DIP Episode 125 - USMLE Step 2CK Rapid Review Series 6 (IM)

Source / episode info

• Episode: 125
• Title: Divine Intervention Episode 125 – USMLE Step 2 CK Rapid Review Series 6 (IM)
• Published: 2019-07-20
• Source: Episode page

One-liner

This episode provides a rapid review covering high-yield topics including HSV meningoencephalitis and subarachnoid hemorrhage differentials, the classic signs of transtentorial herniation (CN III palsy), tuberculosis diagnosis protocols (PPD cutoffs, INH toxicity), rhabdomyolysis management, and the pathophysiology of Type I through IV hypersensitivity reactions.

High-yield summary

• HSV Encephalitis: Classically presents with temporal lobe involvement, seizures, or focal neurological deficits; CSF findings may show lymphocytic pleocytosis (though red blood cells can be seen in SAH).
• Transtentorial Herniation: Compression of CN III causes a "down and out" eye due to paralysis of the levator palpebrae superioris and inferior rectus. Compression of CN VI/CN V leads to other deficits; compression of CN III also impairs parasympathetic function, leading to mydriasis (blown pupil).
• Tuberculosis (TB): The PPD cutoff values are critical: 15 mm for healthy individuals, 10 mm for healthcare workers, and 5 mm for immunocompromised patients (e.g., HIV/IV drug users). Latent TB is treated with Isoniazid (INH) for nine months plus Vitamin B6 supplementation to prevent peripheral neuropathy.
• Rhabdomyolysis: Characterized by muscle injury leading to myoglobinuria, which can cause acute kidney injury (AKI) and electrolyte abnormalities like hyperkalemia and hypocalcemia. Treatment involves aggressive IV fluid resuscitation.
• Hypersensitivity Reactions: Type I is IgE-mediated (e.g., asthma); Type II is antibody-mediated against fixed antigens (e.g., autoimmune hemolytic anemia); Type III is immune complex deposition (e.g., post-streptococcal glomerulonephritis); and Type IV is delayed T-cell mediated (e.g., contact dermatitis).

Learning objectives

• Differentiate between HSV meningoencephalitis, subarachnoid hemorrhage, and other causes of altered mental status with elevated CSF red blood cells.
• Describe the specific cranial nerve deficits associated with transtentorial herniation (CN III palsy).
• Apply appropriate PPD cutoff values for diagnosing tuberculosis based on patient risk factors.
• Recognize the pathophysiology and acute management steps for rhabdomyolysis, including electrolyte correction.
• Classify and provide clinical examples of Type I through IV hypersensitivity reactions.

Board exam buzzwords

Rapid review table

Board-speak -> diagnosis

Differential diagnosis / distinguishing features

Cranial Nerve Palsies due to Herniation

Hypersensitivity Reactions

Management pearls

• For suspected rhabdomyolysis, aggressive intravenous fluid resuscitation (e.g., Normal Saline) is paramount to maintain urine output and prevent myoglobin precipitation in the renal tubules.
• In latent TB infection, treatment requires Isoniazid (INH) for nine months plus mandatory Vitamin B6 supplementation to prevent peripheral neuropathy.
• For close contacts of active TB, the preferred prophylactic agent is Rifampin, but if pregnancy is suspected, Rifampin and Rifampicin are teratogenic; use Rifampin (a third-generation cephalosporin) instead.
• The initial workup for altered mental status with elevated CSF RB Cs must differentiate between hemorrhagic causes (SAH) and infectious/inflammatory causes (HSV).

Don't miss

Integration & clinical reasoning

• Neuroanatomy: Understanding the relationship between the temporal lobe, CN III, and the midbrain (via the cerebral peduncle) is crucial. Compression of the CN III/CN VI complex leads to characteristic signs that help localize the site of increased intracranial pressure or mass effect.
• Pharmacology & Metabolism: The mechanism by which INH depletes Vitamin B6 (pyridoxine) leading to peripheral neuropathy, and the subsequent need for supplementation, links drug metabolism directly to neurological outcomes.
• Pathophysiology: Hypersensitivity reactions demonstrate how immune responses can be highly specific but also cause widespread tissue damage through complex mechanisms (e.g., complement activation in Type III).

OMM / COMLEX integration

• Acute/Unstable Management Priority: In any patient presenting with altered mental status or AKI (rhabdomyolysis), standard emergency management (Airway, Breathing, Circulation, IV fluids) takes absolute priority over OMT assessment.
• Neurovascular Assessment: The detailed understanding of cranial nerve deficits due to herniation is critical for physical exam skills and recognizing signs of rapidly deteriorating intracranial pressure.

Concept connections / cross-references

• For detailed information on CNS infections and meningoencephalitis: [Episode 102]
• For comprehensive review of electrolyte imbalances and acid-base disorders: [Episode 88]
• For general principles of inflammatory myopathies and muscle injury: [Episode 45]

High-yield association table

Key terms glossary

Study optimization

Question pattern recognition

• The "Red Flag" Differential: When a patient presents with vague symptoms (altered mental status, fever), always generate a differential diagnosis that includes both infectious/inflammatory causes and acute vascular events (e.g., HSV vs SAH).
• Classic Triad Recognition: Recognize the classic triad of signs associated with specific pathologies (e.g., "down and out" eye + blown pupil + CN III palsy = herniation).
• Risk Stratification in Screening: Never assume a single test result is definitive; always apply risk stratification rules (like PPD cutoffs) to adjust interpretation.

Test yourself

Common mistakes to avoid

Common traps