Author: Hannah Vollmer

Cranial nerve IV, or the trochlear nerve, is solely responsible for controlling movement of the superior oblique muscle, which functions to help rotate the eye in an inwards motion (intorsion).  As such, CN IV is a general somatic efferent nerve.  

 

Like the oculomotor nerve, the trochlear nucleus originates in the midbrain.  However, instead of immediately going forward towards the eyes, the trochlear nerve crosses to the opposite side, and then passes dorsally (posteriorly or to the back) out of the midbrain, and then proceeds to wrap towards the eyes.  This pathway will prove very important in my CN IV palsies post!

CN V: Trigeminal

While CN IV was short, sweet, and to the point, the trigeminal nerve is anything but!  Trigeminal by definition means threefold, which speaks to the three primary divisions of the trigeminal nerve… which doesn’t sound bad, until you look at all the many subdivisions of these three.  Think you’re ready?  Hold onto your hats, and let’s give it a go!

Cranial nerve V is a mixed nerve, containing both sensory and motor components. For the sensory side, the trigeminal nerve conducts sensory signals via generals somatic afferent fibers from a number of smaller nerves that we’ll discuss further in a moment.  On the motor side, the trigeminal innervates the muscles of mastication (chewing), as well as a few others that I’ll briefly cover.  Due to the embryologic origins of these muscles, the signal is transmitted via special visceral efferent fibers.

Interestingly, rather than having a single nucleus like CN III and IV, the trigeminal nerve actually has four nuclei: three sensory (primary, spinal and mesencephalic) and one motor.  These, essentially, end up passing through a majority of the brainstem, converging into the trigeminal ganglion near the petrous portion of the temporal bone.

From there, as previously mentioned, the trigeminal splits into three branches: the ophthalmic (V1), the maxillary (V2), and the mandibular (V3).

    Ophthalmic

• Sensory nerve
  
• Further divides into:
• Lacrimal nerve (lacrimal gland/upper eyelid)
• Frontal nerve
• Supraorbital nerve (sensory from skin of lower forehead)
  
• Supratrochlear nerve (sensory from nose/upper eyelid)
  
• Nasociliary nerve
• Anterior ethmoid nerve (sensory from nasal cavity)
  
• Posterior ethmoid nerve (sensory from sinuses)
  
• Long/short ciliary nerves (sensory for eye)
  
• Infratrochlear nerve (sensory for conjunctiva and central upper eyelid)

• Sensory nerve
• Further divides into:
• Infraorbital nerve
• External nasal nerve(sensory for skin on side of nose)
  
• Internal nasal nerve(sensory to nasal septum)
  
• Superior labial nerve (sensory for upper lip)
  
• Inferior palpebral nerve (sensory for lower eyelid)
  
• Anterior superior alveolar nerve (sensory for teeth)
  
• Middle superior alveolar nerve (sensory for teeth)
  
• Meningeal nerve
• Pterygopalantine nerve
• Lateral superior posterior nasal (sensory for nose)
  
• Medial superior posterior nasal (sensory for nose)
  
• Nasopalatine
  
• Major palatine (sensory for palate)
  
• Minor palatine (sensory for palate)
  
• Zygomatic nerve
• Anterior zygomaticofacial nerve (sensory from far side of face)
  
• Posterior zygomaticotemporal nerve (sensory from far side of face)

    Mandibular

• Sensory and motor nerve
  
• Sensory:
• Buccal nerve
• Inferior alveolar nerve
• Inferior dental plexus (sensory for teeth and gums)
  
• Mental nerve (sensory for teeth and gums)
• Lingual nerve (sensory for tongue and floor of mouth)
• Motor: (muscles of mastication)
• Mylohyoid
• Anterior belly of digastric
• Tensor palantini
• Masticatory muscles
• Tensory tympani

Phew! We made it!

CN VI: Abducens

For right now, I primarily want to focus on function.

Finally, nerves can be responsible for either somatic (general body) or visceral (internal organ) functions.

 

Okay, I think that covers the basics. Now onto cranial nerves!

 

CN I: Olfactory

In working with pediatrics, this is one of the more common questions that I am asked, and so, for tonight’s post, I’ll try to shed some light on eye turns.  Check it out!

