Dermatomes and myotomes

Dermatomes and myotomes US Medical PG Question 1: A 50-year-old man arrives to the clinic complaining of progressive weakness. He explains that for 3 months he has had difficulty climbing the stairs, which has now progressed to difficulty getting out of a chair. He denies diplopia, dysphagia, dyspnea, muscle aches, or joint pains. He denies weight loss, weight gain, change in appetite, or heat or cold intolerance. He reports intermittent low-grade fevers. He has a medical history significant for hypertension and hyperlipidemia. He has taken simvastatin and losartan daily for the past 6 years. His temperature is 99.0°F (37.2°C), blood pressure is 135/82 mmHg, and pulse is 76/min. Cardiopulmonary examination is normal. The abdomen is soft, non-tender, non-distended, and without hepatosplenomegaly. Muscle strength is 3/5 in the hip flexors and 4/5 in the deltoids, biceps, triceps, patellar, and Achilles tendon reflexes are 2+ and symmetric. Sensation to pain, light touch, and vibration are intact. Gait is cautious, but grossly normal. There is mild muscle tenderness of his thighs and upper extremities. There is no joint swelling or erythema and no skin rashes. A complete metabolic panel is within normal limits. Additional lab work is obtained as shown below: Serum: Na+: 141 mEq/L Cl-: 100 mEq/L K+: 4.3 mEq/L HCO3-: 23 mEq/L Urea nitrogen: 18 mg/dL Glucose: 128 mg/dL Creatinine: 1.0 mg/dL Alkaline phosphatase: 69 U/L Aspartate aminotransferase (AST): 302 U/L Alanine aminotransferase (ALT): 210 U/L TSH: 6.9 uU/mL Thyroxine (T4): 5.8 ug/dL Creatine kinase: 4300 U/L C-reactive protein: 11.9 mg/L Erythrocyte sedimentation rate: 37 mm/h Which of the following is the most accurate diagnostic test?

• A. Statin cessation
• B. Muscle biopsy (Correct Answer)
• C. Autoantibodies
• D. Electromyography
• E. Fine needle aspiration

Dermatomes and myotomes Explanation: ***Muscle biopsy*** - A **muscle biopsy** is the **most accurate diagnostic test** for establishing the definitive diagnosis of myopathy in this patient with **proximal muscle weakness** and markedly elevated **creatine kinase (CK 4300 U/L)**. - While the patient is on long-term statin therapy, the **persistently elevated inflammatory markers** (ESR 37, CRP 11.9) and very high CK raise concern for **statin-associated immune-mediated necrotizing myopathy (IMNM)**, not just simple statin myopathy. - Muscle biopsy can differentiate between **toxic statin myopathy** (which shows minimal inflammation) and **immune-mediated necrotizing myopathy** (which shows necrosis with minimal or patchy inflammatory infiltrates). - This distinction is **clinically critical** because IMNM requires **immunosuppressive therapy** in addition to statin discontinuation, whereas simple statin myopathy resolves with drug cessation alone. *Statin cessation* - While **stopping the statin** is an important **management step**, it is a **therapeutic intervention**, not a diagnostic test. - The question specifically asks for the "most accurate **diagnostic test**," making this an inappropriate answer despite being good clinical practice. - In cases of suspected statin-associated IMNM, simply stopping the statin may not resolve symptoms, necessitating a definitive diagnosis. *Autoantibodies* - **Anti-HMGCR antibodies** are highly specific for statin-associated immune-mediated necrotizing myopathy and would support the diagnosis. - However, **muscle biopsy** remains the **gold standard** as it provides direct histopathologic confirmation and can identify the pattern of muscle injury. - Autoantibody testing would be complementary but not as definitive as tissue diagnosis. *Electromyography* - **EMG** can confirm a **myopathic pattern** (short-duration, low-amplitude motor unit potentials) and help exclude neurogenic causes. - However, it is a **functional test** that shows abnormal electrical activity but does not provide the **specific histopathologic diagnosis** that muscle biopsy offers. - EMG findings are supportive but not definitive for the underlying etiology. *Fine needle aspiration* - **Fine needle aspiration** is used for cytological examination of masses or lymph nodes to diagnose malignancy or infection. - It is **not applicable** to the diagnosis of myopathy or muscle weakness and would not provide useful information in this clinical scenario.

