When it’s NOT the neck, and why this is important
Generalized Lower Motor Neuron (LMN) disorders are difficult to recognize, particularly in the earlier phases of the disease process. They are commonly mistaken for cervical myelopathies (neck problems). When we see tetraparetic pets, our first thought tends to be ‘NECK’! But it’s not always the neck and recognizing this is important because the diagnostics, treatments and prognoses are very different.
Lower Motor Neuron: Functional motor unit comprised of the motor neuron(s) within the CNS, the peripheral motor nerve(s) the motor neurons give rise to, the neuromuscular junction, and the muscle directly innervated by the nerve. We most commonly think of the lower motor neuron with reference to the limbs, but cranial nerves (motor) can also be an example of LMN.
Lower Motor Neuron dysfunction: Dysfunction arises when ANY component of the motor unit is diseased—that could be the motor neuron itself (as in ALS in people), the peripheral motor nerve (as with Guillain-Barre syndrome and Coonhound paralysis), the neuromuscular junction (myasthenia gravis, Tick paralysis, botulism), or muscle itself (any form of primary acquired or congenital myopathy). Clinically, this looks like weakness in the limbs, characterized by loss of muscle tone or mass and poor to absent myotactic reflexes—most obviously, poor or absent withdrawal strength (the patella reflex is often normal in early stages of many LMN disorders, disappearing only late in the problem). The unexpected finding of a normal or even hyper patella reflex in early LMN disorders can be confusing. We’ve all been brainwashed to think that normal or increased patella reflexes always equates to an UPPER MOTOR NEURON problem…Not always! “Pseudohyperreflexia” in the patella reflex occurs when sciatic distribution muscles lack normal opposing tone; the patella reflex therefore appears exaggerated. Poor or absent withdrawal strength in the limbs is the most common earliest indicator of LMN dysfunction. A cervical myelopathy will never cause poor withdrawal in the pelvic limbs.
Why is it important to differentiate cervical myelopathy from LMN disorders?
A few years ago, there was an article in one of the veterinary news magazines about a miraculous last minute diagnosis that saved a dog scheduled to be euthanized. The dog was non-ambulatory tetraparetic and was being managed by ER staff in a Specialty/ER hospital. The working diagnosis was cervical myelopathy, and the owners could not afford MRI and surgery (for suspected cervical IVDD)…they had decided upon euthanasia. At the dramatic last minute, a tick was discovered on the dog, removed, and within 2 days the dog was up and around. While the doc managing the case was hailed as a hero, the irony is that simple clinical signs were overlooked; signs that should have immediately indicated to the doc that the problem was NOT in the neck. A careful (even abbreviated) neurological exam would have revealed LMN signs (poor withdrawal x 4 is never a cervical myelopathy). Additionally, many tick paralysis dogs do lose patella reflexes early…a dead giveaway—no pun intended. This scenario is common. Misdiagnosis can lead to costly unnecessary diagnosis and/or surgery. Misdiagnosis can lead to misguided euthanasia.
LMN disorders can be life threatening in severe cases, but most will have favorable prognosis, and treatment/management doesn’t have the big price tag of MRI and surgery. Treatment is typically medical, or just nursing care and time.
Pure LMN disorders are relatively uncommon, which is why they are frequently missed. Here are examples of the most common LMN disorders seen in practice:
1. Polyradiculoneuritis (coonhound paralysis): Canine. Immune mediated severe inflammation of the peripheral nerve root/nerve. Similar to Guillain-Barre syndrome in people. Signs typically begin in pelvic limbs and progress as an ‘ascending paralysis’. Pain sensation in limbs normal to hyper. Tail, bladder and rectum are normal. Diminished withdrawal strength in all limbs is the most obvious exam finding supporting this problem. Don’t be fooled by a ‘normal’ patella reflex. High protein in otherwise normal CSF supports diagnosis. Clinical diagnosis most common, no definitive testing exists. Severe cases can advance to respiratory paralysis and death. No proven specific treatments. Some academic institutions have experimented with immunoglobulin therapy, which is used in people, but no proven effective results exist in dogs. Good nursing care and PT will aid in functional recovery which can take weeks to months.
2. Myasthenia Gravis: More commonly recognized in dogs, but occurs in cats also. Immune mediated block at the level of the neuromuscular junction (Auto anti-acetylcholine antibody blocks Ach receptors). Prognosis fair to good IF the condition is not associated with thymoma or other paraneoplastic condition. Weakness, dysphagia/regurgitation, megaesophagus, ventroflexion (cats) are the predominant signs. Key exam findings: exercise induced weakness, palpebral reflex that fatigues and may be lost with rapid repetition, megaesophagus in up to 90% of canine cases. Diagnosis supported by clinical response to IV Tensilon. Diagnosis made by detecting high levels of anti-acetylcholine receptor antibody in serum (send out to IDEXX or directly to the Comparative Neuromuscular lab at UC-San Diego). Treatment with pyridostigmine, sometimes alone, more often combined with prednisone and/or other immunomodulating drugs. Treatment at least 6 months, often longer.
3. Tick Paralysis: Neurotoxin liberated in saliva of dermacentor variabilis and dermacentor andersoni (among others) interferes with the release of acetylcholine at the neuromuscular junction, causing a flaccid ascending motor paralysis. Clinical signs are present when tick is attached and typically engorged, and begin to resolve within 48 hours of removal. Progression to full non-ambulatory tetraparesis/plegia is rapid, often within 12-72 hours of onset of signs. No specific diagnostic testing is available. I will often treat a suspect dog with ‘tick-o-cide’ even if I can’t find a tick (ie-malamute). Again, poor withdrawal x 4 is the hallmark exam finding—patella reflexes often gone also in tick paralysis dogs in my experience.
4. Botulism: Block at the neuromuscular junction from block of ACH release. Intoxication results most commonly from ingestion of toxin in uncooked spoiled meats, less commonly from liberation of toxin from colonizing C. botulinum in the GI tract or liver. Similar in presentation to Tick and Coonhound paralysis, onset of signs are rapidly progressive. Unlike Tick or Coonhound paralysis, cranial nerves may be frequently affected. Autonomic effects may also be seen (pupillary abnormalities, urinary retention, constipation for example). Treatment is supportive. Antitoxin can be considered in acute cases, but will not ‘unbind’ toxin once it has been bound. Gastrointestinal lavage considered if contaminating contents still present.
5. Ischemic Neuromyopathy: Embolism of arterial supply chokes nerves and muscle of its blood/oxygen supply creating LMN paresis/paralysis. More common in cats secondary to cardiac disease. Less common in dogs, this is usually secondary to protein losing nephropathies, advanced Cushing’s, paraneoplastic, idiopathic…generally in that order of likelihood. Flaccid paresis, loss of withdrawal strength and absent patella reflexes are common findings. *this one is just one to remember, as pelvic limbs or single limbs are most often effected, so its not really one that is mistaken for cervical myelopathy.