The Split Hand Syndrome in Amyotrophic Lateral Sclerosis
The Split Hand Syndrome in Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder in which there is preferential loss of both the upper and lower motor neurons. At clinical onset, manifestations appear rather focal, resulting in bulbar, cervical or lumbar phenotypes. It has been suggested that neuronal loss spreads contiguously from the site of origin, becoming diffuse and resulting in complex motor deficits. Among the intrinsic hand muscles, wasting predominantly affects the 'lateral (thenar) hand' involving both median innervated muscles (abductor pollicis brevis (APB) and opponens pollicis) and ulnar innervated muscles (first dorsal interosseous (FDI), adductor pollicis and flexor pollicis brevis), with relative sparing of the hypothenar muscles (the abductor digiti minimi (ADM)) (figure 1). This peculiar pattern of dissociated atrophy of the intrinsic hand muscles was termed 'split hand' by Dr Asa Wilbourn.
(Enlarge Image)
Figure 1.
(A) The 'split hand' syndrome of amyotrophic lateral sclerosis. There is clear wasting of the first dorsal interosseous (FDI) and thenar complex but sparing of the hypothenar muscle (black arrows). FDI, thenar and hypothenar muscles are innervated by C8–T1, and FDI and hypothenar muscles by the ulnar nerve. The dissociated hand muscle involvement cannot be anatomically explained. (B) Hand muscle atrophy in C8 radiculopathy. Note the marked atrophy of the FDI as well as the hypothenar muscle (white arrows).
During the 1990s, Wilbourn routinely compared the amplitudes of compound muscle action potentials (CMAPs) and extent of denervation in the thenar muscles, FDI and ADM in patients with ALS. It was noted that the thenar muscles are frequently and substantially more denervated than the hypothenar muscles in ALS. Sometimes the FDI was even more severely denervated than the other muscles, and could be involved before the thenar muscles. As the muscles on the lateral aspect of the hand were preferentially affected, compared with those on the medial aspect of the hand, the disassociation was labelled 'split hand'.
At the Third International Symposium on ALS/motor neuron disease (MND), Wilbourn presented in abstract form 'dissociation of muscle wasting of ulnar innervated hand muscles with motor neuron disease', with a subsequent abstract 'dissociated wasting of medial and lateral hand muscles with motor neuron disease'. The same year Eisen introduced the term 'thenar hand' in ALS, emphasising the preferential wasting of the lateral hand in ALS. But the term 'split hand' was first coined in 1996. These early reports prompted one of the authors (SK) to prospectively examine motor neuron loss in the APB and ADM in ALS using motor unit number estimates. Motor unit loss was significantly greater in APB than ADM, and a simple comparison of CMAP amplitude showed significantly reduced APB/ADM ratios in ALS patients compared with normal subjects and neurological controls. Wilbourn responded to this publication with a letter to the editor entitled 'The split hand syndrome'.
Over the past decade, the split hand has become increasingly recognised as a useful clinical sign of ALS. The thenar complex muscles (APB and FDI) constituting the split hand are innervated through the same spinal segments (C8 and T1) but the FDI and ADM, which are differentially affected, share ulnar innervation. Therefore, the mechanisms underlying the split hand are complex and incompletely understood. However, recent evidence suggests that both cortical and spinal/peripheral mechanisms must be involved in the split hand of ALS.
Introduction and Historical Aspects
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder in which there is preferential loss of both the upper and lower motor neurons. At clinical onset, manifestations appear rather focal, resulting in bulbar, cervical or lumbar phenotypes. It has been suggested that neuronal loss spreads contiguously from the site of origin, becoming diffuse and resulting in complex motor deficits. Among the intrinsic hand muscles, wasting predominantly affects the 'lateral (thenar) hand' involving both median innervated muscles (abductor pollicis brevis (APB) and opponens pollicis) and ulnar innervated muscles (first dorsal interosseous (FDI), adductor pollicis and flexor pollicis brevis), with relative sparing of the hypothenar muscles (the abductor digiti minimi (ADM)) (figure 1). This peculiar pattern of dissociated atrophy of the intrinsic hand muscles was termed 'split hand' by Dr Asa Wilbourn.
(Enlarge Image)
Figure 1.
(A) The 'split hand' syndrome of amyotrophic lateral sclerosis. There is clear wasting of the first dorsal interosseous (FDI) and thenar complex but sparing of the hypothenar muscle (black arrows). FDI, thenar and hypothenar muscles are innervated by C8–T1, and FDI and hypothenar muscles by the ulnar nerve. The dissociated hand muscle involvement cannot be anatomically explained. (B) Hand muscle atrophy in C8 radiculopathy. Note the marked atrophy of the FDI as well as the hypothenar muscle (white arrows).
During the 1990s, Wilbourn routinely compared the amplitudes of compound muscle action potentials (CMAPs) and extent of denervation in the thenar muscles, FDI and ADM in patients with ALS. It was noted that the thenar muscles are frequently and substantially more denervated than the hypothenar muscles in ALS. Sometimes the FDI was even more severely denervated than the other muscles, and could be involved before the thenar muscles. As the muscles on the lateral aspect of the hand were preferentially affected, compared with those on the medial aspect of the hand, the disassociation was labelled 'split hand'.
At the Third International Symposium on ALS/motor neuron disease (MND), Wilbourn presented in abstract form 'dissociation of muscle wasting of ulnar innervated hand muscles with motor neuron disease', with a subsequent abstract 'dissociated wasting of medial and lateral hand muscles with motor neuron disease'. The same year Eisen introduced the term 'thenar hand' in ALS, emphasising the preferential wasting of the lateral hand in ALS. But the term 'split hand' was first coined in 1996. These early reports prompted one of the authors (SK) to prospectively examine motor neuron loss in the APB and ADM in ALS using motor unit number estimates. Motor unit loss was significantly greater in APB than ADM, and a simple comparison of CMAP amplitude showed significantly reduced APB/ADM ratios in ALS patients compared with normal subjects and neurological controls. Wilbourn responded to this publication with a letter to the editor entitled 'The split hand syndrome'.
Over the past decade, the split hand has become increasingly recognised as a useful clinical sign of ALS. The thenar complex muscles (APB and FDI) constituting the split hand are innervated through the same spinal segments (C8 and T1) but the FDI and ADM, which are differentially affected, share ulnar innervation. Therefore, the mechanisms underlying the split hand are complex and incompletely understood. However, recent evidence suggests that both cortical and spinal/peripheral mechanisms must be involved in the split hand of ALS.
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