A 55-year-old woman is brought to the hospital due to a day of worsening dyspnea. The patient has no cough, chest pain, or fever. The day before the onset of dyspnea, she had blurred vision, dry mouth, and mild nausea. The patient takes lisinopril for hypertension. She does not use tobacco, alcohol, or illicit drugs. Temperature is 36.8 C (98.2 F), blood pressure is 120/70 mm Hg, pulse is 65/min, and respirations are 18/min. Pulse oximetry on room air is 93%. The patient is alert but has increased work of breathing. The pupils are 6 to 7 mm and sluggishly reactive. The mucous membranes are dry. Cardiopulmonary and abdominal examinations show no abnormalities. The neck muscles are weak. Muscle strength is 4/5 in the upper extremities. Sensations are normal. Deep tendon reflexes are 2+, and Babinski sign is absent. Arterial blood gas analysis on room air reveals the following: What is the most likely cause of abnormal arterial blood gas findings in this patient?

Question

A 55-year-old woman is brought to the hospital due to a day of worsening dyspnea.  The patient has no cough, chest pain, or fever.  The day before the onset of dyspnea, she had blurred vision, dry mouth, and mild nausea.  The patient takes lisinopril for hypertension.  She does not use tobacco, alcohol, or illicit drugs.  Temperature is 36.8 C (98.2 F), blood pressure is 120/70 mm Hg, pulse is 65/min, and respirations are 18/min.  Pulse oximetry on room air is 93%.  The patient is alert but has increased work of breathing.  The pupils are 6 to 7 mm and sluggishly reactive.  The mucous membranes are dry.  Cardiopulmonary and abdominal examinations show no abnormalities.  The neck muscles are weak.  Muscle strength is 4/5 in the upper extremities.  Sensations are normal.  Deep tendon reflexes are 2+, and Babinski sign is absent.  Arterial blood gas analysis on room air reveals the following:  What is the most likely cause of abnormal arterial blood gas findings in this patient?

Quizplus · Accepted Answer

Explanation: 11ec25c8_d53a_302a_bc81_910dddc5d16b_MD0003_00 This patient with low pulse oximetry measurements has respiratory acidosis, evidenced by a decrease in pH (<7.35) and an increase in PaCO₂ (>40 mm Hg) (ie, hypercarbia), which is consistent with hypoventilation. Because her respiratory rate is normal, hypoventilation is likely due to decreased tidal volume (minute ventilation = respiratory rate × tidal volume). In this patient with dry mouth, sluggish pupils, and weakness of the neck and upper extremities, this presentation is concerning for impaired neuromuscular transmission with diaphragmatic paralysis, possibly due to botulinum toxicity. ₂ Botulism can occur when preformed botulinum toxin (ie, foodborne botulism) is ingested, which impairs the presynaptic release of acetylcholine at the neuromuscular junction and generally results in: presynaptic release Symmetric, descending motor paresis that begins with the cranial nerves, often including the oculomotor nerve (CN III), trochlear nerve (CN IV), and abducens nerve (CN VI) (eg, blurry vision, diplopia, ptosis, mydriasis) and the glossopharyngeal nerve (CN IX) and vagus nerve (CN X) (eg, dysphagia, suppressed gag) Hypoventilation and respiratory acidosis due to diaphragmatic paralysis and/or upper airway compromise Autonomic dysfunction (eg, ileus, orthostatic hypotension, urinary retention) In addition, patients typically have a normal or slow pulse; normal blood pressure; and no fever, sensory defects, or changes in consciousness. Although the etiology of foodborne botulism may be obvious (eg, onset of symptoms within hours of eating home-canned foods), it may not always be immediately apparent, as in this patient. The differential diagnosis includes myasthenia gravis, but in this condition pupillary function is generally spared, autonomic dysfunction is less prominent, and symptom progression is typically less rapid than in botulism. (Choice A) Decreased central respiratory drive, muscle weakness, and cranial neuropathies can be seen with stroke. However, weakness is typically unilateral, Babinski sign is often present, and patients are typically older (age >65) or have other risk factors. (Choice B) Impaired axonal transmission occurs in Guillain-Barré syndrome, a polyneuropathy caused by immune-mediated demyelination of peripheral nerve fibers. It can progress to diaphragmatic paralysis, but it presents with ascending, not descending, weakness. (Choice D) Increased airway resistance can occur in many diseases, most notably chronic obstructive pulmonary disease (COPD) and asthma. Asthma attacks or COPD exacerbations can present with respiratory acidosis but would also likely present with tachypnea and tachycardia without blurry vision or muscle weakness. (Choices E and F) Increased thoracic cage stiffness can occur due to scoliosis or ankylosing spondylitis. Intrinsic skeletal muscle weakness can occur due to muscular dystrophies. These conditions can result in respiratory insufficiency that presents with a restrictive pattern. However, they typically progress slowly, rather than occurring over 1-2 days, and they are accompanied by ascending weakness. Educational objective: Botulism presents with symmetric, descending motor paresis that can progress to involve the diaphragm, causing hypoventilation and respiratory acidosis. References: Public health risk associated with botulism as foodborne zoonoses. (http://www.ncbi.nlm.nih.gov/pubmed/31905908)

A 55-Year-Old Woman Is Brought to the Hospital Due to a Day