Motor Neuron Diseases for NEET PG / FMGE:

Content: MOTOR NEURON DISEASE – FOR NEET PG/FMGE

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Definition:

It is the disease of motor neurons.
It is an irreversible loss of motor neurons and gliosis; in motor cortex, motor nuclei of brainstem and anterior horn of spinal cord.

It can affect any age but most patients are over 40 years old at the time of diagnosis.

90% MNDs are sporadic in nature.

There won’t be any sensory involvement.

Nomenclature:

  1.    UMN lesion of Cortico-bulbar tract: Pseudo Bulbar Palsy.

  2.    LMN lesion Cortico-bulbar tract: Progressive Bulbar Palsy.

  3.    UMN lesion of corticospinal tract: Primary Lateral Sclerosis.

  4.    LMN lesion of corticospinal tract: Progressive Spinal Muscle Atrophy.

  5.    Both UMN and LMN involvement: Amyotrophic Lateral Sclerosis.

Points about: ALS

  -         Most common type of MND.

  -         Men > Women.

  -         Smoking – Independent risk factor for sporadic ALS.

  -         SPLIT HAND phenomenon: Severe changes in thenar eminence and the relative sparing of hypothenar eminence, observed in EMG study.

  -         Coronal T2WI MRI: Bilateral symmetrical hyper intensity along corticospinal tracts forming a ‘Wine Glass’ appearance.

Impaired neuronal viability is due to:

 -         Superoxide dismutase 1 transgenes.

 -         Hexanucleotide repeats.

 -         ALS gene leading to defective axonal cytoskeleton.

Clinical Features:

  -         Asymmetric muscle weakness.

  -         Normal sensations.

  -         Difficulty in swallowing, chewing and tongue movements. Tongue fasciculation may be present.

  -         Death is due to respiratory failure.

Normal in MNDs:

-         Eye movement.

-         Bladder and bowel.

-         Mentation.

-         Sexual function.

Investigations:

-         EMG: Most confirmatory.

-         Biopsy shows atrophy.

-         Diagnosis is considered definitive if 3 out of 4 areas are involved: bulbar, cervical, and thoracic and lumbosacral motor neurons.

Drugs:

  -         Riluzole:
Reduces glutamate induced exitotoxicity.
Recommended dose is 100mg per day.

  -         Edavarone:

Free radical scavenging agent.
It just slows the disease progression. Recommended dose is 30 – 60 mg per day IV Infusion.

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Bell's Palsy

Bell’s Palsy.

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Definition:

Acute Idiopathic Lower Motor Neuron Type of Facial nerve palsy.

Understanding:
Acute – sudden in onset
Idiopathic – the cause is not known
Lower Motor Neuron – Infranuclear Pathology.

So there will be all features of Lower Motor Neuron type of facial nerve palsy.

Etiology:

Incidence: Bell’s palsy is more in women than in men.
Some literature suggests that it is more common in patients with diabetes.
One of the risk factors also includes Pregnancy.

As already explained the cause of Bell’s palsy is unknown i.e. idiopathic but there are some theories which suggest some viral or auto-immune etiology.

Herpes Simplex Virus 1 (HSV1) was frequently detected in endoneurial fluid and posterior auricular muscle in patients with Bell’s palsy, suggesting that reactivation of this virus in geniculate ganglion may be responsible for most of the cases.

Also, reactivation of the varicella-zoster virus is associated with Bell’s Palsy in up to 1/3^rd of the cases.

Clinical Features:

Onset – characteristically Acute (about one-half of cases attain maximum paralysis within 48 h and practically all within 3 or 4 days).

In most of cases, pain behind the ear may precede the paralysis by a day or two.

Impairment of taste is present in most patients but it rarely persists beyond the second week of paralysis.

Hyperacusis can be experienced in the ipsilateral ear which indicates the involvement of the stapedius muscle.

All features of LMN type of facial palsy will be seen, which are:

·       Drooping of corner of the mouth.

·       Drooling of saliva from angle of mouth.

·       Loss of nasolabial fold.

·       Angle of mouth deviates to opposite side of the lesion.

·       Forehead involved i.e. unable to raise an eyebrow on the same side of the lesion.

We already discussed the concept of LMN and UMN facial nerve palsy conceptually in detail in our previous videos. Link for those videos: 

Facial Nerve Basics:

https://www.youtube.com/watch?v=eYCKoMvCqNo&t=43s

Facial Nerve Clinical Co-relation: 

https://www.youtube.com/watch?v=cnuBuskixzA

Then there can also be corneal ulceration due to inability to close the eye during sleep.

Differential Diagnosis:

First: Ramsay Hunt Syndrome: It is caused by reactivation of Herpes Zoster in geniculate ganglion characterized by facial nerve palsy associated with vesicular eruption in external auditory canal. Often the eighth cranial nerve is also involved in Ramsay Hunt Syndrome.

Second: Any Middle Ear disease involving 7^th nerve like cholesteatoma.

Third: Acoustic Neuroma: It is the most common Cerebro-Pontine angle tumor that arise from the Schwann cell sheath of vestibulocochlear nerve.
The characteristic feature of Acoustic Neuroma is that in MRI it shows ice-cream cone appearance.
This due to its extension of tumor into the intracanulicular part of the internal acoustic meatus.

Tumors that invade temporal bone (example: carotid body, dermoid)

Investigations:

It is usually diagnosed clinically after taking proper history.

There is no particular diagnostic procedure or tests for diagnosis of Bell’s Palsy.

MRI often shows swelling and enhancement of the facial nerve.

To rule out alternate diagnosis, appropriate investigations can be done if needed.

Treatment:

Symptomatic measures: Use of paper tape to depress the upper eyelid during sleep. Prevent corneal drying by artificial tears.

The course of Prednisolone, given as 60-80 mg daily during the first 5 days and then tapered over the next 5 days. It shortens the recovery period.

Combination of Prednisolone with anti-virals like acyclovir or valacyclovir has no added benefit. But if given, the dose should be:

Acyclovir: 400 mg 5 times a day for 10 days or
Valacyclovir 1000 mg daily for 5-7 days.

Prognosis:

Majority of patients recovers very well (>90%) if recovers starts within a few days.

Poor prognosis is seen if pt. have lost of taste sensations and hyperacusis.

Those who recover may show signs of anomalous re-innervations like “crocodile tears”.

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