Spinal Muscular Atrophy: Autopsy Based Neuropathological Demonstration
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.293477
Source of Support: None, Conflict of Interest: None
Keywords: Neurogenic atrophy, spinal muscular atrophy, Werdnig–Hoffmann disease
Spinal muscular atrophy (SMA) is characterized by progressive degeneration of lower motor neurons in the spinal cord, which, in severe cases, extends to bulbar motor nuclei and in which there is concomitant skeletal muscle atrophy. The onset of weakness ranges from before birth to adolescence or young adulthood. The weakness is symmetric, with proximal more than distal, and progressive. It is the most common genetic cause of death in infants. Based on the age of onset and clinical course, SMA is classified into four types of which type 1 is a severe form with infantile onset. We report the post-mortem neuropathological findings of a five-and-half month-old boy with type I SMA who succumbed to the illness.
A five-and-half month old boy was admitted with complaints of difficulty in breathing for five days associated with fever, cough, and poor feeding for two days duration. Developmental history was significant with history of paucity of limb movements since birth. No significant family history was present, and child was first-born of a non-consanguineous marriage. On examination, the baby had fast breathing with retraction, non-productive cough and altered sensorium. Bilateral crepitations were present. There was isolated motor development delay in the form of absent neck holding and rolling over. Language, social, and visual developments were appropriate for age. There was generalized floppiness, hypotonia and areflexia. Power was 1/5 in all four limbs. No gross wasting of muscle fibres was noted. Tongue fasciculations were present.
Arterial blood gas analysis revealed chronic respiratory acidosis with metabolic compensation with [pH of 7.25 (N: 7.36–7.44), PaO2 of 85 mmHg (N: 80-100 mmHg), PaCO2 of 58.9 mmHg (N: 35–45 mmHg), HCO3:32.9 mEq/L (N: 22-26 mEq/L)]. Chest X-ray showed mild cardiomegaly and heterogeneous opacity involving lower lobe of right lung. Electrocardiogram showed baseline fasciculations indicating a myopathy. The respiratory distress and shock worsened and the child succumbed within three days of admission despite intravenous antibiotics, respiratory support, correction of electrolyte disturbance and anti-shock measures.
A complete autopsy including brain and spinal cord was performed on this baby with a clinical diagnosis of acute floppy infant likely to be SMA and a differential of glycogen storage disorder like Pompe's disease was kept. No facial dysmorphism was noted.
Brain weighed 630g (average normal weight for this age-644 g). The gyral pattern was normal. No focal lesions or infarcts were seen on coronal sectioning of entire brain. On microscopy, the cortical laminations of cortical neurons were preserved, however hypoxic changes were noted within the cortical neurons [Figure 1]a, [Figure 1]b, [Figure 1]c. The deep grey nuclei including basal ganglia, thalamus, granular and pyramidal neurons of dentate gyrus (CA4–CA1 areas), substantia nigra, pontine nuclei, medulla (inferior olivary nuclei) were normal [Figure 2]a, [Figure 2]b, [Figure 2]c, [Figure 2]d. The cerebellar cortex including granular neurons, Purkinje cells, dentate nuclei were also normal. The nuclei of the cranial nerves including hypoglossal and vagus were unremarkable. The anterior spinal roots were thinner than the posterior roots.
Sections from spinal cord at upper and lower cervical levels, thoracic levels, and lumbar levels were sampled [Figure 3]a. The anterior motor neurons at cervical and thoracic levels demonstrated neuronal loss with few chromatolytic and ballooned neurons with peripheral condensation of Nissl granules [Figure 3]b, [Figure 3]c, [Figure 3]d, [Figure 3]e. Neuronophagia was rarely seen. No microglial nodules were identified. Neuronal loss was further highlighted by NeuNimmunostain. Neurons within the dorsal thoracic nuclei of Clarke's revealed variable degree of degeneration [Figure 4]a and [Figure 4]b. CD68 positive cells were not significantly increased. There was no neuronal loss noted in the posterior nerve root ganglion. No significant gliosis was evident in regions of neuronal loss [Figure 4]c. Sampled portions of anterior nerve roots attached to the spinal cord were atrophic with loss of myelinated fibres [Figure 4]d.
