Comprehensive Guide to Cortical and Brain Stem Stroke Localization

Classification of Strokes

Strokes are divided into two primary categories based on the location of the vascular insult and the resulting clinical presentation:

• Cortical Stroke: These involve the major arteries supplying the cerebral cortex. The specific arteries include:
  • Anterior Cerebral Artery (ACA).
  • Middle Cerebral Artery (MCA), also referred to as the Medial Cerebral Artery.
  • Posterior Cerebral Artery (PCA).
• Brain Stem Stroke: These involve the structures within the midbrain, pons, or medulla.

Cortical Strokes and Focal Deficits

Cortical strokes are characterized by focal motor or sensory symptoms. The location of the deficit corresponds to the specific artery affected:

• Anterior Cerebral Artery (ACA): Involvement results in motor and/or sensory deficits of the leg.
• Middle Cerebral Artery (MCA): Involvement results in deficits of the face.
• Posterior Cerebral Artery (PCA): Involvement results in deficits involving vision or language (if the stroke occurs in the dominant hemisphere).

Signals in the cortical pathways cross over to the contralateral side of the body. Therefore:

• Motor abnormalities in the left leg indicate a stroke in the right ACA.
• Motor weakness on the right side of the face indicates a stroke in the left MCA.

Differentiating Cortical from Brain Stem Strokes

• Focal Deficits: If the clinical presentation involves weakness restricted to a specific area (e.g., only the leg or only the face), it is a cortical stroke.
• Diffuse or Global Deficits: If the patient presents with weakness across an entire side of the body (e.g., the entire left side including the face, arm, and leg), it indicates a brain stem stroke.

Case Study: Cortical Localization

A $67$-year-old male presents with a $2$-hour history of altered mental status. He was found unresponsive while watching Jeopardy. Physical examination findings include:

• Motor strength of the left lower extremity: $4/5$.
• Motor strength of the right lower extremity: $0/5$.
• Motor strength in both upper extremities: $5/5$.
• Sensation to facial muscles: Intact.
• Functional abilities: Able to puff out cheeks, swallow, and smile.
• Reflexes: Gag reflex is intact.
• Anatomy: No tongue or uvular deviation.

Analysis:

• The deficit is focal, involving only the right lower extremity.
• Leg involvement localizes the stroke to the Anterior Cerebral Artery (ACA).
• Because the deficit is on the right side, the stroke occurred in the left hemisphere.
• Diagnosis: Left anterior cerebral artery stroke.

Brain Stem Strokes and the Rule of Fours

The Rule of Fours is a clinical tool used to detect and localize brain stem strokes. It allows for the identification of the level of the brain stem (midbrain, pons, or medulla) and the horizontal location (medial/midline or lateral).

Cranial Nerve Division

The cranial nerves are divided into three sections of the brain stem, with four nerves assigned to each compartment in numerical order (excluding Cranial Nerves $1$ and $2$, which sit above the midbrain):

• Midbrain: Cranial Nerves $3$ and $4$.
• Pons: Cranial Nerves $5$, $6$, $7$, and $8$.
• Medulla: Cranial Nerves $9$, $10$, $11$, and $12$.

Midline vs. Lateral Localization

To determine if a stroke is medial or lateral, use the numerical value of the affected cranial nerve:

• Midline (Medial) Nerves: Cranial nerves that divide evenly into $12$ ($3$, $4$, $6$, and $12$).
• Lateral Nerves: Cranial nerves that do not divide evenly into $12$ ($5$, $7$, $8$, $9$, $10$, and $11$).

Rules of Midline (Medial) Motor Syndromes

There are four motor-related syndromes that localize to the midline. These are easily remembered as the "M" syndromes:

• Medial Longitudinal Fasciculus (MLF): Controls motor signals for lateral gaze and eye movement coordination.
• Motor Tract (Corticospinal Tract): The upper motor neuron pathway. Involvement leads to global weakness on the opposite side of the body.
• Medial Lemniscus: Part of the posterior columns responsible for proprioception and vibration sense.
• Motor Nuclei of Cranial Nerves: Specifically the nuclei for nerves $3$, $4$, $6$, and $12$.

If a patient presents with a loss of proprioception, vibration, or upper motor neuron signs combined with midline cranial nerve deficits, the stroke is midline/medial.

Rules of Lateral Sensory Syndromes

There are four sensory-related syndromes that localize to the lateral aspect of the brain stem. These are remembered as the "S" syndromes:

• Spinothalamic Tract: Responsibility for sensing pain and temperature.
• Spinocerebellar Tract: Involvement leads to problems with Rapid Alternating Movements (RAM).
• Sympathetic Chain: Involvement results in Pseudo-Horner syndrome, characterized by ptosis and anhidrosis.
• Sensory Nuclei of Cranial Nerves: Specifically the nuclei for nerves that do not divide evenly into $12$ ($5$, $7$, $8$, $9$, $10$, and $11$).

Case Study: Brain Stem Localization

A $67$-year-old male presents with a $4$-hour history of diffuse motor weakness on his entire left side. Clinical findings include:

• Global motor weakness (entire left side).
• Inability to move eyes laterally toward the left.

Analysis:

• Initial Classification: The weakness is diffuse/global, not focal, which categorizes this as a brain stem stroke rather than a cortical stroke.
• Medial vs. Lateral: The involvement of the corticospinal tract (motor tract) signifies a midline (medial) stroke. Because the weakness is on the left, the stroke is on the right side.
• Cranial Nerve Involvement: The inability to move eyes laterally indicates a Cranial Nerve $6$ (abducens nerve) palsy, as the lateral rectus muscle is affected. Cranial Nerve $6$ divides evenly into $12$, confirming it is a medial/midline nerve.
• Level of the Brain Stem: In the Rule of Fours, Cranial Nerve $6$ is located in the pons.
• Conclusion: Combining these factors (Medial, Right-sided, Pons) results in the diagnosis of a right medial pontine stroke.
• Note: While clinical syndromes like Wallenberg syndrome exist, they can be described using the localization (e.g., right medial pontine).