A previously well 25-year old female was brought
to the emergency department by her boyfriend
because of progressively blurred vision.
Eye examination reveals loss of horizontal
gaze, intact convergence and nystagmus.
A clinical diagnosis of multiple sclerosis is made and
the patient is started on a course of corticosteroids.
What is the most likely etiology
for her eye examination findings?
Answer choice (A) -
loss of reticular formations
(B) - loss of frontal eye fields
(C) - loss of bilateral MLF
(D) - loss of cranial nerves III
and (E) - loss of cranial nerves VI
Now take a moment to come
to your own conclusion
Now let's go through the question
characteristics that we're dealing with here,
Now what's the subject
we're talking about?
This is a multiple sclerosis patient so
this is a classic neurology question
Now this question is what we call a 2-step question,
You have to make one conclusion first and then take
another deeper step to get to the right answer.
And in this question, the stem is
required to come to the conclusion.
We have to rely on the details
in the clinical vignette
Now let's walk through the question.
The first thing we need to do is determine the type
of eye disorder that the patient is experiencing.
While in the question stem, we learn that the
abnormal eye movements that we hear about
are blurred vision, loss of
horizontal gaze and nystagmus.
Well, when it comes to blurred vision,
this is commonly caused by
disorders of the lens of the eye
But the fact that in this patient,
there also is loss of horizontal gaze
suggests that there is actually misalignment of
the eyes being the cause of the blurred vision.
Now this patient has intact convergence, that
is, adduction of both eyes towards the nose
Now this confirms that both medial rectus
muscles are well innervated and not paralyzed,
thus there is no cranial nerve III palsy.
And the fact then that the combination of
intact convergence and loss of horizontal gaze
actually suggest that there is a
disorder in the conjugate eye movements
With the most likely diagnosis here being
internuclear ophthalmoplegia or INO
Now once we figure that out,
we need to go one step deeper
to determine the anatomical structure
that's affected in this eye disorder
Whereas we call it in neurology,
we need to localize the lesion.
Now, conjugate eye movement requires simultaneous
contraction of oculomotor muscles in both eyes
that are innervated by the the
oculomotor nerve - cranial nerve III,
and also the abducens
nerve- cranial nerve VI
Now, transmission of integrated information
from upstream gaze centers to these nerves
occurs via a white matter tract called the
medial longitudinal fasciculus or also the MLF
on each side of the brainstem
Now this is a structure that's affected
in internuclear ophthalmoplegia.
We have now just localized the lesion
Now, in patients that have multiple sclerosis, they have
generalized demyelination of the white matter tracts,
the MLF being a white matter tract and then
they thus have internuclear ophthalmoplegia
In fact, in multiple sclerosis, the internuclear
ophthalmoplegia is usually bilateral in these MS patients
Thus the answer choice that's
correct here is answer choice (C),
loss of bilateral MLF leading to
the eye findings in the exam.
Now let's discuss some high-yield facts
that we can learn from this question.
Now, multiple sclerosis causes demyelination of white
matter tracts in the brain and also the spinal cord.
Due to the involved generalized lesions
of the central nervous system,
signs and symptoms of MS can really
widely range from one patient to another.
We can see double vision, muscle weakness, trouble
with sensation, trouble with coordination,
and even mental cognition disorders
Now MS usually has what we call
the relapsing and remitting form,
in which patients have isolated attacks
that are worse, and then get a bit better.
Another form, less common is
called the "progressive form"
in which patient just continuously
gets worse and worse,
they don't have these acute flares.
Now multiple sclerosis is typically diagnosed
in young patients and more highly,
there is prevalence in
females than in males.
Now the underlying cause of multiple sclerosis
is unknown and there unfortunately is no cure
But there is symptomatic and disease- modifying
treatments that have proven effectiveness.
Now another high-yield topic is
Now this is a disorder of
horizontal conjugate lateral gaze
in which the affected eye
or eyes cannot adduct.
It's caused by a lesion to the MLF or medial
longitudinal fascuculus in the brainstem
that transmits integrated information from
upstream gaze centers to oculomotor nerves
In MS patients, very important to know, the
lesion to the MLF tends to be bilateral.
Now the MLF is a white matter tract on each side of
the brainstem that coordinates abduction of one eye
and adduction of the other eye to
produce conjugate horizontal gaze
Now the structures the MLF connects are known
as the ipsilateral nucleus of cranial nerve VI
which controls abduction, the contralateral nucleus
of cranial nerve III which controls adduction,
and the ipsilateral paramedian pontine
reticular formation, also called the PPRF.
Now lesions here result in an impaired
adduction in horizontal lateral gaze
but convergence will remain intact which we also
saw in this patient's clinical presentation.