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.

Internuclear Opthalmoplegia

ino.v2Internuclear opthalmoplegia (INO) is an impairment in lateral conjugate gaze (both eyes looking toward one side), caused by a lesion in the medial longitudinal fasciculus (MLF), and associated with multiple sclerosis.

Lateral conjugate gaze requires coordination of adduction (medial rectus muscle, CN III) in one eye and abduction (lateral rectus muscle, CN VI) in the other eye. These movements are coordinated by the paramedian pontine reticular formation (PPRF), also known as the pontine gaze centre. The pathway is as follows:

  1. To look to the left, the right frontal eye field (FEF) sends a signal to the left PPRF.
  2. The left PPRF innervates the left abducens (CN VI) nucleus, which controls the left lateral rectus muscle and causes the left eye to abduct (gaze to the left).
  3. Additionally, the left CN VI nucleus innervates the right oculomotor (CN III) nucleus, which controls the right medial rectus muscle and causes the right eye to adduct (gaze to the left). The MLF is the tract connecting the CN VI nucleus to the contralateral CN III nucleus.

In INO, there is damage to the MLF, giving a deficit in adduction of the corresponding eye during conjugate lateral gaze, but convergence (eye crossing) is classically preserved because that is controlled by a different pathway. In very mild cases of INO, the only deficit is a slowed velocity of the affected eye. For naming, a right INO (as in the sketch) involves damage to the right MLF, which means that the right eye can’t adduct to look to the left, but can abduct to look to the right.

INO may also be associated with gaze abnormalities such as nystagmus, skew deviation, and even abduction or convergence deficits.

The causes of INO include: multiple sclerosis, pontine glioma, and stroke.

  • Flaherty AW, Rost NS. 2007. Eyes and vision. In: Massachusetts General Hospital Handbook of Neurology. Lippincott Williams & Wilkins.
  • Frohman EM, Frohman TC, Zee DS, McColl R, Galetta S. 2005. The neuro-opthalmology of multiple sclerosis. The Lancet Neurology; 4:111.
  • Ropper AH, Brown RH. 2005. Disorders of ocular movement and pupillary function. In: Adams and Victor’s Principles of Neurology. McGraw-Hill.
  • Wilkinson I, Lennox G. 2005. Cranial nerve disorders. In: Essential Neurology. Blackwell.

 

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