Eye turns, or strabismus, can be described in several ways.  The first is by consistency, with the deviation being either constant or intermittent.  The second is by time of onset – whether congenital/early onset or acquired.  Eye turns can also be described by the direction of the eye turn. In this, the eye can turn in (esotropia), out (esotropia), up (hypertropia), or down (hypotropia).  Finally, a single eye can turn, or the eyes can alternate.

At the end of the day, essentially anyone can have an eye turn. However, early onset eye turns may have a genetic component. Additionally, eye turns may be associated with ocular pathology or problems with the development of the brain (ie hydrocephalus).  Inward eye turns that are worse when focusing at near may be associated with a moderate to high far-sighted prescription.

Acquired eye turns are more likely to present after an acquired brain injury, as a result of damage to the nerve pathway.

The biggest sign or symptom of strabismus is simply an eye turn. However, additional signs include closing or covering an eye and blinking excessively.  These both may be indications that your child is seeing double.  The first is used to remove the second image formed by the deviating eye, while the second is used to help focus.

In new onset or acquired strabismus, double vision, or diplopia, is common, as the brain has not adapted to automatically suppress the additional image.  This may be perceived as blur. 

The complete explanation behind early onset eye turns is not fully known, but the basis is this:

All eye movements are controlled by six muscles on each eye.  These 6 muscles in turn are controlled by three cranial nerves (in each eye).  Damage to the nerve or the muscle can cause  deviation from normal eye position and movements.

In the first few months of life, it is very common for your child’s eyes (as well as the rest of their body) to not be perfectly synced.  They’re just trying to get the hang of coordinating and focusing, so this should come as no real surprise.

However, if an eye turn continues after the first 3-4 months, or is constant or always the same eye, it’s worth getting checked out to rule out pathology.  Additionally, a constant, or near constant, eye turn is a risk factor for ambloypia – decreased vision that is not attributable to glasses prescription alone!

Additionally, if you ever notice a new onset eye turn, or your child begins to complain of double or closes an eye for specific tasks, an exam is warranted.

The treatment options for eye turns vary significantly depending on the size, direction, and frequency of deviation.  Large angle deviations often end up requiring surgery to decrease the demand on the muscles.  Smaller angle outward eye turns, or exotropias,  may be managed by vision therapy, additional minus lens power, or prisms.  Inward eye turns, or esotropias, especially those associated with focusing the eyes (accommodation) may be managed by plus lenses, prisms, or in more rare cases vision therapy.

Eye turns up or down (though the eye that is higher, or hyper, is more typically referenced), are less frequent than eye turns in or out.  When they’re small, prisms, or a combination of prisms and vision therapy, may be used for treatment.  Larger deviations, as before, are more likely to be surgically managed.

And there you have it!

Pretty much all allergic reactions in the body have the same basic cause: the body identifies something (an antigen) as a foreign entity, and decides to attack it.  Allergies specifically are associated with a Type 1 Hypersensitivity reaction, mediated by the specific antibody Ig-E.  In this, the Ig-E antibody that is specific for the antigen that you’re allergic to (ie pollen, dander, etc) binds to an immune cell called the mast cell, which contains products that are meant to destroy or remove the antigen.

The most prominent product in mast cells is histamine.  This chemical causes all the typical allergy symptoms – a runny nose, watery eyes, and itching!

The most common symptoms of ocular allergies (as could probably be guessed from above) are:

• Watery eyes
• Itchy eyes
• Swelling
• Mild discharge
• Blurred vision

Treatment

Ocular allergies are treated much in the same way that systemic seasonal allergies are treated.

The first course of action is to remove the offending agent – aka, stay away from  what you’re allergic to!

However, when you’re allergic to the air in the entire state, that’s hard to do.

The second step is to take systemic allergy medications.  More often than not, these can be over the counter.  However, if your allergies are severe, you may need stronger, prescription medications.  If possible, try to stay away from decongestants though, as they can cause dryness and increase ocular irritation.

In the case of ocular allergies, it is often necessary to specifically treat the eyes.  I most commonly advocate for the use of over the counter eye drops before proceeding to prescription mediations.  My favorite drop is Pataday (olopatidine 0.2%), as it is relatively cost effective and only needs to be used once a day!  Other great the counter option are Zaditor and Alaway (both are ketotifen 0.035%), which are used twice a day.  Make sure to wait 5 minutes after using your drops to put in contact lenses or using any other drops.

As ocular allergies may be exacerbated by longer contact lens replacement schedules, I tend to prefer to fit patients with symptomatic ocular allergies in daily lenses.  This reduces the potential for buildup on lenses, and therefore decreases the likelihood of an allergic response.