Snap frozen section of psoas muscle biopsy revealed marked group atrophy of fibres with compensatory hypertrophy of adjacent fibres [Figure 5]a and b]. The atrophic fibres were predominantly rounded. No significant interstitial fibrosis or inflammatory infiltrate was seen. Immunohistochemistry performed for slow and fast myosin highlighted predominant atrophy of type 2 fibres and hypertrophy of type 1 fibres [Figure 5]c and d]. No ragged red fibres or sub-sarcolemmal accumulation of mitochondria was observed on Modified Gomori trichrome stain [Figure 5]e.
The heart weighed 38.5g (average normal weight for this age-31g). The shape of the heart was maintained and no congenital malformations were seen. Left ventricular wall thickness was 0.4 cm and right ventricular wall thickness was 0.2 cm. Microscopically, no significant interstitial fibrosis or inflammatory infiltrate was seen. Cardiac myocytes did not show any vacuolation which is a feature of Pompe's disease. Periodic acid Schiff (PAS) stain also did not highlight any glycogen deposition within the cells. Electron microscopy did not reveal any membrane bound vacuoles, thus ruling out clinically suspected glycogen storage disorder. Liver weighed 198g (average normal weight for this age is 200g) and on microscopy revealed maintained lobular architecture with diffuse microvesicular steatosis. PAS stain, however, did not show positivity in any of the vacuoles. Semi-thin section (1 μg) stained with toluidine blue confirmed the microvesicular steatosis and did not reveal any glycogen vacuoles. Lungs weighed 120 g (average normal weight for this age is 81g) and on cut surface revealed areas of consolidation and hemorrhage. On microscopy, there was extensive diffuse alveolar damage in exudative phase with hyaline membrane formation in alveoli. Bronchopneumonia was also seen with acute inflammatory infiltrate within the alveoli. Gram Twort stain failed to highlight any gram positive or negative organisms. Rest of the organs was unremarkable. Hence, a final autopsy diagnosis of neurogenic atrophy consistent with SMA type 1 along with bronchopneumonia and diffuse alveolar damage was rendered.
SMA is characterized by muscle weakness and atrophy resulting from progressive degeneration and loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The incidence of SMA is estimated to be 1:11,000 births. Guido Werdnig in 1891 and Johann Hoffmann, between 1893 and 1900, described a total of nine cases of a disease where young people were affected by a new form of muscular atrophy. SMA has been classified traditionally into types 1–3, but some experts suggest an expanded classification that includes additional subtypes from type 0 to type 4 on the basis of age of onset and the highest motor function achieved. Type 1 or the Werdnig–Hoffmann disease is a very severe form representing 45% of cases; it is associated with onset after birth but before age six months. Affected infants have delayed milestones with impaired head control, weak cry and cough and inability to sit. Swallowing, feeding, and handling of oral secretion are affected before 1 year of age. Weakness and hypotonia in the limbs and trunks are eventually accompanied by intercostal muscle weakness. Paradoxical breathing is a characteristic feature with flattening of the chest wall (rather than expansion) and protrusion of the abdomen during inspiration. Their life expectancy usually does not exceed two years. Type 2 SMA is an intermediate form with onset is between 7 and 18 months, while patients with type 3 SMA (Kugelberg and Welander syndrome) have onset after 30th month of life. Type 4 has onset between the 10th and the 30th year of life and many have a normal lifespan. The clinical features such as the age of onset, paucity of limb movements with absent neck control as well as pathological features in the index child were consistent of Type 1 SMA.
