Amyotrophic Lateral Sclerosis (ALS) & the corticospinal tract

corticospinal_tract

Amyotrophic Lateral Sclerosis (ALS) is a degenerative disease of the motor neurons in the brain and spinal cord. It progressively affects all the muscles in the body but there is no known cause and no treatment. Only about 5-10% of cases are inherited while the rest are sporadic.

The neurons ALS affects are primarily the upper motor neurons. These are the ones that originate in the brain and travel down the spinal cord. These neurons then synapse with the lower motor neurons in the ventral horn, and it is the lower motor neurons that go directly to the muscles.

In ALS there are both upper motor neuron and lower motor neuron symptoms. As the neurons die, a constellation of symptoms including numbness, weakness and paralysis emerge. Eventually the paralysis progresses leading to inability to speak, swallow and breath. There is no cure for ALS and treatments only help with the symptoms, they do not slow the progression of the disease.

So you may have seen a lot of ice bucket challenges over the last few weeks but please support this cause as it is a horrible disease that up until now had almost no recognition or support. So please donate to The ALS Association (alas.org).

And in case you get tired or jaded seeing your social media full of these videos, watch this one of my father doing it. He’s not an emotional guy, but he has lost more than his fair share of friends to this disease.

donate to help fund ALS research and support from Ali & Mike on Vimeo.

Brachial Plexus Part 1 – anatomical relations

brachial_plexus

The brachial plexus is the bane of many med students’ existence during any sort of neuro block. So many nerves, so many connections, so many seemingly arbitrary names of different sections. It’s just a woven mess of misery. (especially when they start getting into the “where is the lesion” questions)

Thus I’ve decided to have a couple posts about the brachial plexus, hopefully demystifying it to some extent. This first doodle is about the brachial plexus and its anatomical relationship to some of the structures that show why anatomists who named the parts weren’t as crazy as they seem.

Important structures to remember because they explain why parts are named the way they are:

  1. Vertebrae
  2. Anterior and posterior scalene muscles
  3. Subclavian artery
  4. The arm (in its anatomical position)

Vertebrae: There are 7 cervical vertebrae and 12 thoracic vertebrae. To make things confusing the cervical spinal nerves exit ABOVE their named vertebrae (except for C8) while the thoracic, lumbar and sacral exit BELOW. This messes up the whole numbering system because there are SEVEN cervical vertebrae but there are EIGHT cervical spinal nerve roots. The brachial plexus generally includes the nerve roots C5-T1*
* I say generally because there’s are anatomical variations such as a “prefixed” plexus that goes from C4-C8 and a “postfixed” plexus that goes from C6-T2

Scalene Muscles: The brachial plexus is nestled between the scalenes in the neck. At this point the plexus is oriented up and down and therefore the trunks are superior (closest to your noggin), middle, and inferior.

Subclavian Artery/Anatomical Position: The artery is in front of the plexus at the level of the trunks and then the plexus starts to wrap around it (or at least seems to because we don’t keep our arms straight out to our sides in “anatomical position” at all times). The cords are named for their relationship to the artery. One is lateral (again, if the arm was held out to the side), one is posterior and one is medial (think closest to armpit).

 

Subdivisions of the Brachial Plexus

The parts are: Roots/Trunks/Divisions/Cords/Branches or, as I remember them being a classy east coast Canadian: Real/Truckers/Drink/Cold/Beer

Then you might think, “But how do I remember which of the terminal branches comes off where?” For that I think of the two “M” branches being on the M: Musculocutaneous, Median and (M)Ulnar and that the whole thing together can just be said as “MARMU” Pick the mnemonics you want, the brachial plexus is rife with them. I personally just like the sound of the word marmu.

Vertebral Disc Prolapse (slipped disc)

A prolapsed (slipped) disc is when the squishy innards of the disc (nucleus pulposus) bulge out past the stiffer wall of the disc (annulus fibrosis). The problem is that sometimes when this happens, the bulge can impinge the spinal cord or the spinal nerve root. This could result in an anterior cord syndrome (remember this doodle) or it could just knock out the nerve root, resulting in a specific radiculopathy (check out this doodle for where to check for numbness and weakness).

The tricky thing to remember is that though, for example, the L3 root exits at L3, if the L3,4 disc herniates, it doesn’t hit the L3 root but the L4.

Slipped L3,4 disc = L4 nerve injury

The disc hits the nerve after it has branched off the spinal cord, but before it has exited the  vertebral canal.

Innervation of the lower leg

The lower leg (and especially the foot) have a pretty fancy pattern of skin innervation by the terminal branches. For example, the skin of the foot is innervated by 7 separate nerves:

  1. Superficial peroneal nerve
  2. Deep peroneal nerve
  3. Sural nerve
  4. Saphenous nerve
  5. Calcaneal branch of the tibial nerve
  6. Medial branch of plantar nerve
  7. Lateral branch of plantar nerve

Also good to keep in mind that the anterior compartment is innervated by the deep peroneal nerve, the lateral compartment by the superficial peroneal nerve and the posterior compartment by the tibial nerve.

