to
perform eye muscle surgery, i.e., strabismus, head posture or convergence
damping. In addition to improving those three primary goals of surgery
they will also receive the beneficial “secondary” effects due to their
nystagmus being reduced. Those 40-60% of patients in whom there was
no “traditional” reason to perform extraocular muscle surgery could also
benefit from “cutting” the eye muscles and reducing the nystagmus. This is
the reason for the “tenotomy” surgery in this 60% group of patients. In
patients who do not need “repositioning” of the eyes the tendon is simply
cut and reattached at the original insertion, thus no “orthopedic” effect but
only the “neurologic” effect. This is not new surgery, but, an old procedure
for a new group of patients, i.e. a new “indication.”
Measures other than visual acuity are improved after surgery on the
eye muscles, and probably contribute to the visual “well-being.” These
include vision in side gaze, speed of visual recognition time, and improved
binocular field (due to a more normal head posture). Recent data collected
on INS patients having had eye muscle surgery of all types supports the
hypothesis that surgical manipulation of the extraocular muscles in patients
with oculographically diagnosed INS “improves” the oscillation and visual
functions.