Dermatomes and myotomes US Medical PG Question 2: A 25-year-old man comes to the physician for severe back pain. He describes the pain as shooting and stabbing. On a 10-point scale, he rates the pain as a 9 to 10. The pain started after he lifted a heavy box at work; he works at a supermarket and recently switched from being a cashier to a storekeeper. The patient appears to be in severe distress. Vital signs are within normal limits. On physical examination, the spine is nontender without paravertebral muscle spasms. Range of motion is normal. A straight-leg raise test is negative. After the physical examination has been completed, the patient asks for a letter to his employer attesting to his inability to work as a storekeeper. Which of the following is the most appropriate response?

• A. “Yes. Since work may worsen your condition, I would prefer that you stay home a few days. I will write a letter to your employer to explain the situation.”
• B. You say you are in severe pain. However, the physical examination findings do not suggest a physical problem that can be addressed with medications or surgery. I'd like to meet on a regular basis to see how you're doing.
• C. I understand that you are uncomfortable, but the findings do not match the severity of your symptoms. Let's talk about the recent changes at your job. (Correct Answer)
• D. The physical exam findings do not match your symptoms, which suggests a psychological problem. I would be happy to refer you to a mental health professional.
• E. The physical exam findings suggest a psychological rather than a physical problem. But there is a good chance that we can address it with cognitive-behavioral therapy.

Dermatomes and myotomes Explanation: ***"I understand that you are uncomfortable, but the findings do not match the severity of your symptoms. Let's talk about the recent changes at your job."*** - This response acknowledges the patient's reported discomfort while gently highlighting the **discrepancy between symptoms and objective findings**, which is crucial in cases of suspected **somatoform or functional pain**. - It also opens communication about potential **psychosocial stressors** related to his job change, which could be contributing to his symptoms, without dismissing his pain or making a premature diagnosis. *"You say you are in severe pain. However, the physical examination findings do not suggest a physical problem that can be addressed with medications or surgery. I'd like to meet on a regular basis to see how you're doing."* - While this option correctly identifies the lack of physical findings, it can be perceived as dismissive of the patient's pain, potentially damaging the **physician-patient relationship**. - Suggesting regular meetings without a clear plan for addressing his immediate concerns or exploring underlying issues might not be the most effective initial approach. *“Yes. Since work may worsen your condition, I would prefer that you stay home a few days. I will write a letter to your employer to explain the situation.”* - This response would **validate the patient's claim of severe pain** without objective evidence, potentially reinforcing illness behavior and avoiding addressing the underlying issue. - Providing a doctor's note for inability to work without a clear diagnostic basis or understanding of the pain's origin is **medically inappropriate** and could set a precedent for future such requests. *"The physical exam findings do not match your symptoms, which suggests a psychological problem. I would be happy to refer you to a mental health professional."* - Directly labeling the problem as "psychological" can be **stigmatizing and alienating** to the patient, leading to distrust and resistance to care. - While a psychological component might be present, immediately referring to mental health without further exploration of the patient's situation or current stressors is premature and lacks empathy. *"The physical exam findings suggest a psychological rather than a physical problem. But there is a good chance that we can address it with cognitive-behavioral therapy."* - Similar to the previous option, explicitly stating a "psychological problem" can be **stigmatizing**. - Jumping directly to recommending **cognitive-behavioral therapy (CBT)** without a comprehensive discussion and patient buy-in is premature and may lead to non-compliance.

