Dermatomes and myotomes Practice Questions

Q: A 35-year-old gymnast presents with progressive lower extremity weakness after a thoracic spine injury. She has normal hip flexion and knee extension but weakness of knee flexion, ankle dorsiflexion, ankle plantarflexion, and all toe movements. Sensory examination shows normal sensation in the anterior thigh and medial leg but decreased sensation below the knee laterally and posteriorly. She has urinary retention and saddle anesthesia. Evaluate the level and nature of this neurological injury.

Conus medullaris syndrome at T12-L1. ***Conus medullaris syndrome at T12-L1*** - This syndrome occurs at the terminal end of the spinal cord (T12-L1 vertebral level) and presents with **symmetrical** weakness, **urinary retention**, and **saddle anesthesia**. - The sparing of **hip flexion (L2)** and **knee extension (L3/L4)** alongside the loss of **S2-S4 functions** (bladder and distal motor power) confirms the injury is localized to the **sacral cord segments**. *L4-L5 disc herniation with bilateral radiculopathy* - This would typically present with **asymmetric pain** and specific dermatomal loss, but would not naturally cause **saddle anesthesia** or **urinary retention** unless progressing to cauda equina syndrome. - Reflexes for knee extension (L4) would likely be affected, whereas this patient has preserved **knee extension**. *L5-S1 disc herniation with cauda equina syndrome* - While it causes **saddle anesthesia**, cauda equina syndrome involves **lower motor neuron** injury of nerve roots and is usually more **asymmetric and gradual**. - A herniation at L5-S1 is too low to explain the loss of **knee flexion**, which is primarily mediated by the **L5-S2** nerve roots. *Complete spinal cord transection at L1* - A complete transection at L1 would result in the loss of **all motor and sensory function** below that level, including **hip flexion (L2)** and **knee extension (L3)**. - This patient demonstrates **normal hip flexion**, indicating the injury is distal to the upper lumbar segments of the spinal cord. *Bilateral S1 radiculopathy from spinal stenosis* - Bilateral S1 involvement would cause weakness in **plantarflexion** and loss of the **Achilles reflex**, but would spare the **L5-mediated dorsiflexion**. - It does not account for the **urinary retention** or the extensive sensory loss across all **sacral dermatomes** seen in this patient.

Q: A 50-year-old man with cervical spondylosis presents with hand weakness and numbness. Examination shows weakness of finger abduction and adduction, loss of sensation along the medial forearm and fifth finger, and a positive Froment's sign. Biceps, triceps, and brachioradialis reflexes are normal. Analyze the anatomical basis for these findings.

C8 nerve root compression. ***C8 nerve root compression*** - The **C8 nerve root** provides motor innervation to the **intrinsic hand muscles** (interossei), leading to weakness in **finger abduction and adduction** and a **positive Froment’s sign**. - It provides sensory innervation to the **medial hand**, fifth finger, and portions of the **medial forearm**, matching the patient's presentation of numbness in those specific regions. *C6 nerve root compression* - This typically presents with weakness in **elbow flexion** and **wrist extension** (the "extensor Carpi radialis" group). - Sensory loss would be localized to the **thumb and lateral forearm**, and the **brachioradialis reflex** would likely be diminished. *C7 nerve root compression* - Characterized by weakness in **elbow extension** (triceps) and **wrist flexion**, which are not reported in this case. - The **triceps reflex** would be reduced or absent, but it is explicitly stated as normal in this examination. *T1 nerve root compression* - While T1 does innervate intrinsic hand muscles, an isolated T1 lesion would not explain sensory loss in the **fifth finger**, which is primarily a **C8 dermatome**. - T1 compression is more specifically associated with the **medial antebrachial cutaneous nerve** (medial forearm) but less so with the hand's ulnar border compared to C8. *Combined C8-T1 nerve root compression* - This could explain the symptoms, but **C8 compression alone** is the more parsimonious anatomical explanation for sensory loss involving both the fifth finger and the medial forearm. - In cervical spondylosis, nerve roots are typically affected at a **specific disc level**, and isolated C8 involvement is a classic cause of hand intrinsic muscle wasting and medial hand numbness.

Q: A 38-year-old woman presents with neck pain radiating to her thumb and index finger after a motor vehicle accident. Examination reveals weakness of elbow flexion and wrist extension, a diminished brachioradialis reflex, and decreased sensation over the lateral forearm and thumb. She has normal shoulder abduction and grip strength. Analyze the most likely nerve root affected.

