Neuromuscular J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp-2016-313526 on 21 December 2016. Downloaded from REVIEW Differentiating lower motor neuron syndromes Nidhi Garg,1 Susanna B Park,1 Steve Vucic,2 Con Yiannikas,3 Judy Spies,1 James Howells,1 William Huynh,1,4 José M Matamala,1 Arun V Krishnan,4 John D Pollard,1 David R Cornblath,5 Mary M Reilly,6 Matthew C Kiernan1 ► Additional material is ABSTRACT MND from other relatively indolent conditions, published online only. To view Lower motor neuron (LMN) syndromes typically present such as SMA and immune neuropathies. The please visit the journal online (http://dx.doi.org/10.1136/ with muscle wasting and weakness and may arise from pattern of weakness should be documented, includ- jnnp-2016-313526). pathology affecting the distal motor nerve up to the level ing (1) symmetry versus asymmetry, (2) proximal of the anterior horn cell. A variety of hereditary causes versus distal involvement, (3) upper versus lower 1Brain and Mind Centre, Sydney are recognised, including spinal muscular atrophy, distal limb predominance and (4) presence versus absence Medical School, The University hereditary motor neuropathy and LMN variants of of bulbar involvement. Nerve conduction studies of Sydney, Sydney, New South Wales, Australia familial motor neuron disease. Recent genetic advances and electromyography (EMG) are essential to 2Departments of Neurology and have resulted in the identification of a variety of disease- confirm that the disorder is neurogenic and should Neurophysiology, Westmead causing mutations. Immune-mediated disorders, focus on assessing (1) the pattern of involvement, Hospital, The University of including multifocal motor neuropathy and variants of including symmetry and length dependence, (2) Sydney, Sydney, New South fl Wales, Australia chronic in ammatory demyelinating polyneuropathy, presence of focal motor conduction block or 3Department of Neurology, account for a proportion of LMN presentations and are demyelinating features and (3) the presence or Concord and Royal North Shore important to recognise, as effective treatments are absence of subclinical sensory abnormalities. Hospitals, The University of available. The present review will outline the spectrum of Imaging, genetic testing, antibody markers and Sydney, Sydney, New South LMN syndromes that may develop in adulthood and advanced neurophysiological techniques are useful Wales, Australia 4Prince of Wales Clinical School, provide a framework for the clinician assessing a patient adjuncts and form an extension of the clinical The University of New South presenting with predominantly LMN features. assessment. The present review will examine LMN Wales, Sydney, New South syndromes from a clinical perspective as well as Wales, Australia providing an overview of current understanding of copyright. 5Department of Neurology, Johns Hopkins School of pathophysiological mechanisms. Medicine, Baltimore, Maryland, INTRODUCTION USA Lower motor neuron (LMN) syndromes are clinic- 6MRC Centre for Neuromuscular ally characterised by muscle atrophy, weakness and Late-onset SMA Diseases, National Hospital for hyporeflexia without sensory involvement. They SMA represents a group of genetic disorders result- Neurology and Neurosurgery may arise from disease processes affecting the ing in the degeneration of anterior horn cells in the and UCL Institute of Neurology, London, UK anterior horn cell or the motor axon and/or its sur- spinal cord and motor nuclei in the brainstem rounding myelin. Neuromuscular junction path- causing progressive, predominantly proximal fl Correspondence to ology and muscle disorders may mimic a LMN muscle weakness with reduced or absent re exes. Professor Matthew C Kiernan, disorder and form part of the differential diagnosis. They are classified into four types on the basis of Brain and Mind Centre, The LMN syndromes can be broadly classified as age of onset and clinical course (SMA I–IV).1 SMA University of Sydney, 94 Mallett hereditary, sporadic or immune-mediated. I and II are defined by onset in infancy. SMA III is http://jnnp.bmj.com/ Street Camperdown, Sydney, NSW 2050, Australia; Immune-mediated neuropathies, such as multifocal a milder phenotype with signs of weakness present- matthew.kiernan@sydney. motor neuropathy (MMN) and chronic inflamma- ing at or after 1 year of age with patients attaining edu.au tory demyelinating polyneuropathy (CIDP) are the ability to walk unaided.1 It is associated with important to distinguish from sporadic and heredi- significant variability in the age of onset, disease Received 22 September 2016 tary forms, as treatments are available. LMN pre- progression and ambulatory period with some Accepted 21 November 2016 fi Published Online First