25-year-old (female) with progressively blurred vision

00:02 A previously well 25-year old female was brought to the emergency department by her boyfriend because of progressively blurred vision.

00:10 Eye examination reveals loss of horizontal gaze, intact convergence and nystagmus.

00:16 A clinical diagnosis of multiple sclerosis is made and the patient is started on a course of corticosteroids.

00:24 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.

01:08 And in this question, the stem is required to come to the conclusion.

01:12 We have to rely on the details in the clinical vignette Now let's walk through the question.

01:18 The first thing we need to do is determine the type of eye disorder that the patient is experiencing.

01:24 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.

01:34 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.

01:51 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.

02:09 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.

02:39 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.

03:15 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.

03:49 Now let's discuss some high-yield facts that we can learn from this question.

03:53 Now, multiple sclerosis causes demyelination of white matter tracts in the brain and also the spinal cord.

04:01 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.

04:11 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.

04:31 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.

04:41 Now multiple sclerosis is typically diagnosed in young patients and more highly, there is prevalence in females than in males.

04:49 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.

05:01 Now another high-yield topic is internuclear ophthalmoplegia Now this is a disorder of horizontal conjugate lateral gaze in which the affected eye or eyes cannot adduct.

05:14 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.

05:32 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.

06:07 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.