A 58-year-old woman presents to the clinic with an abnormal sensation on the left side of her body that has been present for the past several months. At first, the area seemed numb and she recalls touching a hot stove and accidentally burning herself but not feeling the heat. Now she is suffering from a constant, uncomfortable burning pain on her left side for the past week. The pain gets worse when someone even lightly touches that side. She has recently immigrated and her past medical records are unavailable. Last month she had a stroke but she cannot recall any details from the event. She confirms a history of hypertension, type II diabetes mellitus, and bilateral knee pain. She also had cardiac surgery 20 years ago. She denies fever, mood changes, weight changes, and trauma to the head, neck, or limbs. Her blood pressure is 162/90 mm Hg, the heart rate is 82/min, and the respiratory rate is 15/min. Multiple old burn marks are visible on the left hand and forearm. Muscle strength is mildly reduced in the left upper and lower limbs. Hyperesthesia is noted in the left upper and lower limbs. Laboratory results are significant for: Hemoglobin 13.9 g/dL MCV 92 fL White blood cells 7,500/mm3 Platelets 278,000/mm3 Creatinine 1.3 U/L BUN 38 mg/dL TSH 2.5 uU/L Hemoglobin A1c 7.9% Vitamin B12 526 ng/L What is the most likely diagnosis?

Subacute combined degeneration of spinal cord

A 17-year-old boy is brought to the emergency department after being stabbed with a knife during an altercation. Physical examination shows a 4-cm stab wound on the right lateral border of the T1 spinous process. An MRI of the spinal cord shows damage to the area of the right lateral corticospinal tract at the level of T1. Further evaluation will most likely show which of the following findings?

Absence of right-sided motor function below T1

Absence of left-sided proprioception below T1

Presence of left-sided Babinski sign

Absence of left-sided fine touch sensation below T1

Absence of right-sided temperature sensation below T1

A 23-year-old man presents to the emergency room following a stab wound to the back. He was in a bar when he got into an argument with another man who proceeded to stab him slightly right of the midline of his back. He is otherwise healthy and does not take any medications. He has one previous admission to the hospital for a stab wound to the leg from another bar fight 2 years ago. His temperature is 99°F (37.2°C), blood pressure is 115/80 mmHg, pulse is 100/min, and pulse oximetry is 99% on room air. Cardiopulmonary and abdominal exams are unremarkable; however, he has an abnormal neurologic exam. If this wound entered his spinal cord but did not cross the midline, which of the following would most likely be seen in this patient?

Ipsilateral flaccid paralysis at the level of the lesion

Contralateral spasticity below the level of the lesion

Ipsilateral loss of pain and temperature sensation below the lesion

Contralateral loss of tactile, vibration, and proprioception below the lesion

Contralateral loss of sensation at the level of the lesion

A 27-year-old woman comes to the clinic for blisters on both hands. The patient has a past medical history of asthma, eczema, and a car accident 2 years ago where she sustained a concussion. She also reports frequent transient episodes of blurred vision that clear with artificial tears. When asked about her blisters, the patient claims she was baking yesterday and forgot to take the pan out with oven gloves. Physical examination demonstrates weeping blisters bilaterally concentrated along the palmar surfaces of both hands and decreased pinprick sensation along the arms bilaterally. What is the most likely explanation of this patient’s symptoms?

Syringomyelia at the cervico-thoracic region

Syringomyelia at the lumbar region

Sensory pathways for USMLE Step 1: High-Yield Anatomy Lessons

DCML Pathway - Feeling the Good Vibes

Modalities: Fine touch, vibration, pressure, proprioception.
Receptors: Pacinian (vibration), Meissner's (light touch), Ruffini (stretch), Merkel's (pressure).

⭐ Somatotopy: The dorsal columns have a specific organization: "Legs are Medial, Arms are Lateral." Damage to the medial spinal cord (e.g., from a thoracic lesion) affects the legs first.

Spinothalamic Tract - Ouch, That's Hot!

Function: A major ascending pathway transmitting affective sensations: pain (nociception), temperature, and associated crude touch and pressure.
Pathway Overview: A three-neuron chain from the periphery to the cortex, with a critical decussation (crossing) in the spinal cord, making the pathway contralateral.

Spinal cord cross-section with ascending/descending tracts

⭐ Syringomyelia, a fluid-filled cavity in the central canal, classically damages the crossing fibers in the anterior white commissure. This leads to bilateral loss of pain and temperature in a "cape-like" distribution over the shoulders and arms, while sparing touch and proprioception.

Trigeminal & Spinocerebellar - Face and Unconscious Steps

Trigeminal Pathway (CN V): Sensation for the face (touch, pain, temp).
- 1st Neuron: Trigeminal ganglion.
- 2nd Neuron: Trigeminal nuclei in pons/medulla; decussates.
- 3rd Neuron: VPM of thalamus → somatosensory cortex.
Spinocerebellar Tracts: Unconscious proprioception for coordinated movement.
- Function: Relays limb/joint position to the cerebellum.
- Key Tracts:
  - Dorsal: Uncrossed, individual muscle data.
  - Ventral: Double-crosses, whole limb data.
- All input terminates in the ipsilateral cerebellum.

⭐ Lesions of the spinocerebellar tract result in ipsilateral ataxia (e.g., Friedreich's ataxia).

Dorsal Column-Medial Lemniscal and Spinocerebellar Pathways

Clinical Lesions - Pathway Power Outs

Spinal Cord Lesions: Brown-Séquard, Tabes, Syringomyelia

Cortical Lesion (Sensory Cortex):
- Contralateral sensory loss (hemianesthesia), often with graphesthesia/stereognosis deficits.
Thalamic Lesion (VPL/VPM Nuclei):
- Complete contralateral sensory loss. Chronic evolution may lead to Dejerine-Roussy syndrome (central post-stroke pain).
Spinal Cord Syndromes:
- Brown-Séquard Syndrome (Hemicord Lesion):
  - Ipsilateral loss of vibration, proprioception, and light touch below the lesion.
  - Contralateral loss of pain and temperature starting 1-2 segments below the lesion.
- Tabes Dorsalis (Neurosyphilis):
  - Demyelination of dorsal columns → bilateral loss of proprioception/vibration.
- Syringomyelia (Anterior White Commissure):
  - Bilateral loss of pain and temperature in a "cape-like" distribution (C8-T1).

⭐ In Brown-Séquard syndrome, ipsilateral motor paralysis and dorsal column sensory loss occur at and below the lesion level, but contralateral pain/temp loss spares the segments of entry before crossing.

The Dorsal Column-Medial Lemniscus (DCML) pathway handles fine touch, vibration, and proprioception, decussating in the medulla.

The Anterolateral (Spinothalamic) tract conveys pain, temperature, and crude touch, decussating in the spinal cord.

Facial sensation is carried by the trigeminal pathway to the VPM nucleus of the thalamus; body sensation goes to the VPL nucleus.

All third-order neurons project from the thalamus to the primary somatosensory cortex.

This cortex is somatotopically organized as the sensory homunculus.

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