Symptoms may also be mitigated by strategic hygienic considerations, such as showering in the evening and frequently washing pillowcases to reduce the number of potential allergens around the eyes while sleeping.

Finally, cool compresses are often helpful in reducing the itch that accompanies ocular allergies!

One of my favorite COVID-era eye questions (thanks to 4 months of emergency care, triage, and telehealth calls to round out residency) is, “what constitutes an ocular emergency?”

(Which is closely related to, “do I really need to see a doctor for this?”)

With that in mind, today’s post is going to cover some common signs and symptoms that really do require a trip to your OD!

Flashes and floaters are most commonly associated with a posterior vitreous detachment (not an emergency).  However, they may also be an indication of a retinal tear or retinal detachment.  If you have a sudden increase in flashes and floaters, call your eye doctor up, so that they can take a look!

A sudden change in vision can also be a sign of significant ocular problems.  From sudden blurry vision (that lasts more than a few seconds and doesn’t get better with artificial tears, cleaning glasses, or changing contacts), to a complete loss of vision, these changes should not be taken lightly.  Don’t wait for a few days to see if your vision gets better, call your doctor up immediately!

Unfortunately, not all red eyes are created equally – which is probably why they’ll end up in a series sooner or later.  When does a red eye need to be seen?  When it’s painful or associated with changes in vision. (Okay, and a few other times, but we’ll talk about those later…)

Knowingly getting something in your eye (especially if you suspect/know that it may have penetrated your eye) also gives cause to call up your eye doctor.  They’re able to perform rinses, remove foreign bodies, and prescribe mediations (often antibiotics) to decrease the changes of an infection forming.

If you have other signs or symptoms not listed here, don’t be afraid of checking in with your provider! If you’re afraid there might be a problem, it’s always better to be safe than sorry!

First things first: light.

Everything that you see in everyday life corresponds with an electromagnetic frequency within the visible spectrum of light.

Light enters the eye through the clear cornea, passing through the lens, and ends up at the retina.  Together, the cornea and the lens focus the light to attempt to provide a distinct pattern of light on the retina.  When they aren’t enough, spectacle or contact lenses are added to help focus the light.

When the light hits the retina, it is sensed by special cells called photoreceptors.  The role of photoreceptors is to take the light and transform it into an electrical signal.

Within your eyes, there are different types of photoreceptors: three that require more light to function, and so are utilized in brighter lighting  (cones), and one that requires little light to function, allowing it to work in darker conditions (rods). Each of these have specific wavelengths that they are more sensitive to.   Three different cells with three different specific wavelengths provides for the perception of color in brighter lighting conditions.

Additionally, cones and rods have specific functions.  Cones are responsible for your best vision.  For this reason, they are highly concentrated in the most central part of your vision to allow you to see clearly when looking straight ahead.  Rods, on the other hand, provide less precise vision, and are located further out from the very central point of vision.

The signal produced by the photoreceptors is then transmitted through additional cells in the retina. These cells help to group, enhance, and refine the signal as it makes its way to the retinal ganglion cells that then pass out of the retina as the optic nerve.

The optic nerve (well, after some minor-ish detours) then passes to the first visual center in the brain – the lateral geniculate nucleus.  While it would seem like a primary place for image processing, as this structure is comprised of 6 layers with divided input based on specific types of retinal ganglion cells, relatively little processing occurs here.  Rather, it would seem that the lateral geniculate nucleus simply serves to help combine information from both eyes before going further in the brain, or provides an ideal midpoint for information modulation.

From the lateral geniculate nucleus, the signal passes to the primary visual cortex (by way of optic radiations).  This is where the first visual processing occurs, allowing for the beginnings of depth perception, a determination of object orientation, and detection of movement.

The signal continues to pass though the other 5 layers of the visual cortex, becoming increasingly refined along the way, until full perception of the image is achieved!

However, simply creating an image does not allow you to react to what you see.  Rather, once fully processed in the visual cortex, signals are sent to other areas of the brain (such as Brodmann and Wernicke’s areas) that then give context to the image.  Once context is achieved, signals are set to respond appropriately in some form of action – be it smiling at a funny picture, stepping out of the way to avoid someone walking towards you, or reading words off a page!

