AssessmentsSpinal cord disorders: Pathology review
USMLE® Step 1 style questions USMLE
A 46-year-old male is brought to the emergency department after being involved in a high-speed motor vehicle injury. The patient was the restrained passenger in a stationary vehicle as he was rear-ended by another vehicle traveling approximately 80 miles per hour. The patient’s neck was hyperflexed during the collision and now feels stiff. He is unable to move his arms or legs. Temperature is 37.0°C (98.6°F), pulse is 110/min, respirations are 9/min, blood pressure is 90/65 mmHg, and O2 saturation is 97% on room air. Neurological exam is notable for bilateral loss of pain and temperature sensation as well as bilateral loss of motor function below C6. Sensation to soft touch and vibration is unaffected. Which of the following is the most likely diagnosis?
Contributors:Megan Gullotto, MSMI, Robyn Hughes, MScBMC, Elizabeth Nixon-Shapiro, MSMI, CMI, Alex Aranda, Jake Ryan, Antonia Syrnioti, MD
At the physician’s office, 55-year-old Mario presents complaining of weakness in his hands and feet. These symptoms have gradually progressed over the past couple of months. At first, he struggled to manipulate small objects like buttoning his shirt. Now he also often stumbles while walking and recently fell down the stairs. In addition, his family mentions that his speech has become slightly slurred. He denies any sensory symptoms, memory loss or any bowel or bladder complaints. Later that day, 43-year-old Donna comes in with difficulty walking. She has fallen several times over the last few weeks. Her past medical history is significant for HIV infection, which was diagnosed a few years ago. On neurologic examination, her pupils are small and irregularly shaped and do not react to light, but constrict with accommodation. The sensations of pressure, vibration, fine touch and proprioception are also reduced throughout the lower extremities. She has a wide-based gait and cannot maintain balance with her eyes closed.
Based on the initial presentation, both Mario and Donna have some form of spinal cord disorder. Okay, let’s talk about physiology first real quick. If we zoom in at a cross-section of the spinal cord, we’ll see that it is composed of both grey and white matter. Grey matter is found within the medial portion of the spinal cord and has two dorsal or posterior horns that contain cell bodies of sensory neurons and two ventral or anterior horns that contain cell bodies of motor neurons. In the center of the grey matter there’s a small cavity called the central canal which is filled with cerebrospinal fluid that provides nutrients and mechanical support. Surrounding the grey matter is white matter, which consists of the axons of various neurons and they are organized into tracts that carry information to and from the brain.
For your exams, there are a few main tracts to remember. First, there’s the spinothalamic tract which is an ascending pathway and it’s divided into two parts. The lateral tract carries sensory information for pain and temperature, while the anterior tract carries information for crude touch, or the sense one has been touched, without being able to localize where they were touched. For this to happen, a first order neuron, found inside a dorsal root ganglion carries sensory input from the skin to the dorsal horn of the spinal cord, where it synapses with the second order neuron. And that neuron ascends 1-2 vertebral levels and decussates or crosses to the opposite side of the spinal cord via an area of white matter called the anterior white commissure. The secondary neuron then ascends up the length of the spinal cord via the spinothalamic tracts, and eventually synapse with a 3rd order neuron located in the ventral posterior nucleus of the thalamus. This 3rd order neuron then sends its axon up to the sensory cortex of the brain, letting you know that there’s a sensory signal.
Next, there are two ascending dorsal column tracts: the fasciculus gracilis which carries sensory information from the lower trunk and legs, and the fasciculus cuneatus which carries sensory information from the upper trunk and arms. These tracts both carry sensations like pressure, vibration, fine touch, which is where you can localize where you were touched, and proprioception which is an awareness of your body’s position in space. Once again, for this to occur, a 1st order neuron collects sensory information, but, in this case, ascends up along the same side of the whole length of the spinal cord to reach the lower level of the medulla oblongata where it synapses with the cell body of a 2nd order neuron in the nucleus cuneatus. The 2nd order neuron then sends off an axon that crosses over to the opposite side of the medulla, and travels up to the ventral posterior nucleus of the thalamus to synapse with a 3rd order neuron. And then, the 3rd order neuron then sends up an axon that carries the sensory signals to the sensory cortex of the brain.
Once your brain receives all that information, it may decide that it wants a muscle to contract. So, to do that, it sends a signal from an upper motor neuron in the motor cortex through a descending pathway, known as the corticospinal tract, in the midbrain and cross to the opposite side at the medulla before continuing down the spinal cord. There, it synapses with lower motor neurons in the anterior horn. The lower motor neuron then sends out an axon that synapse with the muscle it innervates. Similar to that, there’s also the corticobulbar tract. Upper motor neurons from the motor cortex travel through the corticobulbar tract to reach the brainstem and cross over to the opposite side of the pons or medulla, where they synapse directly with the nuclei of lower motor neurons that makes up the cranial nerves. And these control the muscles of the head and neck.
Now, for your exams, it’s important to know that with both an upper and a lower motor neuron lesion, the muscle won’t be able to contract, which will result in muscle weakness, or decrease in muscle power. But there are a few key facts that will help you differentiate between the two. So with an upper motor neuron lesion, muscle tone or resistance to passive stretch will be increased, also known as spastic paralysis, muscle bulk will be normal, and deep tendon reflexes will be hyperactive. And there’s also a unique feature, known as the Babinski reflex. So, normally when stroking the lateral aspect of the sole of the foot, there’s plantar or downward flexion of all toes. When there’s dorsiflexion of the big toe and fanning of the other toes, that’s called Babinski reflex. In contrast, in a lower motor neuron lesion, muscle tone will be decreased, which is known as flaccid paralysis, there’ll be muscle atrophy and deep tendon reflexes will be hypoactive or absent. And the unique feature will be fasciculations, which are small muscle twitches under the skin.