SMA is inherited in an autosomal recessive manner. The classical form of the disorder is caused by a genetic mutation in 5q11.2–q13.3, affecting the survival motor neuron (SMN) gene. This gene is duplicated with a telomeric copy (SMN1) and a centromeric analogue copy (SMN2). SMN2 predominantly produces a survival motor neuron protein that is lacking in exon 7, a less stable protein. SMA is caused by loss of SMN1 because SMN2 cannot fully compensate for loss of SMN1-produced protein. However, when the SMN2 (dosage) copy number is increased, the small amount of full-length transcript generated by SMN2 is often able to produce a milder type II or type III phenotype.
In the index case, the atrophic fibres in the muscle biopsy were predominantly rounded in contrast to the angulated shape seen in other forms of neurogenic atrophy such as amyotrophic lateral sclerosis. Muscle biopsy cannot distinguish clearly between SMA subtypes, but certain histological features are associated with disease severity. Muscle biopsy in infants with types 1 and 2 shows large groups of atrophic fibres interspersed with fascicles of hypertrophied and normal fibres as demonstrated in this case. The hypertrophied fibres are usually type 1. In milder cases of type 2 and type 3 SMA, there are typically groups of uniformly atrophic fibres between groups of non-atrophic muscle fibres. Both the atrophic and non-atrophic groups may be of either type 1 or 2 muscle fibres. The groups of atrophic fibres vary in size. Muscle biopsy features in type 4 SMA are similar to type 3. Harding et al. had demonstrated the neuropathological findings in five genetically confirmed cases of Type 1 SMA. Similar to our case, degeneration of neurons was predominantly seen in the anterior horn cells and Clarke's column. However, unlike our case, two cases showed degenerative changes in the cerebral cortex, thalamus and brain stem and cranial nerve nuclei XII, X, VII, VI, III. The dorsal root ganglion showed residual nodules of Nageotte and frequent ballooned chromatolytic sensory neurons. Similar findings were reported by Ito et al. In addition, there was expression of phosphorylated NFP in the ballooned neurons with prominent SMN immunostaining in the prenatal period and loss in the later stages. Autopsy pathology of SMA has been reported rarely in literature., Kuru and colleagues describe abundant gliosis in areas of neuronal loss and glial bundles in the nerve roots in anterior spinal roots. Long clinical course and late onset could explain the presence of these which were not evident in the present case.
One of the interesting challenges in considering the pathogenesis of SMA is the selective vulnerability of certain cell populations. The current hypothesis is that there may be differential thresholds between different cell types, with spinal motor neurons the most sensitive to a reduction in SMN protein expression. Individuals with only 1 copy of the truncated SMN2 gene have the lowest level of SMN protein compatible with survival, which may unmask the vulnerability of cell types with lesser quantitative requirements for the protein.
The diagnosis of SMA is confirmed by genetic test demonstrating homozygous SMN1 deletion which is 100% specific and 95% sensitive for SMA. If mutational analysis is negative, laboratory investigations including electrophysiological tests such as electromyography and nerve conduction tests should be performed. If electromyography suggests a motor neuron disease, then further testing for SMN mutations should be undertaken. Mutational analysis for SMN1 gene could not be performed in the present case. However, the neuropathological features at autopsy are classical. Relatively few such reports are documented in the literature which prompted us to report this instructive case.
Until recently, treatment of SMA was primarily supportive. The identification of SMN2 as a molecular target and generation of animal models for preclinical testing has significantly transformed its therapeutics. Lately, the therapeutic strategies focus on increasing the amount of SMN protein through small molecules, oligonucleotides and gene replacement.
In conclusion, the report adds observational data to the literature as there are very few autopsy studies demonstrating the neuropathological findings of SMA leading to its complications. The case illustrates one such rare example with a brief review of the literature. Thus, in a child presenting with protean manifestation of delayed milestones, hypotonia and floppiness, genetic testing for SMN gene must be performed. Characteristics features of neurogenic atrophy in muscle biopsy if performed are helpful supportive clues for diagnosis. Lastly, role of genetic counselling is equally important as it has autosomal recessive inheritance.
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The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
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