Dermatomes and myotomes US Medical PG Question 3: A 78-year-old woman is accompanied by her family for a routine visit to her primary care provider. The family states that 5 months prior, the patient had a stroke and is currently undergoing physical therapy. Today, her temperature is 98.2°F (36.8°C), blood pressure is 112/72 mmHg, pulse is 64/min, and respirations are 12/min. On exam, she is alert and oriented with no deficits in speech. Additionally, her strength and sensation are symmetric and preserved bilaterally. However, on further neurologic testing, she appears to have some difficulty with balance and a propensity to fall to her right side. Which of the following deficits does the patient also likely have?

• A. Hemiballismus
• B. Hemispatial neglect
• C. Intention tremor
• D. Contralateral eye deviation
• E. Truncal ataxia (Correct Answer)

Dermatomes and myotomes Explanation: ***Truncal ataxia*** - This patient's symptoms of **difficulty with balance** and a **propensity to fall to her right side** are highly suggestive of truncal ataxia. - While she had a stroke, her preserved speech, symmetric strength and sensation, and alertness rule out typical hemiparesis or aphasia, pointing towards a **cerebellar lesion** affecting balance and coordination. *Hemiballismus* - This condition involves **flailing, high-amplitude, involuntary movements** typically affecting one side of the body. - The patient's description of balance issues and falling, without mention of such specific movements, makes hemiballismus less likely. *Hemispatial neglect* - Characterized by the **inability to attend to one side of the environment**, usually the left side following a right parietal stroke. - The patient's presentation does not describe an indifference to one side of her visual or personal space. *Intention tremor* - An **intention tremor** is a tremor that worsens during purposeful movement towards a target. - While it can be associated with cerebellar dysfunction, the primary deficit described is imbalance and falling to one side, not specifically a tremor. *Contralateral eye deviation* - This typically occurs in acute stroke scenarios as part of a **gaze preference**, where the eyes deviate towards the side of the lesion (or away from the hemiparesis). - The patient is 5 months post-stroke and is alert with no acute focal deficits, making acute eye deviation unlikely as a chronic presenting symptom here.

Dermatomes and myotomes US Medical PG Question 4: A 41-year-old woman presents with back pain for the past 2 days. She says that the pain radiates down along the posterior right thigh and leg. She says the pain started suddenly after lifting a heavy box 2 days ago. Past medical history is irrelevant. Physical examination reveals a straight leg raise (SLR) test restricted to 30°, inability to walk on her toes, decreased sensation along the lateral border of her right foot, and diminished ankle jerk on the same side. Which of the following nerve roots is most likely compressed?

• A. Fourth lumbar nerve root (L4)
• B. Second sacral nerve root (S2)
• C. Third sacral nerve root (S3)
• D. Fifth lumbar nerve root (L5)
• E. First sacral nerve root (S1) (Correct Answer)

Dermatomes and myotomes Explanation: ***First sacral nerve root (S1)*** - **Inability to walk on toes** (weakness of gastrocnemius and soleus), **decreased sensation along the lateral border of the foot**, and a **diminished ankle jerk** are classic signs of S1 radiculopathy. - The radiating pain down the posterior leg, restricted straight leg raise due to a sudden onset after lifting, points towards a **disc herniation** compressing the S1 nerve root. *Fourth lumbar nerve root (L4)* - Compression of L4 typically causes **weakness in knee extension** (quadriceps), diminished patellar reflex, and sensory loss over the medial aspect of the shin. - The patient's symptoms (inability to walk on toes, diminished ankle jerk) are not consistent with L4 nerve root involvement. *Second sacral nerve root (S2)* - S2 radiculopathy primarily affects sensation in the posterior thigh and calf and can cause **weakness in knee flexion** and **plantarflexion**, but the complete constellation of symptoms (especially ankle jerk reflex) is more indicative of S1. - Isolated S2 compression without S1 involvement is less common with these specific signs. *Third sacral nerve root (S3)* - S3 nerve root compression typically presents with **perineal numbness** and issues with bowel or bladder function due to its involvement in these functions. - The described motor and sensory deficits are not characteristic of S3 radiculopathy. *Fifth lumbar nerve root (L5)* - L5 radiculopathy is characterized by **weakness in foot dorsiflexion** (foot drop) and toe extension, leading to inability to walk on heels, and sensory loss on the dorsum of the foot. - While L5 compression can cause radiating pain and a restricted straight leg raise, the specific deficit of **inability to walk on toes** and a **diminished ankle jerk** are not typical of L5 involvement.