sentations of motor neuron disease (MND) are patients only developing walking dif culties in 21 December 2016 most often sporadic, but several genetic mutations adulthood.2 Adult-onset SMA (SMA IV) typically on September 30, 2021 by guest. Protected have been described which can be associated with presents in the third or fourth decade of life with a LMN preponderance. Other hereditary forms of slowly progressive and relatively benign course.3 LMN syndromes include the spinal muscular atro- Respiratory insufficiency may occur in SMA IV, but phies (SMAs) and distal hereditary motor neuropa- is usually mild and life expectancy is normal.1 thies (dHMNs). The increasing availability of The vast majority of SMA is autosomal recessive next-generation sequencing (NGS), including the (AR) in inheritance and related to mutations in the ability for multiple genes to be sequenced in paral- SMN1 gene located on chromosome 5q13. Most lel, has resulted in an increase in the discovery of cases are homozygous for a deletion of exon 7 novel genetic mutations. (94%), but a small percentage are compound het- The clinical evaluation of a patient presenting erozygous for a deletion in SMN1 and an intragenic with a LMN syndrome includes a thorough assess- mutation of SMN1.4 Targeted molecular genetic fi To cite: Garg N, Park SB, ment of disease onset and progression. This is par- testing is the rst-line investigation for SMA to Vucic S, et al. J Neurol ticularly important to ascertain as a rapid rate of detect homozygous deletions of SMN1 exon 7 Neurosurg Psychiatry decline may support a diagnosis of MND and gene. However, if only a single deletion is detected, 2017;88:474–483. remains an important factor in distinguishing sequencing the SMN1 gene should be performed to 474 Garg N, et al. J Neurol Neurosurg Psychiatry 2017;88:474–483. doi:10.1136/jnnp-2016-313526 Neuromuscular J Neurol Neurosurg Psychiatry: first published as 10.1136/jnnp-2016-313526 on 21 December 2016. Downloaded from assess for a point mutation. Overall, 4–5% of patients with clin- Despite significant advances in molecular genetics, a disease- ically typical SMA have no identifiable mutation in SMN1.5 causing mutation is only identified in ∼15% of patients with a Non-5q SMA can be inherited in an autosomal dominant (AD), typical presentation of dHMN.13 Mutations in the HSPB1, AR or X-linked pattern with marked clinical and genetic hetero- HSPB8 and BSCL2 genes are the most frequent causes of AD geneity. NGS technology has facilitated the discovery of a dHMN. Mutations in HSPB1 and HSPB8 are associated with a number of non-5q causative genes associated with SMA.6 classical length-dependent motor neuropathy beginning in the lower limbs which may present in childhood (dHMN type I) or adulthood (dHMN type II).14 Several phenotypes associated Spinobulbar muscular atrophy (Kennedy’s disease) with mutations in BSCL2 have been described and include (1) Spinobulbar muscular atrophy or Kennedy’s disease is the most dHMN type II with a length-dependent motor neuropathy, (2) common adult-onset SMA. It is a polyglutamine genetic dis- dHMN V presenting with a predominantly upper limb distal order caused by a CAG trinucleotide repeat expansion in the phenotype, (3) dHMN with pyramidal signs and (4) Silver syn- androgen receptor gene on the X-chromosome.7 Degeneration drome with atrophy of the intrinsic hand muscles, pyramidal of motor neurons in the spinal cord and brainstem results in a 14 signs and lower limb spasticity. The upper limb-onset pheno- slowly progressive disorder characterised by weakness and type (dHMN V) may also result from mutations in GARS with atrophy of facial, bulbar and limb muscles without upper motor most cases presenting in their second decade with progressive neuron (UMN) signs (figure 1). Cramps, leg weakness, tremor weakness and wasting of the thenar eminence and first dorsal and orolingual fasciculations (see online supplementary video 15 interossei muscles. Cramping and pain in the hands on expos- S1) with bulbar symptoms are the most common presenting 15 ure to cold may be an early manifestation. The GARS mutation symptoms. The syndrome affects only men, although female may also present with a classical length-dependent neuropathy carriers may experience mild symptoms such as cramps.8 beginning in the lower limbs, further highlighting the variabi- Symptom onset is typically between 30 and 50 years of age, but lity in genotype–phenotype correlations. It remains unclear why there is marked variability in age of presentation.9 Weakness is mutations in ubiquitously expressed proteins may result in such typically noted first in the lower limbs and may be symmetrical 11 variable and ‘focal’ phenotypes. Bulbar involvement is rare in or asymmetrical, affecting