17.4 Neuromuscular Disorders & Headache

Neuromuscular Disorders & Headache

This section pairs two exam staples: the major pediatric neuromuscular disorders, distinguished largely by the pattern of weakness, inheritance, and key bedside or laboratory clues, and the approach to pediatric headache, which mirrors the red-flag reasoning already introduced for febrile seizures and meningitis.

Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the dystrophin gene on the X chromosome, so it affects boys almost exclusively. About one-third of cases arise from new (de novo) mutations rather than maternal carrier inheritance, a detail the exam likes to use as a trap in a stem with a negative family history.

• Onset: typically ages 2-5 years, with delayed walking, a waddling gait, frequent falls, and difficulty climbing stairs or rising from the floor
• Gowers sign: when asked to rise from a seated or lying position on the floor, the child uses their hands to walk up their own thighs and legs to compensate for weak proximal hip and thigh extensor muscles; this is one of the single most recognizable exam findings in pediatrics
• Calf pseudohypertrophy: the calves appear enlarged but are firm, because fatty and fibrous tissue is replacing degenerating muscle rather than true muscle bulk
• Creatine kinase (CK): markedly elevated, often 10-100 times the upper limit of normal, and elevated even before overt clinical weakness; an incidentally very high CK in a young boy, discovered on labs drawn for another reason, should trigger a DMD workup
• Diagnosis: confirmed by genetic testing for a dystrophin gene deletion or duplication, which identifies the mutation in most cases; muscle biopsy showing absent dystrophin on immunohistochemistry is reserved for cases where genetic testing is inconclusive
• Course: progressive proximal weakness, with loss of independent ambulation typically by age 10-13, followed by cardiomyopathy and respiratory muscle failure; corticosteroids (prednisone or deflazacort) are the mainstay treatment to slow progression, and newer exon-skipping or gene-targeted therapies apply to specific mutation subsets

Becker muscular dystrophy is the milder allelic counterpart of DMD: later onset, slower progression, and partially functional rather than absent dystrophin. It is a common distractor answer against DMD on exam stems.

Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive disorder caused by deletion or mutation of the SMN1 gene on chromosome 5, leading to progressive degeneration of anterior horn cells (lower motor neurons) in the spinal cord. Because this is a pure lower motor neuron disease, the exam-defining clue is symmetric proximal weakness with hypotonia, areflexia, and tongue fasciculations, but entirely preserved cognition.

SMA is now included in newborn screening programs in many countries, because early treatment dramatically changes outcomes. Disease-modifying therapies, including nusinersen (an intrathecal antisense oligonucleotide), onasemnogene abeparvovec (a one-time intravenous gene replacement therapy), and risdiplam (an oral SMN2 splicing modifier), have transformed what was historically a uniformly fatal Type 1 course into a much more favorable prognosis when treatment is started early.

Guillain-Barre Syndrome

Guillain-Barre syndrome (GBS) is an acute inflammatory demyelinating polyneuropathy, typically post-infectious, occurring 1-3 weeks after a gastrointestinal illness (classically Campylobacter jejuni) or a nonspecific upper respiratory infection.

• Ascending weakness: symmetric weakness that begins distally in the legs and progresses upward, potentially involving the trunk, arms, and cranial nerves, with facial diplegia being common
• Areflexia or hyporeflexia: loss of deep tendon reflexes is a hallmark finding and helps distinguish GBS from central causes of weakness
• Sensory symptoms, such as paresthesias and pain, are often present subjectively, but objective sensory loss on exam is usually mild relative to the degree of motor weakness
• Autonomic instability (labile blood pressure, arrhythmia) and respiratory muscle weakness are the two features that make GBS dangerous; vital capacity and negative inspiratory force should be monitored serially, since a child can progress to needing mechanical ventilation
• Cerebrospinal fluid (CSF): the classic finding is albuminocytologic dissociation, meaning elevated protein with a normal white cell count; this finding may be absent in the first week of illness, so a normal CSF early on does not rule out GBS
• Treatment: intravenous immunoglobulin (IVIG) or plasmapheresis are equally effective; corticosteroids are not effective in GBS, a frequently tested negative

A classic exam differentiator: myasthenia gravis causes fatigable weakness without sensory symptoms or areflexia; botulism causes a descending paralysis with early autonomic findings and normal or preserved early reflexes; transverse myelitis produces a sensory level and bowel or bladder dysfunction. GBS is distinguished by its ascending pattern, areflexia, and albuminocytologic dissociation together.

Pediatric Headache: Primary vs. Red-Flag Secondary Features

Most children presenting with headache have a primary headache disorder, migraine or tension-type headache, and do not need neuroimaging. Pediatric migraine differs slightly from the adult pattern: it is often shorter in duration, sometimes as brief as 1-2 hours compared with the adult criterion of 4 hours or longer, and more frequently bilateral rather than strictly unilateral.

The exam expects recognition of red flags that warrant neuroimaging to exclude a secondary cause, such as a mass lesion, hydrocephalus, or vascular event:

• Worst headache of life, or thunderclap onset with sudden, maximal-intensity pain
• Headache that wakes the child from sleep, or is worse in the early morning with vomiting, the classic pattern of raised intracranial pressure from a posterior fossa tumor
• Progressive increase in frequency or severity over weeks
• Focal neurologic signs, ataxia, or a new cranial nerve palsy
• Papilledema on fundoscopic exam
• Headache triggered or worsened by Valsalva maneuvers such as coughing, straining, or bending over, suggesting a Chiari malformation or posterior fossa mass
• Change in personality, decline in school performance, or abnormal growth and endocrine findings, suggesting a lesion in the hypothalamic-pituitary region
• Fever with neck stiffness, suggesting meningitis (see section 17.3)
• Very young age, roughly under 3-6 years, with a new headache, since younger children have a higher pretest probability of an underlying secondary cause
• A new or changed headache pattern in a child with a ventriculoperitoneal (VP) shunt, raising concern for shunt malfunction, or in a child with neurofibromatosis type 1 or tuberous sclerosis, both of which carry elevated tumor risk

The decision logic tested on this exam is straightforward: a child whose headache history and exam are reassuring, with a normal neurologic exam, no red flags, and a family history of migraine, needs clinical diagnosis and headache-specific management rather than imaging. Any red flag above shifts the answer toward urgent magnetic resonance imaging (MRI) of the brain, which is preferred over computed tomography (CT) when the situation is not emergent, in order to avoid radiation exposure and better visualize the posterior fossa.