Ready or not, a new school year is upon us.  However, for many, it looks very different than years in the past, as virtual schooling has become a reality for children of all ages across the country.  Children from Kindergarten through high school (and even through college)  are now needing to sit still at home in front of computer screens for hours at a time, completing virtual instruction.

Unfortunately, this mode of education poses significant problems for the visual system.

In a previous series, I discussed Computer Vision Syndrome in adults.  However, this condition is not limited by age.  Rather, as screen time increases, children are reporting  increasing symptoms of Computer Vision Syndrome as well.

As a review from before, some common symptoms of computer vision syndrome are:

• Headaches (especially around the eyes)
• Eye strain
• Eye fatigue
• Double vision
• Blurry vision
• Dry eyes

A Facebook group that I am a part of (VTODs on Facebook) has compiled some tips and tricks for optimal ocular health in this new situation, and so, for today, I’d like to share some of them with you!

1. Blue light filters: Blue light filters block blue light (which is emitted from screens), which has been shown to have effects on circadian rhythms and potentially cause ocular fatigue secondary to the screen flicker rates.  If you’re ordering new specs for your child, consider adding a blue blocking filter.  If your child doesn’t wear glasses, blue light filters are sold as prescription-less glasses, as well as filters that can be put on screens!
2. Reading glasses: When using screens for long periods of time, it may be beneficial for your child to use low powered reading glasses to relax how hard they have to focus their eyes to clearly view the screen.  Low plus (+1.25-+1.50) lenses can be bought over the counter at most stores.
3. 20/20/20 Rule:  As with adult Computer Vision Syndrome, encourage your child to look at something 20 feet away for 20 seconds every 20 minutes to give their eyes a break!
4. Distance: Make sure all screens are at least an arms length away from your child!
5. TV: Consider casting your child’s virtual class to a TV screen for easier viewing.
6. Activities: Try to have your child do some form of physical activity (jumping jacks, jump rope, running, etc) for at least 5 minutes to use some energy before having to sit still
7. Windows:  If possible, have your child’s school set up be next to a window so that they can easily look far away.
8. Go outside:  During any breaks, go outside!  This helps relax eyes, and is good for overall health!

So, as I discussed in my Introduction: Hannah Vollmer, OD post, one of my greatest optometric passions is neuro-optometric rehabilitation, which is the field that I completed a year long residency in.  During my residency, I had the opportunity to work in an in-patient rehabilitation hospital, which was one of the most rewarding experiences in my career to date.

As a neuro-optometry resident, I worked with a significant number of people who were in the process of recovering from strokes. Prior to residency, I honestly had little understanding of the effects of strokes or the potential treatment strategies to be implemented in stroke survivors, despite having four years of graduate level, medically based education and a positive family history for the condition.

Maybe I lived under a rock, but I’m guessing I’m not alone.

Over the past year, I have learned a lot about working with stroke survivors, but I’m still nowhere near an expert.  Nevertheless, in today’s post, I’ll try to share a few things that I’ve learned over the past year, especially regarding how strokes impact visually related tasks.

Enjoy!

Strokes: In Focus

What is a Stroke?

I suppose talking about what a stroke is would be a good first step…

Medically, strokes are called cerebrovascular accidents, or CVAs.  This term does a good job at explaining what a stroke is: a condition in the brain (cerebro) that occurs as a result of problems with blood flow (vascular).

Strokes can be either ischemic or hemorrhagic.

Ischemic strokes occur secondary to a blockage that prevents blood from entering an area of the brain.  This is the most common type of stroke, accounting for nearly 90% of all strokes.

Hemorrhagic strokes, on the other hand, occur when a blood vessel breaks or leaks within the brain.  These are relatively rare, and are most often associated with aneurysms (essentially an outpocketing of the blood vessel) or very high blood pressure.

Who Gets Strokes?

According to the CDC, approximately 795,000 people in the US alone have a stroke each year.  This equates to one person every about 40 seconds.

Out of these, approximately 140,000 people die (1 every 4 minutes).

Around 610,000 are first time strokes.  The other 185,000 are a recurrence.

Crazy, right?

Strokes are more common in patients with vascular problems, such as high blood pressure, high cholesterol, and diabetes.  Higher risk is also associated with smoking and obesity.

While strokes are often associated with older age, over 1/3 of the people who are hospitalized with strokes are under the age of 65.

Takeaway: Anyone can have a stroke at any time.  However, certain conditions or behaviors significantly increase the risk of having a stroke!

