A 28-year-old woman comes to the clinic reporting progressive fatigue, mild shortness of b… — USMLE Step 2 CK

A 28-year-old woman comes to the clinic reporting progressive fatigue, mild shortness of breath on exertion, and tingling in her fingertips over the past two weeks. She has a long-standing history of binge-eating disorder characterized by eating large amounts of food with a sense of loss of control, without any compensatory purging behaviors. Three weeks ago, her psychiatrist initiated a new medication that is also indicated for migraine prophylaxis to help manage her binge-eating episodes and promote weight loss. Her body mass index (BMI) is $32.4 \text{ kg/m}^2$ . Her temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), pulse is $88/\text{min}$ , blood pressure is $118/76\text{ mm Hg}$ , and respirations are $18/\text{min}$ .

Laboratory studies reveal the following:

Test	Result
Serum sodium	$141\text{ mEq/L}$
Serum potassium	$3.8\text{ mEq/L}$
Serum chloride	$115\text{ mEq/L}$
Serum bicarbonate	$16\text{ mEq/L}$
Blood urea nitrogen	$14\text{ mg/dL}$
Serum creatinine	$0.9\text{ mg/dL}$

Arterial blood gas analysis on room air shows:

Test	Result
pH	$7.31$
PaCO2	$32\text{ mm Hg}$
PaO2	$95\text{ mm Hg}$

Which of the following best describes the acid-base disturbance in this patient resulting from her new psychiatric medication?

Normal anion gap metabolic acidosis with appropriate respiratory compensationAnswer
B
Normal anion gap metabolic acidosis with concurrent respiratory alkalosis
C
Normal anion gap metabolic acidosis with concurrent respiratory acidosis
D
High anion gap metabolic acidosis with appropriate respiratory compensation
E
Metabolic alkalosis with appropriate respiratory compensation

Answer

Normal anion gap metabolic acidosis with appropriate respiratory compensation

The correct answer identifies a normal anion gap metabolic acidosis with appropriate respiratory compensation. The clinical vignette describes a patient with binge-eating disorder who was recently started on topiramate, an anticonvulsant frequently used off-label for weight reduction in binge-eating disorder and indicated for migraine prophylaxis. Topiramate inhibits carbonic anhydrase in the proximal renal tubule, leading to decreased bicarbonate reabsorption and a subsequent Type 2 renal tubular acidosis. This presents as a normal anion gap metabolic acidosis (hyperchloremic acidosis). The patient's anion gap is calculated as

141 - (115 + 16) = 10 \text{ mEq/L}

, which is within the normal limits of 8-12 mEq/L. To determine if the respiratory compensation is adequate, Winter's formula is applied: Expected PaCO2 =

(1.5 \times 16) + 8 \pm 2 = 32 \pm 2 \text{ mm Hg}

. Because the measured PaCO2 is exactly

32 \text{ mm Hg}

, the patient has an appropriately compensated simple metabolic acidosis without any concurrent respiratory disorders.

Step-by-Step Solution

Identify the primary psychiatric medication and its associated adverse effects.

The patient with binge-eating disorder is taking topiramate (indicated for weight loss and migraine prophylaxis). A classic side effect of topiramate is paresthesias and a type 2 renal tubular acidosis due to carbonic anhydrase inhibition.

Understanding the pharmacotherapy for eating disorders is required to anticipate the physiologic complications presented in the laboratories.

Determine the primary acid-base disturbance and calculate the serum anion gap.

The pH is 7.31 (acidemia) and serum bicarbonate is 16 mEq/L, indicating a primary metabolic acidosis. The anion gap is 141 - (115 + 16) = 10 mEq/L, which is a normal anion gap.

Calculating the anion gap differentiates the cause of the metabolic acidosis. A normal anion gap confirms the topiramate-induced bicarbonate loss rather than an unmeasured anion accumulation.

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO2 = (1.5 × HCO3) + 8 ± 2. Substituting the values: (1.5 × 16) + 8 = 24 + 8 = 32 ± 2 mm Hg. The measured PaCO2 is 32 mm Hg.

Comparing the measured PaCO2 to the expected PaCO2 determines if a secondary respiratory acid-base disorder is present. Since they match perfectly, there is appropriate respiratory compensation.

Key Concept

Topiramate is a pharmacologic option for binge-eating disorder that can cause a normal anion gap metabolic acidosis (Type 2 RTA) due to carbonic anhydrase inhibition. Winter's formula [Expected PaCO2 = (1.5 × HCO3) + 8 ± 2] is used to verify appropriate respiratory compensation in metabolic acidosis.

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