C6 nerve root. ***C6 nerve root*** - The **C6 nerve root** provides sensory innervation to the **lateral forearm, thumb, and index finger**, matching this patient's dermatomal sensory loss. - Motor deficits in **elbow flexion** (brachioradialis) and **wrist extension**, along with a diminished **brachioradialis reflex**, are classic findings of C6 radiculopathy. *C5 nerve root* - **C5** involvement typically presents with weakness in **shoulder abduction** (deltoid) and a diminished **biceps reflex**, rather than wrist extension issues. - Sensory loss in a C5 lesion is usually localized to the **lateral arm** (over the deltoid) rather than the hand or thumb. *C7 nerve root* - **C7** radiculopathy produces weakness in **elbow extension** (triceps) and **wrist flexion**, which are not reported in this clinical scenario. - It is associated with a diminished **triceps reflex** and sensory loss typically involving the **middle finger**. *C8 nerve root* - Injury to the **C8 nerve root** results in impaired **grip strength** and weakness of the **finger flexors**, which were noted as normal in this patient. - The sensory deficit for C8 would be found on the **medial side of the hand**, specifically the **little and ring fingers**. *Combined C5-C6 nerve roots* - A combined lesion would likely involve significant **weakness in shoulder abduction** (C5) and a diminished **biceps reflex**, which were not observed here. - The clinical presentation is restricted to the specific distribution of a **single nerve root**, making isolated C6 a more precise diagnosis than a combined injury.

Question 1

A 58-year-old diabetic man with multiple thoracic vertebral compression fractures presents with progressive lower extremity dysfunction. He has spastic paraparesis with hyperreflexia, bilateral Babinski signs, and a sensory level at T10. However, he also has areflexic bladder, saddle anesthesia, and absent bulbocavernosus reflex. Upper extremities are completely normal. Synthesize the anatomical explanation for this mixed upper and lower motor neuron presentation.

Accepted Answer

Conus medullaris syndrome with additional cord compression

Answer

Complete spinal cord transection at T10 level

Answer

Cauda equina syndrome with coincidental cord pathology

Answer

Combined central cord and conus medullaris injury

Answer

Diabetic lumbosacral radiculoplexus neuropathy with cord compression

Question 2

A 42-year-old office worker develops progressive bilateral leg pain, weakness, and numbness over months. Examination reveals asymmetric weakness: right leg has weak hip flexion and knee extension with diminished patellar reflex; left leg has weak ankle dorsiflexion and toe extension with normal reflexes. Sensory examination shows patchy loss in L4 distribution on the right and L5 distribution on the left. MRI shows multilevel degenerative disc disease at L3-L4 and L4-L5 with foraminal stenosis. Evaluate the pathophysiological explanation for this clinical pattern.

Accepted Answer

Unilateral L4 radiculopathy with contralateral L5 radiculopathy

Answer

Sequential nerve root compression at different levels bilaterally

Answer

Bilateral L5 radiculopathy with asymmetric presentation

Answer

Central canal stenosis with differential nerve root vulnerability

Answer

Polyradiculopathy from inflammatory or infiltrative process

Question 3

A 35-year-old gymnast presents with progressive lower extremity weakness after a thoracic spine injury. She has normal hip flexion and knee extension but weakness of knee flexion, ankle dorsiflexion, ankle plantarflexion, and all toe movements. Sensory examination shows normal sensation in the anterior thigh and medial leg but decreased sensation below the knee laterally and posteriorly. She has urinary retention and saddle anesthesia. Evaluate the level and nature of this neurological injury.

Accepted Answer

Conus medullaris syndrome at T12-L1

Answer

L4-L5 disc herniation with bilateral radiculopathy

Answer

L5-S1 disc herniation with cauda equina syndrome

Answer

Complete spinal cord transection at L1

Answer

Bilateral S1 radiculopathy from spinal stenosis

Question 4

A 50-year-old man with cervical spondylosis presents with hand weakness and numbness. Examination shows weakness of finger abduction and adduction, loss of sensation along the medial forearm and fifth finger, and a positive Froment's sign. Biceps, triceps, and brachioradialis reflexes are normal. Analyze the anatomical basis for these findings.

Accepted Answer

C8 nerve root compression

Answer

C6 nerve root compression

Answer

C7 nerve root compression

Answer

T1 nerve root compression

Answer

Combined C8-T1 nerve root compression

Question 5

A 38-year-old woman presents with neck pain radiating to her thumb and index finger after a motor vehicle accident. Examination reveals weakness of elbow flexion and wrist extension, a diminished brachioradialis reflex, and decreased sensation over the lateral forearm and thumb. She has normal shoulder abduction and grip strength. Analyze the most likely nerve root affected.

Accepted Answer

C6 nerve root

Answer

C5 nerve root

Answer

C7 nerve root

Answer

C8 nerve root

Answer

Combined C5-C6 nerve roots

Dermatomes and myotomes — MCQs

Dermatomes and myotomes — MCQs

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