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Dr. P here, we've got an 80-year-old man with a

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3 00:00:05,340 --> 00:00:09,920 "lump" in the undersurface of his foot.

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Let's start scrolling, but I think the

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take-home message on this case is the

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specificity of the diagnosis and the signals

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that help you come to that conclusion.

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On the left, T1-weighted image, simple

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fat-weighted, and we have run into a blob.

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It looks a little bit like a peanut.

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Or maybe even a snowman. You know, there's

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the head of the snowman and there's

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the base of the snowman right there.

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And then we go to the simple

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T2 spin echo image right there.

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And there's no fat suppression there, so that

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gives you an idea of the signal of this blob.

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It's got some low signal and then towards

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the center of it, even though it's

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kind of squished together between the

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metatarsals, between M3 and M4, you've

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got some high signal in the middle there.

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Then we go over to the proton density

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fat suppression, in which everything

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is almost always hyperintense.

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And yet, this is a little hyper-

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intense, but not overwhelmingly so.

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It doesn't look like, say, pure water.

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If you look up high in that area that was high

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on T2, it's still high, but not that high.

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And then around it, certainly the signal

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intensity is a mixture of intermediate and high,

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and high-ish signal intensity,

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but certainly not fluid.

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So that's extremely helpful in

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generating the differential diagnosis.

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Because when you're between the metatarsals,

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what you have to think about is what lives there?

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Nerves?

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Bursa?

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Vessels.

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So is this vascular?

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Probably not.

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There's no methemoglobin here, there's

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no deoxyhemoglobin anywhere, it doesn't

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look like blood, and it certainly doesn't

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look like a varix, which will have some

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pooling or a blood-fluid level inside it.

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And besides, I've probably seen two in 35 years.

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So, that's not particularly common.

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That's a long shot.

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Then you've got bursitis.

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That is very common.

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In fact, most people over the age of 50,

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especially women that are high-heel wearers, will

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have intermetatarsal bursitis and capsulitis.

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But the key to the diagnosis of bursitis

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is the signal is overwhelmingly fluid-like.

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That is not the case here.

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There's a little fluid-like signal here, but it's

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not really pure fluid, and there isn't much of it.

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So that really leaves you with one

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conclusion, and that is the favored

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diagnosis, which is known as Morton's neuroma.

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Now, what is a Morton's neuroma?

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It is not a neuroma in the

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sense that it's not a neoplasm.

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We've got schwannoma, we've got neurofibroma,

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neurofibrosarcoma, and there are some other

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really weird neural tumors that I won't

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bore you with right now, but the diagnosis

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of Morton's neuroma is a reactive one.

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You're talking about perineural fibrosis with

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disorganization of the neural structures.

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They undergo some liquefaction, and so you may see

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some central areas of low signal intensity due to

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nerve degeneration and some inflammatory tissue,

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but then the bulk of the inflammatory tissue

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in the periphery is more fibrous, more dark.

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Look at that blob, and look

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at how dark and fibrous it is.

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It looks almost collagenous, like some of those

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plantar fibromas or fibromatosis that you've seen.

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You've got these funny-looking septa

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right here, and it gets fairly large.

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Now let's go to the post-contrast T1, and when we

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look at the post-contrast images, in the middle,

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you've got the neural tissues with that T1.

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liquefaction phenomena of disorganized

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neural cells and neural structures.

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And then the tissue that is fibrotic,

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the perineurofibrosis, is inflammatory.

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Because this is a friction event, much like you

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have a friction event when you have carpal tunnel

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syndrome, or you have tarsal tunnel syndrome.

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This is one of the entrapment neuropathies.

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Now, what could cause this?

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Well, you put your feet in some

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tight-fitting shoes with pointy toes, and

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you squish the metatarsals and the digits

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together, and then they start to rub and rub,

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and as they do so, they induce this phenomenon.

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So, it is more common in women.

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It likes M3 in particular.

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It occurs much less frequently in

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M1, M2 because of the nerve anatomy.

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Between M3 and M4, two nerves kind of meet

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whereas between M1 and M2 you only have

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one nerve coursing through that area.

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So I have seen them there, but it's not common,

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and it's also quite uncommon between M4 and M5.

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The Morton's neuroma.

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Perineural fibrosis as a reactive phenomenon.

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I use size as an important

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criteria to say Morton's neuroma.

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And that size number is between six and

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eight millimeters in either transverse or

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dorsal and ventral, uh, or plantar direction.

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If it's less than that or ill-defined, then I

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simply call it, uh, perineurofibrosis or PNF

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to drive folks away from trying to excise it.

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Now if you look carefully on this

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case, there is something brewing very

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quietly between M2 and M3 right there.

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I'm going to blow it up a little bit

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for you so you can see it better.

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It enhances very little, if at all.

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It's slightly globular.

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It doesn't meet that six-millimeter criteria

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or eight-millimeter criteria, six to eight.

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And therefore I'd say PNF, perineurofibrosis M2

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M3 and giant Morton's neuroma between M3 and M4.

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Dr. P. out.