Femoral Triangle

The femoral triangle is a convenient triangle where the femoral nerve, artery and vein pass from the abdomen to the leg. The best part about this is that they’re all quite superficial, making it a great place to stick things in (place catheters, nerve blocks, etc).

Because the femoral triangle is often getting poked at for various reasons, it’s important to know what’s where because you don’t want to be hitting the nerve when you meant for the artery (or vice versa).

The triangle is made up by the sartorius, adductor longus and inguinal ligament and if you just remember NAVVAN.

DANGER ZONE!!! (the cavernous sinus)

The danger zone on the face is a little triangle from the corners of the mouth up to the bridge of the nose. The reason it has such an epic name is because due to its venous drainage (from the facial veins and pterygoid plexus) there’s the possibility of infection traveling from that area into the cavernous sinus.

The cavernous sinuses (there’s one on each side) is an area posterior to the maxillary sinuses and lateral to the pituitary. It receives blood from the superior and inferior ophthalmic veins, superficial cortical veins and the basilar plexus. The blood then drains into the petrosal sinuses (you guessed it, there’s a superior and inferior one of those too) and then those drain into the internal jugular vein.

The thing about the cavernous sinuses a whole lot of important stuff passes through it.

  • CN III (occulomotor)
  • CN IV (trochlear)
  • CN V1 (ophthalmic branch of trigeminal)
  • CN V2 (maxillary branch of trigeminal)
  • CN VI (abducens)
  • Internal carotid (and the sympathetic fibres on the carotid)

This means that if you are so unfortunate as to have infection tract back into it, there can be some nasty consequences like meningitis and cavernous sinus thrombosis which will generally present as problems involving those nerves.

The abducens and carotid are more medial and thought to be more bathed in the warm loving venous drainage meaning these are generally the first to show signs of a problem a-brewin’.

Find that lesion! (deep tendon reflexes of the arm)

The arm has too many muscles. It also has too many nerves.

The problem is that someone comes in with weakness or numbness and you need to think, “Where in the brain/spine/nerve root/bits of brachial plexus/terminal branch is the actual problem?!”

One of the ways to help suss this out is by testing the deep tendon reflexes.

  • Only biceps reflex absent – might be problem with musculocutaneous nerve or C5
  • Both brachioradialis and triceps absent – problem with the radial nerve
  • Only triceps absent – potential problem with C7

Of course you should correlate the reflex findings with the sensory findings and motor strength (remember the good old ASIA exam for testing specific nerve roots) to help determine if it is a terminal branch issue or something higher up.

And don’t forget to grade those reflexes!

Grade Description
0 Absent
1+ Diminished
2+ Normal
3+ Brisk
4+ Very brisk +/- Clonus

Generally upper motor neuron (UMN) lesions result in hyperreflexia while lower motor neuron (LMN) lesions result in hyporeflexia.

Median Nerve Distribution

The median nerve provides sensory and motor innervation to the anterior compartment of the forearm and hand.

  • Motor branches
    • Pronator teres
    • Flexor carpi radialis
    • Flexor carpi sublimis
  • Anterior interosseus (motor)
    • Flexor pollicis longus
    • Flexor digitorum profundus to 2nd & 3rd fingers
    • Pronator quadratus
  • Palmar cutaneous
    • Sensory distribution: Skin over thenar eminence
  • Terminal motor
    • Abductor pollicis brevis
    • Opponens pollicis
    • Lumbricals: 1st & 2nd
    • ± Flexor pollicis brevis: Also innervated by ulnar nerve
  • Terminal sensory
    • Sensory to palmar surface of thumb, 2nd, 3rd & lateral 1/2 of 4th finger

Physical exam for different muscles supplied by median nerve

  • Flexor digitorum superficialis: patient flexes fingers at PIP joint against resistance
  • Flexor digitorum profundus: patient flexes fingers at DIP joint against resistance
  • Flexor pollicis longus (anterior interosseous nerve): flexes distal phalanx of thumb against resistance
  • Abductor pollicis brevis: patient abducts thumb at right angles to palm against resistance
  • Opponens pollicis: patient touches base of little finger with the thumb, examiner tries to pull  apart
  • 1st lumbrical interosseous: patient extends finger at the PIP joint against resistance with the MCP joint hyperextended and fixed

Neural control of micturition

Nerve control of the bladder

  1. Sympathetic: hypogastric nerve (T10-L2), involuntary control of bladder neck and intrinsic sphincter
  2. Parasympathetic: pelvic nerve (S2-S4), involuntary contraction of detrusor
  3. Somatic: pudendal nerve (S2-S4), voluntary contraction/relaxation of external sphincter
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