Nystagmus
Blockage Syndrome
The nystagmus blockage
syndrome (NBS) denotes a particular type of
nystagmus that begins in early infancy
accompanied by esotropia.
NBS is characterized by a
reduction of the nystagmus when esotropia
increases. As the viewing eye follows a target
moving laterally towards the primary position
and then into abduction, the nystagmus increases
and the esotropia decreases. The name "nystagmus
blockage syndrome" reflects the prevalent
assumption that patients block their nystagmus
by adducting one eye.
The diagnosis of NBS is
difficult to make because :
(1) precise and uniform
diagnostic criteria are lacking
(2) similar, more common
disorders, such as esotropia associated with
manifest latent nystagmus (MLN), are mistaken
for NBS
(3) the
diagnosis usually is made by clinical
observation rather than by accurate oculography.
The decrease of the
nystagmus by adduction or
convergence may be etiologically significant in
causing
esotropia and introduced
the term ‘‘nystagmus blockage syndrome.’’
This syndrome is
defined
as an esotropia with an onset in infancy (often
preceded by nystagmus), a pseudoabducens palsy,
straightening of the eyes under surgical levels
of anesthesia, and the appearance of a coarse
manifest nystagmus during the induction phase of
anesthesia. A head turn may be elicited by
covering either eye, and manifest nystagmus
occurs as the fixating eye moves from adduction
toward abduction.
The etiologic concept of
nystagmus ‘‘blockage’’ and its relationship to
infantile esotropia initially became widely
accepted . In fact, many believed that
most cases of infantile esotropia could be
explained on the basis of this mechanism.
However, criticisms ranged
from whether the blockage mechanism existed at
all
to the demand that the
diagnosis of esotropia caused by nystagmus
blockage be applied only to patients in whom an
inverse relationship between the angle of
esotropia and the intensity of nystagmus could actually be demonstrated.
Since manifest congenital
nystagmus occurs infrequently with infantile
esotropia and latent or manifest-latent
nystagmus is rarely found
without
this condition, most
cases diagnosed in the past as having the
nystagmus blockage syndrome probably had
infantile esotropia associated with latent or
manifest-latent congenital nystagmus. In this
instance there is no reason to believe that the
decreased nystagmus intensity in adduction is
actually caused by dampening through
superimposed adduction innervation, that is, by
a blockage mechanism. In fact, there is evidence
to the contrary.
The differentiation of these
cases from the true syndrome may, in certain
instances, be complicated because patients with
manifest congenital nystagmus who develop
esotropia from sustained convergence may convert
their manifest nystagmus waveform with onset of
the esotropia to a manifest-latent nystagmus
waveform. Such patients will always show an
increase in the magnitude of the nystagmus with
abduction of the fixating eye, which is
characteristic of manifest-latent but not of
manifest congenital nystagmus .
Surgery
Indications for surgery exist when the
esodeviation is constant. Of various surgical
approaches, a recession of both medial rectus
muscles, which may be combined with
posterior fixation sutures,
is advocated .
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