CLINICAL CASE | 12-Year-Old Boy With Horizontal Gaze Palsy With Progressive Scoliosis
A 12-year-old boy presented with an inability to follow a horizontal moving visual stimulus. His eyes remain fixated in the forward direction, bilaterally. He is able to follow a visual stimulus moving vertically with his eyes. Testing for somatic sensory function did not reveal any change in sensation. Limb motor testing revealed mild gait instability, a tremor during reaching, and progressive scoliosis.
Figure 2–1A1 is a midsagittal magnetic resonance imaging (MRI) scan from the patient. This is a T1-weighted image. Brain tissue is represented as shades of gray (gray matter darker than white matter) and cerebrospinal fluid, black. MRI revealed several midline structural defects in the pons and medulla. The bracket (Figure 2–1A1, B1) is located dorsal to the pons and medulla. Cerebrospinal fluid penetrates into this region on the midline because of the presence of a shallow groove (sulcus). Note that this sulcus is not present in the healthy person’s MRI (Figure 2–1B1). The MRIs in the transverse plane (second and third rows) are T2-weighted images, where cerebrospinal fluid and fluid within the eyes appear white. The MRI through the upper medulla (A2) reveals an abnormally flattened appearance in the patient compared with the healthy brain (B2). The image through the caudal medulla reveals an aberrant deep midline groove (A3, arrow; B3).
Diffusion tensor imaging (DTI; this is discussed in Box 2–2) was performed on the patient’s MRI. This method permits imaging of neural pathways in the brain. On this image we can follow the corticospinal tract, the principal motor control pathway, which originates from the motor cortex. Figure 2–1A4 shows the DTI image from the patient. There are two parallel pathways. The arrow points to the caudal medulla, where the motor pathway decussation normally occurs. Furthermore, there are no other decussations along the pathway. B4 is a DTI from a healthy person. The arrow points to the decussation of the corticospinal tract in the caudal medulla where we see the tracts from each side traversing to the other. We will learn about these pathways in later chapters.
Finally, clinical neurophysiological testing was conducted to assess the integrity of the touch and motor pathways (see Figure 2–2). To determine the function of the sensory pathway, the skin is electrically stimulated and the neural response is recorded with electrodes placed on the scalp over the region of the somatic sensory cortex. This test revealed an ipsilateral, not the customary contralateral, response. To determine the function of the corticospinal tract, transcranial magnetic stimulation (TMS) is used to activate the motor cortex and the motor pathway. In the patient, TMS of the motor cortex produced an ipsilateral muscle response, not the typical contralateral response observed in healthy subjects.
After reading the conclusion and explanations below, you should be able to answer the following questions.
1. At what dorsoventral level do the axons of ...