Kennedy’s disease (KD) is an x-linked recessive spinal muscular atrophy that is characterized by slowly progressing bulbar and proximal atrophy, as opposed to distal, and limb weakness.¹ It presents with symptoms similar to most neuromuscular disorders, but early clinical diagnosis is challenging due to its slow progression and rarity of cases involving sensory loss or neuropathic pain.² Moreover, it is characterized by clinical features, such as gynecomastia, testicular atrophy, erectile dysfunction, and diabetes mellitus. Furthermore, since the first case reported by Kennedy in 1968,³ cases have been rarely reported both worldwide and in Korea.^4,5 This disease has an x-linked recessive inheri-tance pattern and is thought to be caused by a mutation in the androgen receptor gene that causes an abnormal increase in trinucleotide Cytosine-Adenine-Guanine (CAG) repeats, which results in the abnormal aggregation of the androgen receptor and dysfunction of the cytoskeletal system within nerve cells due to impaired tubulin regulation.^4,6 Although the disease can be confirmed through molecular genetic analysis, the turnaround time is more than a month.⁴

The most common symptom of KD is muscle spasms, followed by lower limb muscle weakness, gynecomastia, and upper limb muscle weakness.⁷ We report the case of a patient who presented with chronic LBP and mild muscle weakness as chief complaints and suspected with KD based on thorough physical examination and electrophysiological testing. KD was subsequently confirmed through molecular genetic testing.

A 52-year-old male patient presented to a secondary hospital in 2018 due to chronic LBP and persistent muscle weakness that began since 2013. The patient did not have notable history or family history. Blood test and radiologic examination were performed under the suspicion of myopathy and degenerative spinal disorder. And result, only creatinine phosphokinase (CPK) was increased and lumbar spine MRI showed degenerative changes. The patient used to run a secondhand shop, which involved frequent lifting and moving of heavy objects. Considering this occupational history, suspected a renal lesion caused by rhabdomyolysis or chronic LBP caused by degenerative spinal disorder and provided conservative treatment; however, the patient did not respond. In the past few years, chronic LBP was exacerbated, and the patient’s exercise endurance declined.

Subsequently, the patient was referred to the rehabilita-tion medicine department at our hospital in 2023. Physical examination showed mild muscle weakness in some proximal muscles (i.e., grade 4 for quadriceps femoris and biceps on manual muscle test), increased deep tendon reflex, and tongue muscle atrophy, but the patient did not show fasciculation and pathological reflex. Overall, a motor neuron disorder was suspected. Lumbar spine MRI was performed to rule out central nervous lesion and radiculopathy, and there was no significant difference from the previous examination in 2018, such as showing only some degenerative changes (Fig. 1).

Figure 1. T2 weighted images of the lumbar spine show degenerative changes.

In terms of the electrophysiological examination, the motor nerve conduction test showed decreased latency and amplitude in some nerves but was close to normal compared to that in the sensory nerve conduction test. Sensory nerve conduction tests showed decreased latency and amplitude in most nerves (Table 1). F-wave and H-reflex test results were unremarkable.

Table 1 . Findings of Nerve Conduction Study.

Nerve	Stimulation	Right				Left
		Latency (ms)	Amplitude (mV)	CV (m/s)		Latency (ms)	Amplitude (mV)	CV (m/s)
Motor
Median at APB	Wrist	4.43	10.2			4.48	9.8
	Elbow	8.33	9.2	56.8		8.39	9.6	53.2
Ulnar at ADM	Wrist	3.33	11.8			3.02	11.0
	Below elbow	6.51	11.6	66.7		6.51	10.3	55.6
Peroneal at EDB	Ankle	4.01	2.2			4.90	1.8
	Fibular head	10.42	1.5	44.0		10.94	1.9	46.5
Tibial at AH	Ankle	3.91	19.8			3.70	18.0
	Fibular head	11.67	15.8	45.4		11.30	15.2	46.2
Sensory
Median at Digit II	Wrist	3.44	6.1	41		3.49	8.8	40
Ulnar at Digit V	Wrist	2.76	7.1	51		2.60	6.3	54
Sural	Calf	3.28	5.1	43		3.23	4.8	43
Superficial peroneal	Lateral leg	3.33	2.5	42		3.39	3.9	41

CV: conduction velocity, APB: abductor pollicis brevis, EDB: extensor digitorum brevis, AH: abductor hallucis..

Needle electromyography (EMG) revealed widespread reduction in motor unit action potential recruitment with large amplitude and long duration. Fasciculation potentials, diffuse fibrillation potentials and positive sharp waves were detected.¹

Thus, molecular genetic testing was performed under the suspicion of KD. CAG repeats were increased to 46, based on which KD was confirmed (Fig. 2).

Figure 2. The number of CAG repeats increased to 46, confirming the diagnosis of Kennedy’s disease.

A study on 57 patients with KD in the United States reported that the most common symptom was muscle spasms, followed by lower limb muscle weakness, gynecomastia, and upper limb muscle weakness. In a study on 46 British patients with KD, the most common symptom was lower limb weakness, followed by upper limb weakness, tremor, bulbar weakness, and gynecomastia.⁷ In a Korean case report, the chief complaint was lower limb weakness and dysarthria.^4,5 Muscle biopsy, a painful procedure, is commonly performed for the diagnosis of KD since molecular genetic testing, which is the definitive diagnostic test for this diseases, is costly and time-consuming.⁴ Thus, patient consent is difficult to obtain without a strong suspicion of diagnosis. However, unlike other motor neuron diseases (MNDs), sensory nerve action potentials on EMG have reduced amplitude or are unobtainable in KD, while compound muscle action potentials are characteristically normal. Needle EMG reveal muscle fiber fasciculations and prominent abnormal spontaneous activity, similar to other MNDs.¹

The patient in our case had chief complaints of chronic LBP and mild muscle weakness only, without the other common symptoms, such as gynecomastia, testicular atrophy, erectile dysfunction, and diabetes mellitus.³ Furthermore, the patient also had no notable family history. This seems to be similar to previous reports that 74% of patients with KD have no family history.⁸ However, physical examination revealed more significant muscle weakness affecting the proximal than the distal muscles, along with prominent increase in deep tendon reflex, tongue atrophy, and elevated CPK on blood test. Thus, we performed EMG to differentiate myopathy and MND. As characteristic EMG findings of KD were observed, we were able to confirm KD through molecular genetic testing.

A non-negligible percentage (12.8%) of chronic LBP cases is caused by other diseases, including congenital abnormalities and deformities,⁹ and 1% of the cases may be caused by a severe disease, including spondylitis, spinal tumor, and genetic disorders, as opposed to a simple structural issue. This highlights the importance of detailed history taking and neurological testing.¹⁰

The reported patient had only received symptomatic treatment for chronic LBP and mild muscle weakness before presenting to our hospital; however, his symptoms progressed without marked improvement. This case is significant since KD, an uncommon genetic disorder, was diagnosed subsequently based on detailed history taking and neurological testing.