Dermatomes and myotomes US Medical PG Question 5: A 22-year-old man is rushed to the emergency department after a motor vehicle accident. The patient states that he feels weakness and numbness in both of his legs. He also reports pain in his lower back. His airway, breathing, and circulation is intact, and he is conversational. Neurologic exam is significant for bilateral lower extremity flaccid paralysis and impaired pain and temperature sensation up to T10-T11 with normal vibration sense. A computerized tomography scan of the spine is performed which shows a vertebral burst fracture of the vertebral body at the level of T11. Which of the following findings is most likely present in this patient?

• A. Intact vibration sense
• B. Bowel incontinence (Correct Answer)
• C. Flaccid paralysis at the level of the lesion
• D. Spasticity below the lesion
• E. Impaired proprioception sense

Dermatomes and myotomes Explanation: ***Bowel incontinence*** - The presented symptoms of acute **bilateral lower extremity flaccid paralysis**, **impaired pain and temperature sensation**, and a T11 **vertebral burst fracture** are highly indicative of **anterior cord syndrome**. - **Anterior cord syndrome** characteristically involves damage to the **anterior two-thirds of the spinal cord**, affecting the **corticospinal tracts** (motor control), **spinothalamic tracts** (pain and temperature sensation), and the **autonomic fibers** that control bladder and bowel function, leading to **bowel and bladder dysfunction**. *Intact vibration sense* - The sensation of **vibration** and **proprioception** is carried by the **dorsal columns** (posterior part of the spinal cord), which are typically **spared** in **anterior cord syndrome**. - Therefore, **intact vibration sense** is an expected finding, but the question asks for the **most likely finding** that represents a significant complication of the syndrome. *Flaccid paralysis at the level of the lesion* - While **flaccid paralysis** is present in the lower extremities, it occurs **below the level of the lesion** due to damage to the descending motor tracts (corticospinal tracts). - Flaccid paralysis *at* the level of the lesion would typically involve damage to the **lower motor neurons** at that specific segment, which is not the primary feature described for a burst fracture causing **anterior cord syndrome**. *Spasticity below the lesion* - **Spasticity** typically develops much **later** in spinal cord injuries, after the initial phase of **spinal shock** resolves (usually weeks to months). - In the acute phase following a significant spinal cord injury, **flaccid paralysis** is the more common finding below the lesion, reflecting spinal shock. *Impaired proprioception sense* - Similar to vibration sense, **proprioception** is primarily mediated by the **dorsal columns**, which are generally **spared** in **anterior cord syndrome**. - Therefore, **proprioception** would likely be **intact**, not impaired, in this specific type of spinal cord injury.

Dermatomes and myotomes Flashcard 1 of 5

Question: What dermatome is found at the posterior half of the skull? _____

Answer: C2

Dermatomes and myotomes Flashcard 2 of 5

Question: Which type of sensory receptor is found on finger tips and superficial skin? _____

Answer: Merkel discs

Dermatomes and myotomes Flashcard 3 of 5

Question: ID Structure: _____

Answer: Main sensory nucleus of V

Dermatomes and myotomes Flashcard 4 of 5

Question: ID Structure: _____

Answer: Medial lemniscus

Dermatomes and myotomes Flashcard 5 of 5

Question: What dermatome is found at the inguinal ligament? _____

Answer: L1