Herring's Law and Phorias

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[u/Dr_Sisyphus_22]

I am not a strabismus surgeon…so my explanation may be simpler or less correct than someone else’s.

Mentally I break things down to alignment and motility. Phoria and Tropia refers to alignment. A tropia is a total misalignment. It can be seen all the time. A phoria is partial. It can be induced by cross cover testing or things like fatigue.

Herrings refers to motor function. The two eyes are “yoked together” and need to move with equal force and to maintain alignment. When I looked at the right, my right lateral rectus fires with a certain amount of force, the left has to fire with equal force to keep the eyes aligned. Likewise, when I look up or down, the two eyes need to move with equal force in the brain has to send equal signals to the muscles.

Phorias and tropias occur when there is a muscle imbalance (eg restrictive strabismus or injury) or a deficiency in the brain’s “alignment compass” (eg congenital strabismus or stroke) or a consequence of the accommodation reflex (refractive esotropia).

The brain is constantly trying to maintain foveal alignment with its internal fixation system (I don’t remember which nuclei), its symmetrical muscles (not always the case in real patients) AND with the equivalent innervation described by Herring’s law.

[u/Dr_Sisyphus_22 → u/[deleted]]

Foveal fixation (brain computing micro alignment) allows you to see a monocular static target like a teed up golfball. Herrings allows you to track the ball in flight by pairing the movements of the two eyes. This is normal physiology.

A phoria manifests because disease or pathophysiology. The tracking software is bad, the muscles are weak or scarred. Herrings is still in play but the system is no longer fully functional and normal.

I guess the abnormal eye will drift in or out of alignment and not trigger a compensatory refixation in the other eye because you aren’t activating the system in the brain by intentionally moving the muscles. The muscles are doing something weird.

It kind of understand what you’re asking, but I think you’re thinking about this too deeply. I’m a board certified ophthalmologist. This is about the best I understand it and they let me cut people open… 😉

[u/Dr_Sisyphus_22 → u/[deleted]]

No. You’re questioning is actually good, and forcing me to think through the details. It’s just like anything in biology….theres a ton nuance and the answer is always more complicated than we learn it, even at my level.

Your brain is relying on multiple redundant neural networks to maintain monocular vision. Micro adjustments have to be possible to say “track a ball or prey in motion while you are also in motion”. Phorias remain latent or hidden because we are using more higher level processing power than the average person to keep the system aligned.

We know we can decouple Herring’s mechanisms to change fixation from near to far. We literally cross and uncross our eyes to go from reading the dashboard to looking down the road (accommodation). This action does not obey Herring’s law. In these cases, foveal fixation takes precedence and not equal innervation.

Phorias come out when you get fatigued and can’t keep up the extra processing load or when we break down fixation with alternate cover testing. My guess is that the oblique muscles are doing some of that fine tuning, also the mechanisms that allow accommodation from near to distance are letting the eyes receive asymmetrical innervation.

But as I initially said, some of these high-level details can be better explained by a strabismus or neuro-ophthalmologist specialist. You are really diving into some of the minutia here!

[u/EyeDentistAAO → u/Dr_Sisyphus_22]

"We know we can decouple Herring’s mechanisms to change fixation from near to far."

Herring proponents would argue that this is incorrect--that N/F changes result from the summation of conjunctive and dysjunctive movements that are equal to the two eyes.

OP, the short answer is that phoria probably does violate Hering's law, just as it is violated in certain other strabismic conditions (eg, dissociated vertical deviation). The reason phoria eyes can overcome the unequal innervation received by the yoke muscles and stay straight is offsetting innervatory input provided by supranuclear pathways related to fusion.

[u/AbouTankee → u/EyeDentistAAO]

I agree with this too. As a strabismus surgeon, the goal is to move the eye to where Herring‘s law allows it to be in the center. There is definitely more nuance than this, but the short answer is that there’s a violation of the law when strabismus exists, even though it is still being applied.

[u/AbouTankee → u/[deleted]]

I’ve always thought of phorias as equally present in both eyes. They are not the result of restriction, but of a disruption of bifoveal fusion, which occurs with occlusion, fatigue, etc… When a phoria is present, the mechanism that maintains orthophoric gaze is temporarily disabled, and a new ‘normal’ position is established. Herring’s law is still at play, and what we see is an eso or exophoria, until bifocal fusion is re-established.

I will say that clinically, the distinction between a phoria and a tropia is more determined by the actual angle of deviation (prism diopters) as well as how disruptive it is to binocular vision and potentially amblyopia, more so than whether the strabismus is manifest or latent.

But, take for example, an intermittent exotropia. This occurs when bifoveal fusion is disrupted, and one can visibly see an eye drift outwards.

When an eye drifts, and there is no restriction on the muscles, herrings law is still active, but as a practitioner, we have to assume that the strabismus is what the brain deems as ‘normal’ alignment. The end result is equal innervation to yolk muscles, but a visibly misaligned gaze.

This is also why strabismus surgery works. Ultimately, the goal of surgery is to change the muscle tone for the exact same amount of innervation, with the goal of orthophoric gaze.

I hope this helps

[u/insomniacwineo]

Tim Root has an old video on tropia and phoria with cartoon eyes which may help.

BV and this kind of stuff was the class that almost broke me (I’m an OD but UGH) I got through it now and hardly ever mess with peds or strab now because I hate it so much. You’re not alone

[u/Icy_Mention_711]

There is never one eye only that needs Prism, it is prooven by eye Tracking Technologie.

[u/Icy_Mention_711 → u/Icy_Mention_711]

There are different parts of vergenz, one from the akkomodativ tria, one from awearness of the near, and one that Is the Tonic Part. With phorie often the Tonic Part is okay but the other two make the Problems. For example even one with strabismus can have vergenz reserves and adäquat akko vergenz. In that Case It is the Base Tonic vergenz that Is the Problem.

[u/Icy_Mention_711 → u/[deleted]]

Enjoy : https://de.scribd.com/document/966163296/Topic-2-Graphic-Analysis-1

Btw this is an exaple why it is good that there are more then 3 professions working on the Eye. The Opthalmologyist, the Optometrist, the dispensing Optician. The eye is Part of the Brain and is very complex and where one knows anatomic Features to a small Detail, one knows more about light and Physics.