Symptoms of a Stroke

Considering how little information I hear about strokes on a regular basis, I feel like this is one aspect that the public health community has made huge strides in – knowing the symptoms of strokes.

Remember the acronym? FAST?

For a review, it stands for:

• Facial droop
• Arm weakness
• Slurred speech
• Time to act

These are some of the most common symptoms associated with a stroke.  However, it’s important to know that these can also occur as part of what’s called a transient ischemic attack, or TIA.  In these cases, as the name implies, the symptoms are transient, or short lived.  Just because the symptoms are not permanent, though, does not negate the seriousness of the presentation.  TIAs are considered a medical emergency, as they often precede a full-scale stroke.

Despite the widespread knowledge of the FAST acronym, these symptoms fail to include any visual symptoms that may precede or accompany a stroke.  I guess that’s where I come in…

Visual Symptoms of a Stroke

First things first: strokes may often be preceded by visual TIAs.

These visual phenomena can be described in a number of different ways, such as:

• Vision loss (typically in one eye)
• Double vision
• Dizziness/sensation of world moving
• Loss of part of the visual field

As with all TIAs, these symptoms are relatively short lived, normally lasting under an hour, but are still suggestive of an impending full-scale stroke, with the greatest risk being in the first 24 hours after the event.

While these three symptoms may precede a stroke as visual TIAs, they may also present consistently, either before or with a stroke.

For instance, sudden vision loss in one eye may be a result of occlusion (blockage) of the central retinal artery, or a branch of the central retinal artery (CRAO or BRAO).  These conditions are both considered a stroke of the eye and require immediate medical attention due to the strong association between these conditions and risk of large-scale stroke.

New onset, persistent double vision, from my experience, is more likely to occur in conjunction with the full ischemic or hemorrhagic event.  This annoying symptom presents secondary to damage to the neural pathways responsible for controlling the four cranial nerves that innervate the six extraocular muscles.  With improper innervation, the musculature becomes imbalanced, changing the natural position of gaze for one eye, creating the symptom of double vision.  This eye turn may or may not be easily observable, depending on the direction and degree of turn.

Dizziness, or a sensation of the world shaking or moving (oscillopsia), again may present as a symptom of the stroke itself.  Oscillopsia occurs as a result of damage to a part of the brain that normally inhibits movement of the eyes (if you inhibit an inhibitor, you get movement).  Visually, this presents as nystagmus, or a shaking of the eyes.  The direction that the eyes move is directly determined by the location of the stroke itself.  This is one of the most frustrating visual symptoms for patients who have had strokes, as it not only reduces vision (because the eyes never sit still long enough to fully focus on an object), but also causes dizziness (frequently with nausea and vomiting) and disorientation.

Loss of part of the visual field is another significant visual sign associated with strokes.  This symptom may occur secondary to damage to a number of areas in the visual pathway – whether it be in a lobe of the brain that the nerve fibers traveling from the eye to the visual cortex pass through, or in the visual cortex itself – with symptoms varying by the specific location affected.  Interestingly, due to the crossing of nerve fibers from each eye relatively early in the ocular pathway, visual field deficits generally present as loss on the right or left halves of both eyes.  This is referred to as a homonymous hemianopsia – homonymous meaning same, hemianopsia meaning half of the visual field.  Due to the oddly reversed nature of physical placement of retinal nerve fibers and the corresponding visual field, a stroke on the left side of the brain will be associated with right sided visual loss, and vice versa.

What to Do?

If you, or someone you know, experiences any of these symptoms, immediate action is imperative!  When you’re working with brain tissue, time is money.  Or, in this case, life.  The sooner an impending, or even large-scale stroke is caught, the better the outcome.  If caught soon enough (under 5 hours from onset), a medication may be administered to get rid of the blockage, potentially reducing or completely reversing the effects of the stroke.

However, unfortunately, simply going to the hospital isn’t always enough, as not all stroke centers are created equal (though, if you only have the option of hospital or no hospital – get to the hospital).  Optimal treatment would be at a comprehensive stroke center, which is required to have:

• The ability to treat ALL types of strokes
• 24/7 access to minimally invasive procedures to treat stroke
• Neuroscience ICU
• Neurosurgery

Treatment after a stroke varies widely by symptoms and their severity.  For the sake of time, though, I’ll discuss stroke rehabilitation (specifically the role of optometrists in stroke rehab) in a later post.  Stay tuned!

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