CLINICAL CASE | Friedreich Ataxia
A 10-year-old boy first presented to his pediatrician with difficulty walking. He was an only child, of French-Canadian parents. His parents reported that recently he has difficulty standing still, is constantly shifting his position, and difficulty running. They reported normal motor development initially, but that now he seems clumsy. On examination, he was observed to have a wide-based gait with occasional shifting of position to maintain balance. Sitting and standing were noted to be associated with titubation. He was referred to a child neurologist. After further workup and genetic testing, he was diagnosed with Friedreich ataxia, a progressive spinocerebellar ataxia. Friedreich ataxia is an autosomal recessive disorder, due to a chromosome 9 mutation that, in virtually all cases, is an expansion of a GAA trinucleotide repeat within the gene that codes for the mitochondrial protein frataxin.
He was seen regularly by his neurologist who noted progression in motor signs to include upper extremity ataxia, dysdiadochokinesia, and intention tremor. When asked to maintain a standing posture with his eyes closed, he began to sway and loose his balance (positive Romberg sign). He had no tendon reflexes, such as the knee-jerk or biceps reflexes. Friedreich ataxia patients often have cardiomyopathy, and most patients die as a result of cardiac arrhythmia or congestive heart failure.
Figure 13–1A is an MRI from a young Friedreich ataxia patient. Compared with the MRI from a healthy person (Figure 13–1B), the most notable feature is narrowing of the cervical spinal cord.
Answer the following questions based on your reading of the case and the chapter.
1. Why does the patient have a positive Romberg sign, and how is this related to the loss of tendon reflexes?
2. What could account for the wide-based gait and ataxia? Key neurological signs and corresponding damaged brain structures Proprioceptive and reflex signs
The cervical spinal cord in the patient is thin compared with a healthy person. Thinning is produced by degeneration of large-diameter somatic sensory afferents; unmyelinated fibers are spared. The degeneration is in the dorsal columns. Whereas the degeneration is accompanied by gliosis, it is insufficient to maintain the normal size of the cord. The dorsal roots show thinning as well. With this large-diameter fiber loss, there is associated loss of tendon reflexes and limb proprioception. Patients with this sensory loss rely on vision to help maintain balance. Often patients have impairment in tactile sensation. Ataxia
The loss of limb proprioceptive information leads to incoordination. Further, there is loss of neurons in Clarke's column, which transmit proprioceptive and other mechanosensory information to the cerebellum; these neurons give rise to the dorsal spinocerebellar tract (Figure 13–6B). Together these factors contribute to ataxia. To compensate for boththe balance impairment and impaired lower extremity coordination, patients adopt a broad-based, slowed gait. Interestingly, Friedreich ataxia patients may not show extensive cerebellar degeneration for most of the course of the disease. Thus, the ataxia may be more ...