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• Split weakness signs in ALS
  Michael Swash
  Published on: 23 April 2025

• Published on: 23 April 2025

  Split weakness signs in ALS

  • Michael Swash, Neurologist/Professor of Neurology Royal London Hospital and Institute of Neuroscience, University of Lisbon,Portugall

   I note the clinical analysis of differential weakness in ALS in elbow flexion (biceps brachii) compared to elbow extension (triceps) reported by Khalaf et al.1 This is described as analogous to similar 'split' muscle weakness around the ankle joint and, particularly, as that found in flexor digitorum indicis (FDI) compared to abductor digit minim (ADM) in the hand in the disease. It should be remembered that, although characteristic of ALS, this differential pattern of weakness has repeatedly been found not to be unique to ALS, even from the first descriptions.2,3 As the authors, and Vucic in his editorial remark,1,4 the cause of this interesting pattern of weakness in ALS remains uncertain. The finding of an association between the pattern of weakness and increased excitability in the upper motor neuron system in ALS does not necessarily provide primary support for an upper motor neuron (UMN) causation.  Nonetheless this pattern of weakness must be important in the disease. It is worth remembering that differential susceptibility to neurogenic lower motor neuron weakness is also a characteristic feature of some peripheral neuropathies, e.g., the Charcot-Marie-Tooth syndromes. Furthermore, differential muscle weakness and atrophy is a characteristic finding that is important in clinical diagnosis in the myriad different genetically determined muscular dystrophies.5 Although the causation of this differential susceptibility of certain muscles in this la...

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   I note the clinical analysis of differential weakness in ALS in elbow flexion (biceps brachii) compared to elbow extension (triceps) reported by Khalaf et al.1 This is described as analogous to similar 'split' muscle weakness around the ankle joint and, particularly, as that found in flexor digitorum indicis (FDI) compared to abductor digit minim (ADM) in the hand in the disease. It should be remembered that, although characteristic of ALS, this differential pattern of weakness has repeatedly been found not to be unique to ALS, even from the first descriptions.2,3 As the authors, and Vucic in his editorial remark,1,4 the cause of this interesting pattern of weakness in ALS remains uncertain. The finding of an association between the pattern of weakness and increased excitability in the upper motor neuron system in ALS does not necessarily provide primary support for an upper motor neuron (UMN) causation.  Nonetheless this pattern of weakness must be important in the disease. It is worth remembering that differential susceptibility to neurogenic lower motor neuron weakness is also a characteristic feature of some peripheral neuropathies, e.g., the Charcot-Marie-Tooth syndromes. Furthermore, differential muscle weakness and atrophy is a characteristic finding that is important in clinical diagnosis in the myriad different genetically determined muscular dystrophies.5 Although the causation of this differential susceptibility of certain muscles in this latter group of disorders remains uncertain, as does the pattern of susceptibility to denervation in ALS, local factor such as structural differences or even patterns of usage in causation are implied. The causation of differential neurogenic weakness and, indeed, of UMN weakness in ALS, is likely to be complex, dependent on local factors, motor nerve dysfunction and spinal and UMN factors.

   

  May I also point out that there is nothing new in all this? Kinnier Wilson6 gives a clear description in his textbook. He remarks that in the classic spinal type of Charcot syndrome: 

   

  “In the majority the small hand muscles are first affected, on either side indifferently. If the process begins in one arm it soon spreads to the other, either of a homologous place or another level. With loss of substance power is reduced, varying degrees of the two being found in one or other segment of the limbs. Muscles of the thenar and hypothenar groups waste as whole, that is, globally, and not in part……………..The process goes on to implicate one muscle after another proximally, wrist and finger flexors before extensors as a rule, and biceps before triceps, but a jump from the hand to the deltoid or shoulder muscle is not uncommon. When it begins in the forearm the long extensors are involved soonest, particularly those for the fingers and the ulnar side of the wrist: the supinators may hold out till the biceps weaken.”

   

  Wilson describes various other dissociated patterns of muscle involvement in a very detailed clinical description of the spinal type of ALS, a description that attests to the precision with which he and his contemporaries examined their patients, and their subsequent careful clinical notetaking. It is always wise to remember the skill of our forebears!

   

  1. Khalaf R, Martin S, Ellis C, et al. Relative preservation of triceps over biceps strength in upper limb-onset ALS: the ’split elbow’. J Neurol Neurosurg Psychiatry. In Press 2019. doi:10.1136/jnnp-2018-319894. [Epub ahead of print: 07 Mar 2019].

  2. Wilbourn AJ, Sweeney PJ. Dissociated wasting of medial and lateral hand muscles with motor neuron disease. Can J Neurol Sci 1994;21 (suppl 2):S9

  3. Schelhaas HJ, van de Warrenburg BPC, Kremer HPH, Zwarts MJ. The “split hand” phenomenon: evidence of a spinal origin. Neurology 2003;61:1619-1620

  4. Vucic S. Split elbow sign: more evidence for a corticospinal origin. J Neurol Neurosurg Psychiatry. doi:10.1136/jnnp-2019-320534

  5. Swash M. Six issues in muscle disease. J Neurol Neurosurg Psychiatry 2017;88:603-607 doi:10.1136/jnnp-2017-315771

  6. Kinnier Wilson SA. Amyotrophic lateral sclerosis: spinal types. In Neurology. Edited by A Ninian Bruce. London. Butterworth & Co. 2nd edition. 1954 p1145

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  Conflict of Interest:
  None declared.

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