LMN paresis and paralysis: Acquired myasthenia gravis

44 LMN paresis and paralysis

Acquired myasthenia gravis


Tick paralysis

Envenomation by the Ixodes (Australia) or Dermacentor (USA) ticks causes a sudden onset, rapidly ascending flaccid paralysis by preventing release of Ach from the nerve terminus. Cats appear resistant to the salivary neurotoxin of Dermacentor spp. Clinical cases of tick paralysis in Australia coincide with the larger tick population in the spring and summer. Dogs and cats are affected most commonly in late winter and early spring prior to developing humoral immunity.

Ascending flaccid paralysis occurs 5–9 days after the attachment of the Dermacentor tick, progressing to tetraplegia within 3 days. Spinal reflexes are absent (patellar reflex lost before withdrawal reflex). A bark reduced in volume and changed in character, facial weakness and jaw weakness are found.

Signs commence 5–7 days after the attachment of the Ixodes tick, depending on the rate of feeding rather than the absolute number of ticks. High ambient temperatures shorten the course of the disease while in cold weather, signs may not commence until 2 weeks after tick attachment. Ascending flaccid paralysis progresses from hind to forelimbs within a few hours, leaving the animal in lateral recumbency. Spinal stretch reflexes such as the patellar reflex are quickly lost, followed by the withdrawal reflex which weakens as the disease progresses. The gag reflex is absent leading to pseudoptyalism. Voice change and regurgitation are common and early findings. Hypothermia develops. Centrally mediated sympathetic stimulation produces peripheral vasoconstriction and systemic hypertension with secondary pulmonary oedema. Tachycardia or reflex bradycardia and pupil dilation also occur. The pupillary light reflex eventually fails. Respiratory rate falls but tidal volume remains unchanged. Respiration is laboured, with a grunt on expiration from closure of the vocal folds. Cats have a similar presentation with initial signs of agitation, coughing, retching, a changed voice and prominent mydriasis.

Some dogs affected by Ixodes ticks have focal weakness of laryngeal, pharyngeal and oesophageal muscles without appendicular weakness. Megaoesophagus with or without dysphagia may be the sole presenting sign.

Diagnosis is aided by clinical improvement seen after tick removal. Engorged ticks may have fallen off by the time the animal is presented.


Myopathies are characterized by exercise intolerance, a stiff, stilted gait, lameness alternating between limbs, lameness worsening with exercise (cf. orthopaedic disease), muscle swelling or atrophy and pain. Weakness may occur but proprioception is normal and spinal reflexes are generally intact. Dysphagia, regurgitation, masticatory muscle atrophy and a weak bark also occur. AST, ALT and CK are not uniformly elevated in myopathies. If raised, their decline may be used to monitor any response to treatment.

Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on LMN paresis and paralysis: Acquired myasthenia gravis

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